Start, stop and pause flows, and show what their nodes print

Flows can now be taken off the engine and put back. Stopped state lives in a
runtime.json beside the flow, not in the flow document: the canvas autosaves
that document, so a stopped flow would otherwise start itself again on the
next edit. A stopped flow gets no subscriptions, schedules or webhooks, its
nodes are skipped by the scheduler, and running it answers 409. Pausing holds
a flow's nodes while its values keep arriving, so the canvas still shows what
is coming in.

Node code is user code and print is how it says things, so stdout is teed
through a contextvar sink active only during a node execution — one event per
execution, capped, so a chatty node cannot outrun the stream. A node that
fails sends its traceback the same way, trimmed to the author's own frames.
The dock gains a logs panel and a pause control; the dashboard replaces its
placeholder with what is running, stopped or failing; the edge inspector can
send the last message again.

Single-stepping is deferred and noted: the scheduler keeps no progress between
calls, so a step button would re-run the same node rather than advance.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

NOTEPAD.md

@@ -11,7 +11,12 @@ Deferring because out of scope is fine, but don't mention deferring than.
 - BUG/UI: enlarge the icon in the sidebar slightly
 - BUG/UI: clicking outside the panel does not discard the flow edit panel
 - BUG/UI: the graph showed in the node edit panel should also be shown for a specific edge inside the pop-up panel when clicking the edge
-- BUG/UI: the "run" button in the toolbar should be stateful; i.e. when a flow is running (i.e. with nodes that automatically emit data such as triggers), it should be possible to pause the flow and resume it. Also for flows without a trigger, the button should change state to allow interrupting a running node
+- FEAT/FLOW: single-stepping a paused flow. Pause and resume are in; a step button needs the
+  scheduler to keep its per-run progress between calls, which the one-shot executor does not —
+  without that it re-runs the first ready node instead of advancing. Needs a persistent
+  per-flow work queue that a step pops from and resume drains.
+- FEAT/UI: interrupting a node that is already running. Pause holds nodes that have not been
+  submitted yet; one already executing runs to completion.
 - BUG/UI: the enlarged panel (for code editing) should still maintain its floating style
 - BUG/UI: `SidePanel`'s mobile branch does not set `data-testid` on the `SheetContent`, so
   `[data-testid=node-panel]` does not exist on a phone. Mobile specs cannot address the panel.
@@ -57,9 +62,9 @@ Deferring because out of scope is fine, but don't mention deferring than.
 
 ## Blocked
 
-- FEAT/UI: a "Bug" icon on the node error bubble opening the console at the full error. The
-  bubble now shows one line and the traceback only reaches the server log; there is no
-  console in the roadmap yet for it to open.
+- FEAT/UI: a "Bug" icon on the node error bubble opening the logs panel at that node's
+  traceback. The panel now has the traceback; wiring the bubble to open and filter it needs
+  the panel's open state lifted into `FlowEditor`.
 - FEAT/INFRA: MQTT broker and InfluxDB compose services for local development. The node types
   exist; a local broker would make them testable without external hardware.
 - CHORE/INFRA: `bun install` inside the frontend Docker build intermittently fails with

ROADMAP.md

@@ -52,6 +52,11 @@ external interfaces. See `docs/architecture/structure.canvas` → *Backend – M
 - [x] REST + WebSocket API over the engine: create/read/update flows, edit node
       source, run, and stream values, node status and execution events
 - [x] Dependency-loop detection and graph validation surfaced as API errors
+- [x] Per-flow start/stop, stored in a `runtime.json` beside the flow so it
+      survives a restart and stays out of the autosaved document; pause/resume
+      holds a flow's nodes while its values keep arriving
+- [x] Node log streaming: what a node prints, and the traceback of one that
+      fails, reach the editor as `node_log` events
 - [ ] MQTT broker / InfluxDB compose services for local development
 - [x] Git-based versioning of the flow store (one commit per saved change)
 - [x] Draft/publish split: edits autosave to `flow.draft.json` / `nodes.draft/`,
@@ -94,6 +99,10 @@ React + Vite, primarily desktop but usable on mobile. See `docs/architecture/str
 - [x] Validation shown on the node it belongs to, and summarised in the dock
 - [x] Publish control and draft markers in the flow bar, discard in the flow
       panel, and a conflict dialog when another client got there first
+- [x] Dashboard showing which flows run, which are stopped and which have
+      errors, with a switch per flow
+- [x] Logs panel in the canvas dock, pause/resume beside Run, and replaying an
+      edge's last message from the inspector
 - [ ] Device assignment per node, selectable from compatible devices
 - [ ] Test-node affordance on the canvas
 - [ ] User management screens

backend/app/api/routes/flows.py

@@ -57,6 +57,8 @@ class FlowDetail(BaseModel):
     nodes: list[NodeStatusPublic] = []
     issues: list[ValidationIssue] = []
     has_draft: bool = False
+    enabled: bool = True
+    paused: bool = False
 
 
 class ValidationResult(BaseModel):
@@ -81,6 +83,10 @@ class TriggerRequest(BaseModel):
 
 def _detail(controller: FlowController, definition: FlowDef) -> FlowDetail:
     name = definition.name
+    running = {
+        "enabled": controller.is_enabled(name),
+        "paused": controller.is_paused(name),
+    }
     if controller.store.has_draft(name):
         # Report the draft the editor is showing, not the version running
         # underneath it — otherwise a node the author just broke looks fine.
@@ -90,11 +96,13 @@ def _detail(controller: FlowController, definition: FlowDef) -> FlowDetail:
             nodes=preview.nodes,
             issues=preview.issues,
             has_draft=True,
+            **running,
         )
     return FlowDetail(
         definition=definition,
         nodes=controller.node_statuses(name),
         issues=controller.flow_issues(name),
+        **running,
     )
 
 
@@ -106,6 +114,14 @@ def _read_flow(controller: FlowController, name: str) -> FlowDef:
         raise HTTPException(status_code=404, detail=f"No flow named '{name}'")
 
 
+def _require_enabled(controller: FlowController, name: str) -> None:
+    if not controller.is_enabled(name):
+        raise HTTPException(
+            status_code=409,
+            detail=f"Flow '{name}' is stopped — start it before running it",
+        )
+
+
 def _flow_state(controller: FlowController, name: str) -> FlowStatePublic:
     return FlowStatePublic(
         values={
@@ -137,6 +153,8 @@ def read_flows(controller: FlowControllerDep) -> Any:
                 node_count=len(definition.nodes),
                 error_count=sum(1 for s in statuses if s.status == "error"),
                 has_draft=controller.store.has_draft(name),
+                enabled=controller.is_enabled(name),
+                paused=controller.is_paused(name),
             )
         )
     return FlowsPublic(data=summaries, count=len(summaries))
@@ -316,6 +334,43 @@ async def save_node_source(
     )
 
 
+# -----------------------------------------------------------------------------
+# Running, stopped, paused
+# -----------------------------------------------------------------------------
+
+
+@router.post("/{name}/start", response_model=FlowDetail)
+async def start_flow(name: str, controller: FlowControllerDep) -> Any:
+    """Let the engine run this flow again."""
+    _read_flow(controller, name)
+    await controller.set_enabled(name, True)
+    return _detail(controller, _read_flow(controller, name))
+
+
+@router.post("/{name}/stop", response_model=FlowDetail)
+async def stop_flow(name: str, controller: FlowControllerDep) -> Any:
+    """Take this flow off the engine: no subscriptions, schedules or webhooks."""
+    _read_flow(controller, name)
+    await controller.set_enabled(name, False)
+    return _detail(controller, _read_flow(controller, name))
+
+
+@router.post("/{name}/pause", response_model=Message)
+def pause_flow(name: str, controller: FlowControllerDep) -> Any:
+    """Hold the flow's nodes so its messages can be stepped through."""
+    _read_flow(controller, name)
+    controller.pause_flow(name)
+    return Message(message=f"Paused flow '{name}'")
+
+
+@router.post("/{name}/resume", response_model=Message)
+async def resume_flow(name: str, controller: FlowControllerDep) -> Any:
+    """Let the flow carry on, running whatever was held back."""
+    _read_flow(controller, name)
+    await run_in_threadpool(controller.resume_flow, name)
+    return Message(message=f"Resumed flow '{name}'")
+
+
 # -----------------------------------------------------------------------------
 # Validation and execution
 # -----------------------------------------------------------------------------
@@ -342,6 +397,7 @@ async def run_flow(
     on the canvas. Nothing is deployed by running it.
     """
     _read_flow(controller, name)
+    _require_enabled(controller, name)
     inputs = {qualify(name, key): value for key, value in body.inputs.items()}
     if controller.store.has_draft(name):
         await run_in_threadpool(controller.run_preview, name, inputs)
@@ -358,6 +414,7 @@ async def trigger_node(
     controller: FlowControllerDep,
 ) -> Any:
     """Feed values into a single node."""
+    _require_enabled(controller, name)
     try:
         await run_in_threadpool(
             controller.trigger_node, f"{name}.{node_id}", body.values
@@ -429,6 +486,8 @@ async def flow_events(websocket: WebSocket, token: str = "") -> None:
                 "values": controller.values(),
                 "nodes": [s.model_dump() for s in controller.node_statuses()],
                 "issues": [i.model_dump() for i in controller.issues],
+                "paused": controller.paused_flows(),
+                "logs": list(event_bus.recent_logs),
             }
         )
 

backend/app/flow/controller.py

@@ -20,6 +20,7 @@ from types import ModuleType
 from typing import Any, cast
 
 from fastapi import FastAPI
+from fastapi.concurrency import run_in_threadpool
 
 from app.flow.events import EventBus
 from app.flow.messages import MessageSpec, qualify
@@ -188,6 +189,7 @@ class FlowController:
         self.pipeline: Pipeline | None = None
         self.loaded: dict[str, LoadedNode] = {}
         self.issues: list[ValidationIssue] = []
+        self.disabled: set[str] = set()
         self._lock = asyncio.Lock()
 
     # -------------------------------------------------------------------------
@@ -200,13 +202,24 @@ class FlowController:
     async def stop(self) -> None:
         await self._teardown()
 
+    async def set_enabled(self, flow: str, enabled: bool) -> None:
+        """Stop or start one flow. Rebuilding is what applies it."""
+        await run_in_threadpool(self.store.write_enabled, flow, enabled)
+        await self.reload()
+
     async def reload(self) -> None:
         """Rebuild the whole pipeline from what is currently stored."""
         async with self._lock:
             await self._teardown()
 
+            published = self.store.read_all()
+            self.disabled = {
+                flow.name
+                for flow in published
+                if not self.store.read_enabled(flow.name)
+            }
             nodes, loaded, initial_values, flow_inputs = self._build_flows(
-                [(flow, False) for flow in self.store.read_all()]
+                [(flow, False) for flow in published]
             )
 
             self.loaded = loaded
@@ -216,6 +229,7 @@ class FlowController:
                 events=self.events,
                 max_workers=self.max_workers,
                 initial_values=initial_values,
+                disabled_flows=self.disabled,
             )
             self.issues = _collect_issues(loaded, self.pipeline, flow_inputs)
 
@@ -226,6 +240,8 @@ class FlowController:
                 "type": "pipeline_rebuilt",
                 "issues": [issue.model_dump() for issue in self.issues],
                 "nodes": [status.model_dump() for status in self.node_statuses()],
+                # A rebuild is a fresh pipeline, so nothing is paused any more.
+                "paused": self.paused_flows(),
             }
         )
 
@@ -255,6 +271,10 @@ class FlowController:
             node = entry.node
             if node is None:
                 continue
+            # A stopped flow gets no subscriptions, schedules or webhooks —
+            # that is what stopping it means.
+            if entry.flow in self.disabled:
+                continue
             try:
                 if isinstance(node, MqttNode) and node.mode == MqttNode.Mode.SUBSCRIBER:
                     await node.start_subscription()
@@ -392,9 +412,7 @@ class FlowController:
                 for entry in loaded.values()
                 if entry.flow == name
             ],
-            issues=[
-                issue for issue in issues if not issue.flow or issue.flow == name
-            ],
+            issues=[issue for issue in issues if not issue.flow or issue.flow == name],
         )
 
     def compile_check(self, flow: str, node_id: str, code: str) -> str | None:
@@ -416,6 +434,24 @@ class FlowController:
     # Execution
     # -------------------------------------------------------------------------
 
+    def is_enabled(self, flow: str) -> bool:
+        return flow not in self.disabled
+
+    def is_paused(self, flow: str) -> bool:
+        return self.pipeline is not None and flow in self.pipeline.paused_flows()
+
+    def paused_flows(self) -> list[str]:
+        return self.pipeline.paused_flows() if self.pipeline else []
+
+    def pause_flow(self, flow: str) -> None:
+        if self.pipeline is not None:
+            self.pipeline.pause(flow)
+
+    def resume_flow(self, flow: str) -> None:
+        """Blocking — call from a worker thread: held-back nodes run on resume."""
+        if self.pipeline is not None:
+            self.pipeline.resume(flow)
+
     def run_flow(self, flow: str, inputs: dict[str, Any] | None = None) -> None:
         """Run every node of one flow. Blocking — call from a worker thread."""
         if self.pipeline is None:

backend/app/flow/events.py

@@ -9,6 +9,7 @@ from __future__ import annotations
 
 import asyncio
 import logging
+from collections import deque
 from collections.abc import AsyncIterator
 from contextlib import asynccontextmanager
 from typing import Any
@@ -16,6 +17,7 @@ from typing import Any
 logger = logging.getLogger(__name__)
 
 QUEUE_SIZE = 256
+LOG_HISTORY = 400
 
 
 class EventBus:
@@ -24,6 +26,9 @@ class EventBus:
     def __init__(self) -> None:
         self._loop: asyncio.AbstractEventLoop | None = None
         self._subscribers: set[asyncio.Queue[dict[str, Any]]] = set()
+        # Kept whether or not anyone is listening, so opening the log panel
+        # shows what just happened rather than an empty box.
+        self.recent_logs: deque[dict[str, Any]] = deque(maxlen=LOG_HISTORY)
 
     def bind(self, loop: asyncio.AbstractEventLoop) -> None:
         """Attach the bus to the running event loop (called once at startup)."""
@@ -31,6 +36,9 @@ class EventBus:
 
     def publish(self, event: dict[str, Any]) -> None:
         """Publish an event from any thread."""
+        if event.get("type") == "node_log":
+            # deque.append is atomic, so worker threads need no lock here.
+            self.recent_logs.append(event)
         loop = self._loop
         if loop is None or not self._subscribers:
             return

backend/app/flow/logs.py

@@ -0,0 +1,108 @@
+"""Capture what a node prints, so the editor can show it.
+
+Node code is written by the user and ``print`` is the obvious way to look at a
+value, but a node runs on a pool thread and its output would otherwise land
+unattributed in the server log. ``sys.stdout`` is replaced once by a tee that
+also hands what it is given to whichever capture is active on *this* thread —
+so only node executions are captured, and everything else passes through
+untouched.
+"""
+
+from __future__ import annotations
+
+import io
+import sys
+import traceback
+from collections.abc import Callable, Iterator
+from contextlib import contextmanager
+from contextvars import ContextVar
+from typing import TextIO
+
+# Set for the duration of one node execution, on the thread running it.
+_sink: ContextVar[Callable[[str], None] | None] = ContextVar(
+    "fluksio_node_log_sink", default=None
+)
+
+
+class _Tee(io.TextIOBase):
+    """Writes through to the real stream, and to the active capture."""
+
+    def __init__(self, real: TextIO) -> None:
+        self._real = real
+
+    def write(self, text: str) -> int:
+        sink = _sink.get()
+        if sink is not None and text:
+            sink(text)
+        return self._real.write(text)
+
+    def flush(self) -> None:
+        self._real.flush()
+
+    def isatty(self) -> bool:
+        return self._real.isatty()
+
+
+def install() -> None:
+    """Put the tee in place. Safe to call more than once."""
+    if not isinstance(sys.stdout, _Tee):
+        sys.stdout = _Tee(sys.stdout)
+    if not isinstance(sys.stderr, _Tee):
+        sys.stderr = _Tee(sys.stderr)
+
+
+@contextmanager
+def capture(sink: Callable[[str], None]) -> Iterator[None]:
+    """Send everything printed on this thread to ``sink`` for the duration."""
+    token = _sink.set(sink)
+    try:
+        yield
+    finally:
+        _sink.reset(token)
+
+
+def node_traceback() -> str:
+    """The exception being handled, from the node's own code onward.
+
+    The frames above it are the engine calling the node, which is noise to the
+    person who wrote it — the same trimming ``_short_error`` does for the one
+    line shown on the node itself.
+    """
+    exc_type, exc, tb = sys.exc_info()
+    if exc is None:
+        return ""
+    frames = traceback.extract_tb(tb)
+    start = next(
+        (i for i, frame in enumerate(frames) if frame.filename.startswith("<node ")),
+        0,
+    )
+    return "".join(
+        ["Traceback (most recent call last):\n"]
+        + traceback.format_list(frames[start:])
+        + traceback.format_exception_only(exc_type, exc)
+    )
+
+
+class Collector:
+    """Gathers one execution's output, with a ceiling on how much it keeps."""
+
+    __slots__ = ("chunks", "truncated", "_limit", "_size", "_max_bytes")
+
+    def __init__(self, limit: int = 200, max_bytes: int = 8192) -> None:
+        self.chunks: list[str] = []
+        self.truncated = False
+        self._limit = limit
+        self._max_bytes = max_bytes
+        self._size = 0
+
+    def __call__(self, text: str) -> None:
+        if len(self.chunks) >= self._limit or self._size >= self._max_bytes:
+            self.truncated = True
+            return
+        self.chunks.append(text)
+        self._size += len(text)
+
+    @property
+    def text(self) -> str:
+        out = "".join(self.chunks)
+        return out[: self._max_bytes] if len(out) > self._max_bytes else out

backend/app/flow/nodes.py

@@ -13,6 +13,7 @@ import httpx
 import numpy as np
 from pydantic import BaseModel, ConfigDict, Field
 
+from app.flow import logs
 from app.flow.messages import MessageSpec, qualify
 
 if TYPE_CHECKING:
@@ -236,7 +237,10 @@ class Node:
 
         # A source node asked to inject nothing produces its own data.
         if not outputs and not self.requires:
-            outputs = self.f(params=self.params) or {}
+            collected = logs.Collector()
+            with logs.capture(collected):
+                outputs = self.f(params=self.params) or {}
+            self._pipeline.publish_log(self, collected, "")
 
         return self._pipeline.trigger(self, self._to_messages(outputs))
 

backend/app/flow/pipeline.py

@@ -9,6 +9,7 @@ allowed: each publication triggers the consumers, and the latest value wins.
 from __future__ import annotations
 
 import logging
+import threading
 import time
 from collections import deque
 from concurrent.futures import Future, ThreadPoolExecutor, wait
@@ -16,6 +17,7 @@ from typing import Any, Literal
 
 from pydantic import BaseModel
 
+from app.flow import logs
 from app.flow.events import EventBus
 from app.flow.messages import flow_of
 from app.flow.nodes import Node
@@ -55,6 +57,9 @@ class Pipeline:
         "_edges",
         "_execution_order",
         "_downstream_cache",
+        "_disabled",
+        "_paused",
+        "_gate_lock",
     )
 
     def __init__(
@@ -64,8 +69,14 @@ class Pipeline:
         events: EventBus | None = None,
         max_workers: int | None = None,
         initial_values: dict[str, Any] | None = None,
+        disabled_flows: set[str] | None = None,
     ) -> None:
         self._nodes = nodes or []
+        # Stopped flows are stored and survive a restart; paused ones are a
+        # debugging state that a rebuild is meant to clear.
+        self._disabled = frozenset(disabled_flows or ())
+        self._paused: set[str] = set()
+        self._gate_lock = threading.Lock()
         # An empty state backend is falsy, so this cannot be ``state or ...``:
         # that would quietly hand the pipeline a second, private state and
         # leave everyone reading the shared one seeing nothing.
@@ -309,14 +320,34 @@ class Pipeline:
         if self._events is not None:
             self._events.publish(event)
 
+    def publish_log(self, node: Node, collected: logs.Collector, error: str) -> None:
+        """One event per execution, so a chatty node cannot outrun the stream."""
+        text = collected.text + error
+        if not text:
+            return
+        self._publish(
+            {
+                "type": "node_log",
+                "flow": node.flow,
+                "node": node.id,
+                "text": text,
+                "level": "error" if error else "info",
+                "truncated": collected.truncated,
+                "ts": time.time(),
+            }
+        )
+
     def _execute_node(self, node: Node, state: StateBackend) -> dict[str, Any] | None:
         """Run one node and record its outputs. Never raises."""
         started = time.perf_counter()
+        collected = logs.Collector()
         try:
             with state.lock():
                 inputs = {k: state[k] for k in node.requires if k in state}
 
-            result = node.execute(inputs)
+            with logs.capture(collected):
+                result = node.execute(inputs)
+            self.publish_log(node, collected, "")
 
             if result:
                 ts = time.time()
@@ -355,6 +386,9 @@ class Pipeline:
         except Exception as exc:
             # One failing node must not take the rest of the graph down.
             logger.exception("Node '%s' failed", node.id)
+            # The one-line error goes on the node; the traceback goes to the
+            # log panel, which is where there is room to read it.
+            self.publish_log(node, collected, logs.node_traceback())
             self._publish(
                 {
                     "type": "node_error",
@@ -366,6 +400,37 @@ class Pipeline:
             )
             return None
 
+    # -------------------------------------------------------------------------
+    # Running, stopped, paused
+    # -------------------------------------------------------------------------
+
+    def is_disabled(self, flow: str) -> bool:
+        return flow in self._disabled
+
+    def paused_flows(self) -> list[str]:
+        with self._gate_lock:
+            return sorted(self._paused)
+
+    def pause(self, flow: str) -> None:
+        """Hold this flow's nodes. Values still arrive; nothing acts on them."""
+        with self._gate_lock:
+            self._paused.add(flow)
+        self._publish({"type": "flow_paused", "flow": flow, "paused": True})
+
+    def resume(self, flow: str) -> None:
+        with self._gate_lock:
+            self._paused.discard(flow)
+        self._publish({"type": "flow_paused", "flow": flow, "paused": False})
+        # Whatever was held back is free to run now.
+        self._execute_parallel(self.flow_nodes(flow), self._state, check_ready=True)
+
+    def _gate_blocks(self, node: Node) -> bool:
+        """Is this node's flow held back from executing?"""
+        if node.flow in self._disabled:
+            return True
+        with self._gate_lock:
+            return node.flow in self._paused
+
     def _execute_parallel(
         self,
         nodes_subset: set[Node] | None,
@@ -395,10 +460,16 @@ class Pipeline:
 
         def submit_ready(executor: ThreadPoolExecutor) -> None:
             for n in target_nodes:
-                if n not in submitted and n not in skipped and is_ready(n):
+                if n in submitted or n in skipped:
+                    continue
+                # Gate before the readiness check, so a held-back node does not
+                # spend the synchronous claim it would need once it may run.
+                if self._gate_blocks(n):
+                    continue
+                if is_ready(n):
                     submitted.add(n)
                     node_futures[n] = executor.submit(self._execute_node, n, state)
-                elif n not in submitted and n.synchronous and in_degree[n] == 0:
+                elif n.synchronous and in_degree[n] == 0:
                     # Not ready now; a later trigger may make it ready.
                     skipped.add(n)
 
@@ -433,9 +504,19 @@ class Pipeline:
         return self._execute_parallel(nodes, self._state, check_ready=False)
 
     def trigger(self, node: Node, outputs: dict[str, Any] | None) -> StateBackend:
-        """Publish a node's outputs and run everything downstream of it."""
+        """Publish a node's outputs and run everything downstream of it.
+
+        A stopped flow drops the event: its subscriptions and schedules are torn
+        down anyway, and anything still arriving from another thread would be
+        work the flow was explicitly told not to do. A *paused* flow still
+        publishes, so the incoming value is visible on the canvas, and holds
+        the nodes downstream of it for stepping.
+        """
         state = self._state
 
+        if node.flow in self._disabled:
+            return state
+
         if outputs:
             ts = time.time()
             with state.lock():

backend/app/flow/schemas.py

@@ -116,6 +116,8 @@ class FlowSummary(BaseModel):
     node_count: int = 0
     error_count: int = 0
     has_draft: bool = False
+    enabled: bool = True
+    paused: bool = False
 
 
 class FlowsPublic(BaseModel):

backend/app/flow/store.py

@@ -14,6 +14,7 @@ unpublished — which is what every flow written before this existed looks like.
 
 from __future__ import annotations
 
+import json
 import logging
 import shutil
 import subprocess
@@ -132,6 +133,36 @@ class FlowStore:
     def _draft_node_file(self, flow: str, node_id: str) -> Path:
         return self._draft_nodes_dir(flow) / f"{node_id}.py"
 
+    def _runtime_file(self, name: str) -> Path:
+        return self._flow_dir(name) / "runtime.json"
+
+    # -------------------------------------------------------------------------
+    # Runtime state
+    # -------------------------------------------------------------------------
+
+    def read_enabled(self, name: str) -> bool:
+        """Whether the engine should run this flow. Missing means yes."""
+        path = self._runtime_file(name)
+        if not path.exists():
+            return True
+        try:
+            return bool(json.loads(path.read_text()).get("enabled", True))
+        except (ValueError, OSError):
+            logger.warning("Unreadable runtime state for flow '%s'", name)
+            return True
+
+    def write_enabled(self, name: str, enabled: bool) -> None:
+        """Stop or start a flow, in a file the editor never writes.
+
+        This is deliberately not a field on the flow document: that one is
+        autosaved from the canvas, so a stopped flow would start itself again
+        on the next edit.
+        """
+        path = self._runtime_file(name)
+        path.parent.mkdir(parents=True, exist_ok=True)
+        path.write_text(json.dumps({"enabled": enabled}, indent=2) + "\n")
+        self._commit(f"{'Start' if enabled else 'Stop'} flow '{name}'")
+
     # -------------------------------------------------------------------------
     # Flows
     # -------------------------------------------------------------------------

backend/app/main.py

@@ -9,6 +9,7 @@ from starlette.middleware.cors import CORSMiddleware
 
 from app.api.main import api_router
 from app.core.config import settings
+from app.flow import logs
 from app.flow.controller import FlowController
 from app.flow.events import event_bus
 from app.flow.secrets import init_secrets
@@ -34,6 +35,8 @@ def _state_backend() -> StateBackend:
 async def lifespan(app: FastAPI) -> AsyncIterator[None]:
     """Start the flow engine alongside the API."""
     event_bus.bind(asyncio.get_running_loop())
+    # Node code is user code, and `print` is how it says things.
+    logs.install()
     init_secrets(settings.SECRETS_FILE, settings.SECRET_KEY)
 
     controller = FlowController(

backend/pyproject.toml

@@ -79,6 +79,9 @@ ignore = [
 # contract the engine calls them with.
 "app/flow/nodes.py" = ["ARG001", "ARG002"]
 "tests/flow/*" = ["ARG001"]
+# Printing is what this one is about: node code is user code, and `print` is
+# how it says things.
+"tests/flow/test_logs.py" = ["ARG001", "T201"]
 
 [tool.ruff.lint.pyupgrade]
 # Preserve types, even if a file imports `from __future__ import annotations`.

backend/tests/api/routes/test_flows.py

@@ -193,6 +193,71 @@ def test_discarding_a_draft_restores_what_is_running(
     client.delete(f"{PREFIX}/reverted", headers=superuser_token_headers)
 
 
+def test_stopping_a_flow_takes_it_off_the_engine(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    saved = client.put(
+        f"{PREFIX}/halted", headers=superuser_token_headers, json=a_flow("halted")
+    ).json()
+    client.post(
+        f"{PREFIX}/halted/publish",
+        headers=superuser_token_headers,
+        json={"version": saved["definition"]["version"]},
+    )
+
+    stopped = client.post(f"{PREFIX}/halted/stop", headers=superuser_token_headers)
+    assert stopped.status_code == 200
+    assert stopped.json()["enabled"] is False
+    # Nothing of it is loaded, so there is nothing to run.
+    assert (
+        client.post(
+            f"{PREFIX}/halted/run", headers=superuser_token_headers, json={"inputs": {}}
+        ).status_code
+        == 409
+    )
+
+    started = client.post(f"{PREFIX}/halted/start", headers=superuser_token_headers)
+    assert started.json()["enabled"] is True
+    assert (
+        client.post(
+            f"{PREFIX}/halted/run", headers=superuser_token_headers, json={"inputs": {}}
+        ).status_code
+        == 200
+    )
+
+    client.delete(f"{PREFIX}/halted", headers=superuser_token_headers)
+
+
+def test_pausing_is_reported_back(
+    client: TestClient, superuser_token_headers: dict[str, str]
+) -> None:
+    saved = client.put(
+        f"{PREFIX}/held", headers=superuser_token_headers, json=a_flow("held")
+    ).json()
+    client.post(
+        f"{PREFIX}/held/publish",
+        headers=superuser_token_headers,
+        json={"version": saved["definition"]["version"]},
+    )
+
+    assert (
+        client.post(f"{PREFIX}/held/pause", headers=superuser_token_headers).status_code
+        == 200
+    )
+    assert (
+        client.get(f"{PREFIX}/held", headers=superuser_token_headers).json()["paused"]
+        is True
+    )
+
+    client.post(f"{PREFIX}/held/resume", headers=superuser_token_headers)
+    assert (
+        client.get(f"{PREFIX}/held", headers=superuser_token_headers).json()["paused"]
+        is False
+    )
+
+    client.delete(f"{PREFIX}/held", headers=superuser_token_headers)
+
+
 def test_unconnected_input_is_surfaced(
     client: TestClient, superuser_token_headers: dict[str, str]
 ) -> None:

backend/tests/flow/test_logs.py

@@ -0,0 +1,124 @@
+"""What a node prints reaches the editor, attributed to that node."""
+
+import sys
+from typing import Any
+
+from app.flow import logs
+from app.flow.messages import DType, MessageSpec
+from app.flow.nodes import Node
+from app.flow.pipeline import Pipeline
+
+
+def run_with_capture(nodes: list[Node], bus: "RecordingBus") -> None:
+    """Run a graph the way the app does, tee first.
+
+    The app installs the tee once at startup; pytest replaces ``sys.stdout``
+    around each test, so it is installed here rather than in a fixture.
+    """
+    logs.install()
+    Pipeline(nodes=nodes, events=bus).run({})
+
+
+class RecordingBus:
+    """Stands in for the event bus without an event loop behind it."""
+
+    def __init__(self) -> None:
+        self.events: list[dict[str, Any]] = []
+
+    def publish(self, event: dict[str, Any]) -> None:
+        self.events.append(event)
+
+
+def make_node(node_id: str, f, provides=()) -> Node:
+    node = Node(f=f, provides=list(provides), name=node_id)
+    node.assign_flow("demo", node_id)
+    return node
+
+
+def logs_of(bus: RecordingBus) -> list[dict[str, Any]]:
+    return [event for event in bus.events if event["type"] == "node_log"]
+
+
+def test_what_a_node_prints_is_reported_against_it():
+    def talkative(params):
+        print("value looks fine")
+        return {"temp": 20.0}
+
+    bus = RecordingBus()
+    node = make_node("chatty", talkative, provides=[MessageSpec(name="temp")])
+    run_with_capture([node], bus)
+
+    captured = logs_of(bus)
+    assert len(captured) == 1
+    assert captured[0]["node"] == "demo.chatty"
+    assert captured[0]["level"] == "info"
+    assert "value looks fine" in captured[0]["text"]
+
+
+def test_a_quiet_node_produces_no_log_event():
+    bus = RecordingBus()
+    node = make_node(
+        "quiet",
+        lambda params: {"temp": 20.0},
+        provides=[MessageSpec(name="temp", dtype=DType.FLOAT)],
+    )
+    run_with_capture([node], bus)
+
+    assert logs_of(bus) == []
+
+
+def test_a_failing_node_reports_its_traceback():
+    # Compiled the way the controller compiles node source, because the frame
+    # trimming keys on that filename.
+    namespace: dict[str, Any] = {}
+    exec(
+        compile(
+            'def process(params):\n    print("about to fail")\n'
+            '    raise RuntimeError("boom")\n',
+            "<node demo.broken>",
+            "exec",
+        ),
+        namespace,
+    )
+
+    bus = RecordingBus()
+    run_with_capture([make_node("broken", namespace["process"])], bus)
+
+    captured = logs_of(bus)
+    assert len(captured) == 1
+    assert captured[0]["level"] == "error"
+    # Both what it printed and where it broke, which the node bubble has no
+    # room for.
+    assert "about to fail" in captured[0]["text"]
+    assert "RuntimeError: boom" in captured[0]["text"]
+    # The frames above the node belong to the engine that called it.
+    assert "pipeline.py" not in captured[0]["text"]
+
+
+def test_a_flood_is_truncated_rather_than_streamed():
+    def noisy(params):
+        for index in range(1000):
+            print(f"line {index}")
+        return None
+
+    bus = RecordingBus()
+    run_with_capture([make_node("noisy", noisy)], bus)
+
+    captured = logs_of(bus)
+    # One event per execution, whatever the node prints.
+    assert len(captured) == 1
+    assert captured[0]["truncated"] is True
+    assert len(captured[0]["text"]) <= 8192
+
+
+def test_printing_outside_a_node_still_reaches_the_real_stream(capsys):
+    logs.install()
+    print("server talking")
+    assert "server talking" in capsys.readouterr().out
+
+
+def test_installing_twice_does_not_stack_tees():
+    logs.install()
+    once = sys.stdout
+    logs.install()
+    assert sys.stdout is once

backend/tests/flow/test_runtime_control.py

@@ -0,0 +1,103 @@
+"""Stopping a flow takes it off the engine; pausing holds its nodes."""
+
+from pathlib import Path
+
+from app.flow.messages import DType, MessageSpec
+from app.flow.nodes import Node
+from app.flow.pipeline import Pipeline
+from app.flow.store import FlowStore
+
+
+def spec(name: str) -> MessageSpec:
+    return MessageSpec(name=name, dtype=DType.FLOAT)
+
+
+def make_node(node_id: str, flow: str, f, requires=(), provides=()) -> Node:
+    node = Node(f=f, requires=list(requires), provides=list(provides), name=node_id)
+    node.assign_flow(flow, node_id)
+    return node
+
+
+def a_chain(flow: str, ran: list[str]) -> list[Node]:
+    """source → middle: two nodes, so stepping has somewhere to stop."""
+
+    def source(params):
+        ran.append(f"{flow}.source")
+        return {"temp": 20.0}
+
+    def middle(temp, params):
+        ran.append(f"{flow}.middle")
+        return {"warm": temp > 10}
+
+    return [
+        make_node("source", flow, source, provides=[spec("temp")]),
+        make_node(
+            "middle",
+            flow,
+            middle,
+            requires=[spec("temp")],
+            provides=[MessageSpec(name="warm", dtype=DType.BOOL)],
+        ),
+    ]
+
+
+def test_a_stopped_flow_runs_nothing_and_its_neighbours_carry_on():
+    ran: list[str] = []
+    pipeline = Pipeline(
+        nodes=a_chain("stopped", ran) + a_chain("running", ran),
+        disabled_flows={"stopped"},
+    )
+
+    pipeline.run({})
+
+    assert not any(name.startswith("stopped.") for name in ran)
+    assert {"running.source", "running.middle"} <= set(ran)
+
+
+def test_a_stopped_flow_ignores_a_trigger_from_its_own_nodes():
+    ran: list[str] = []
+    nodes = a_chain("stopped", ran)
+    pipeline = Pipeline(nodes=nodes, disabled_flows={"stopped"})
+
+    # What an MQTT subscription or a webhook would do.
+    nodes[0].inject({"temp": 20.0})
+
+    assert ran == []
+    assert "stopped.temp" not in pipeline.state
+
+
+def test_a_paused_flow_still_takes_values_but_acts_on_none_of_them():
+    ran: list[str] = []
+    nodes = a_chain("demo", ran)
+    pipeline = Pipeline(nodes=nodes)
+
+    pipeline.pause("demo")
+    nodes[0].inject({"temp": 20.0})
+
+    # The value is there to look at; nothing downstream of it ran.
+    assert pipeline.state["demo.temp"] == 20.0
+    assert ran == []
+
+
+def test_resuming_runs_what_was_held_back():
+    ran: list[str] = []
+    pipeline = Pipeline(nodes=a_chain("demo", ran))
+
+    pipeline.pause("demo")
+    pipeline.run({})
+    assert ran == []
+
+    pipeline.resume("demo")
+    assert ran == ["demo.source", "demo.middle"]
+    assert pipeline.paused_flows() == []
+
+
+def test_stopped_survives_a_restart(tmp_path: Path):
+    store = FlowStore(tmp_path / "flows")
+    assert store.read_enabled("heating") is True
+
+    store.write_enabled("heating", False)
+    assert store.read_enabled("heating") is False
+
+    # A second store over the same directory is what a restart looks like.
+    assert FlowStore(tmp_path / "flows").read_enabled("heating") is False

frontend/src/client/schemas.gen.ts

@@ -166,6 +166,16 @@ export const FlowDetailSchema = {
             type: 'boolean',
             title: 'Has Draft',
             default: false
+        },
+        enabled: {
+            type: 'boolean',
+            title: 'Enabled',
+            default: true
+        },
+        paused: {
+            type: 'boolean',
+            title: 'Paused',
+            default: false
         }
     },
     type: 'object',
@@ -266,6 +276,16 @@ export const FlowSummarySchema = {
             type: 'boolean',
             title: 'Has Draft',
             default: false
+        },
+        enabled: {
+            type: 'boolean',
+            title: 'Enabled',
+            default: true
+        },
+        paused: {
+            type: 'boolean',
+            title: 'Paused',
+            default: false
         }
     },
     type: 'object',

frontend/src/client/sdk.gen.ts

@@ -3,7 +3,7 @@
 import type { CancelablePromise } from './core/CancelablePromise';
 import { OpenAPI } from './core/OpenAPI';
 import { request as __request } from './core/request';
-import type { FlowsReadFlowsResponse, FlowsReadNodeTypesResponse, FlowsReadFlowData, FlowsReadFlowResponse, FlowsSaveFlowData, FlowsSaveFlowResponse, FlowsDeleteFlowData, FlowsDeleteFlowResponse, FlowsPublishFlowData, FlowsPublishFlowResponse, FlowsDiscardDraftData, FlowsDiscardDraftResponse, FlowsRenameFlowData, FlowsRenameFlowResponse, FlowsReadNodeSourceData, FlowsReadNodeSourceResponse, FlowsSaveNodeSourceData, FlowsSaveNodeSourceResponse, FlowsValidateFlowData, FlowsValidateFlowResponse, FlowsRunFlowData, FlowsRunFlowResponse, FlowsTriggerNodeData, FlowsTriggerNodeResponse, FlowsReadFlowStateData, FlowsReadFlowStateResponse, FlowsReadMessageHistoryData, FlowsReadMessageHistoryResponse, LoginLoginAccessTokenData, LoginLoginAccessTokenResponse, LoginTestTokenResponse, LoginRecoverPasswordData, LoginRecoverPasswordResponse, LoginResetPasswordData, LoginResetPasswordResponse, LoginRecoverPasswordHtmlContentData, LoginRecoverPasswordHtmlContentResponse, PrivateCreateUserData, PrivateCreateUserResponse, SecretsReadSecretsResponse, SecretsSaveSecretData, SecretsSaveSecretResponse, SecretsDeleteSecretData, SecretsDeleteSecretResponse, UsersReadUsersData, UsersReadUsersResponse, UsersCreateUserData, UsersCreateUserResponse, UsersReadUserMeResponse, UsersDeleteUserMeResponse, UsersUpdateUserMeData, UsersUpdateUserMeResponse, UsersUpdatePasswordMeData, UsersUpdatePasswordMeResponse, UsersRegisterUserData, UsersRegisterUserResponse, UsersReadUserByIdData, UsersReadUserByIdResponse, UsersUpdateUserData, UsersUpdateUserResponse, UsersDeleteUserData, UsersDeleteUserResponse, UtilsTestEmailData, UtilsTestEmailResponse, UtilsHealthCheckResponse } from './types.gen';
+import type { FlowsReadFlowsResponse, FlowsReadNodeTypesResponse, FlowsReadFlowData, FlowsReadFlowResponse, FlowsSaveFlowData, FlowsSaveFlowResponse, FlowsDeleteFlowData, FlowsDeleteFlowResponse, FlowsPublishFlowData, FlowsPublishFlowResponse, FlowsDiscardDraftData, FlowsDiscardDraftResponse, FlowsRenameFlowData, FlowsRenameFlowResponse, FlowsReadNodeSourceData, FlowsReadNodeSourceResponse, FlowsSaveNodeSourceData, FlowsSaveNodeSourceResponse, FlowsStartFlowData, FlowsStartFlowResponse, FlowsStopFlowData, FlowsStopFlowResponse, FlowsPauseFlowData, FlowsPauseFlowResponse, FlowsResumeFlowData, FlowsResumeFlowResponse, FlowsValidateFlowData, FlowsValidateFlowResponse, FlowsRunFlowData, FlowsRunFlowResponse, FlowsTriggerNodeData, FlowsTriggerNodeResponse, FlowsReadFlowStateData, FlowsReadFlowStateResponse, FlowsReadMessageHistoryData, FlowsReadMessageHistoryResponse, LoginLoginAccessTokenData, LoginLoginAccessTokenResponse, LoginTestTokenResponse, LoginRecoverPasswordData, LoginRecoverPasswordResponse, LoginResetPasswordData, LoginResetPasswordResponse, LoginRecoverPasswordHtmlContentData, LoginRecoverPasswordHtmlContentResponse, PrivateCreateUserData, PrivateCreateUserResponse, SecretsReadSecretsResponse, SecretsSaveSecretData, SecretsSaveSecretResponse, SecretsDeleteSecretData, SecretsDeleteSecretResponse, UsersReadUsersData, UsersReadUsersResponse, UsersCreateUserData, UsersCreateUserResponse, UsersReadUserMeResponse, UsersDeleteUserMeResponse, UsersUpdateUserMeData, UsersUpdateUserMeResponse, UsersUpdatePasswordMeData, UsersUpdatePasswordMeResponse, UsersRegisterUserData, UsersRegisterUserResponse, UsersReadUserByIdData, UsersReadUserByIdResponse, UsersUpdateUserData, UsersUpdateUserResponse, UsersDeleteUserData, UsersDeleteUserResponse, UtilsTestEmailData, UtilsTestEmailResponse, UtilsHealthCheckResponse } from './types.gen';
 
 export class FlowsService {
     /**
@@ -225,6 +225,90 @@ export class FlowsService {
         });
     }
     
+    /**
+     * Start Flow
+     * Let the engine run this flow again.
+     * @param data The data for the request.
+     * @param data.name
+     * @returns FlowDetail Successful Response
+     * @throws ApiError
+     */
+    public static startFlow(data: FlowsStartFlowData): CancelablePromise<FlowsStartFlowResponse> {
+        return __request(OpenAPI, {
+            method: 'POST',
+            url: '/api/v1/flows/{name}/start',
+            path: {
+                name: data.name
+            },
+            errors: {
+                422: 'Validation Error'
+            }
+        });
+    }
+    
+    /**
+     * Stop Flow
+     * Take this flow off the engine: no subscriptions, schedules or webhooks.
+     * @param data The data for the request.
+     * @param data.name
+     * @returns FlowDetail Successful Response
+     * @throws ApiError
+     */
+    public static stopFlow(data: FlowsStopFlowData): CancelablePromise<FlowsStopFlowResponse> {
+        return __request(OpenAPI, {
+            method: 'POST',
+            url: '/api/v1/flows/{name}/stop',
+            path: {
+                name: data.name
+            },
+            errors: {
+                422: 'Validation Error'
+            }
+        });
+    }
+    
+    /**
+     * Pause Flow
+     * Hold the flow's nodes so its messages can be stepped through.
+     * @param data The data for the request.
+     * @param data.name
+     * @returns Message Successful Response
+     * @throws ApiError
+     */
+    public static pauseFlow(data: FlowsPauseFlowData): CancelablePromise<FlowsPauseFlowResponse> {
+        return __request(OpenAPI, {
+            method: 'POST',
+            url: '/api/v1/flows/{name}/pause',
+            path: {
+                name: data.name
+            },
+            errors: {
+                422: 'Validation Error'
+            }
+        });
+    }
+    
+    /**
+     * Resume Flow
+     * Let the flow carry on, running whatever was held back.
+     * @param data The data for the request.
+     * @param data.name
+     * @returns Message Successful Response
+     * @throws ApiError
+     */
+    public static resumeFlow(data: FlowsResumeFlowData): CancelablePromise<FlowsResumeFlowResponse> {
+        return __request(OpenAPI, {
+            method: 'POST',
+            url: '/api/v1/flows/{name}/resume',
+            path: {
+                name: data.name
+            },
+            errors: {
+                422: 'Validation Error'
+            }
+        });
+    }
+    
     /**
      * Validate Flow
      * Report what would keep this flow from running.

frontend/src/client/types.gen.ts

@@ -51,6 +51,8 @@ export type FlowDetail = {
     nodes?: Array<NodeStatusPublic>;
     issues?: Array<ValidationIssue>;
     has_draft?: boolean;
+    enabled?: boolean;
+    paused?: boolean;
 };
 
 /**
@@ -87,6 +89,8 @@ export type FlowSummary = {
     node_count?: number;
     error_count?: number;
     has_draft?: boolean;
+    enabled?: boolean;
+    paused?: boolean;
 };
 
 /**
@@ -382,6 +386,30 @@ export type FlowsSaveNodeSourceData = {
 
 export type FlowsSaveNodeSourceResponse = (NodeStatusPublic);
 
+export type FlowsStartFlowData = {
+    name: string;
+};
+
+export type FlowsStartFlowResponse = (FlowDetail);
+
+export type FlowsStopFlowData = {
+    name: string;
+};
+
+export type FlowsStopFlowResponse = (FlowDetail);
+
+export type FlowsPauseFlowData = {
+    name: string;
+};
+
+export type FlowsPauseFlowResponse = (Message);
+
+export type FlowsResumeFlowData = {
+    name: string;
+};
+
+export type FlowsResumeFlowResponse = (Message);
+
 export type FlowsValidateFlowData = {
     name: string;
 };

frontend/src/components/Flow/EdgeInspector.tsx

@@ -1,10 +1,13 @@
-import { ArrowRight, Trash2 } from "lucide-react"
+import { useMutation } from "@tanstack/react-query"
+import { ArrowRight, RotateCcw, Trash2 } from "lucide-react"
 import { motion } from "motion/react"
 import { useEffect, useRef, useState } from "react"
 
+import { FlowsService } from "@/client"
 import { Button } from "@/components/ui/button"
 import { Popover, PopoverAnchor, PopoverContent } from "@/components/ui/popover"
 import { ScrollArea } from "@/components/ui/scroll-area"
+import useCustomToast from "@/hooks/useCustomToast"
 import { cn } from "@/lib/utils"
 import { displayName } from "./deriveEdges"
 import { useLiveValue } from "./liveStore"
@@ -77,6 +80,8 @@ export type InspectedEdge = {
   /** Node titles, so the popover names the two ends in the user's own words. */
   from: string
   to: string
+  /** The producing node's id, which is what replaying the message goes through. */
+  sourceId: string
   x: number
   y: number
 }
@@ -96,6 +101,19 @@ export function EdgeInspector({
   onUnbind: (message: string) => void
 }) {
   const live = useLiveValue(edge?.message)
+  const { showErrorToast } = useCustomToast()
+  // Publishing the value again from the node that produced it runs everything
+  // downstream exactly as the original did.
+  const replay = useMutation({
+    mutationFn: (value: unknown) =>
+      FlowsService.triggerNode({
+        name: flow,
+        nodeId: edge?.sourceId ?? "",
+        requestBody: { values: { [edge?.message ?? ""]: value } },
+      }),
+    onError: () => showErrorToast("The message could not be sent again."),
+  })
+
   if (!edge) return null
 
   const scalar = live === undefined ? null : formatScalar(live.value)
@@ -115,6 +133,20 @@ export function EdgeInspector({
           <Marquee text={edge.from} className="flex-1 font-medium" />
           <ArrowRight className="size-3.5 shrink-0 text-muted-foreground" />
           <Marquee text={edge.to} className="flex-1 text-right font-medium" />
+          {live === undefined ? null : (
+            <Button
+              variant="ghost"
+              size="icon-sm"
+              className="-my-1 shrink-0 text-muted-foreground"
+              onClick={() => replay.mutate(live.value)}
+              disabled={replay.isPending}
+              aria-label="Send this message again"
+              title="Send this message again"
+              data-testid="replay-message"
+            >
+              <RotateCcw />
+            </Button>
+          )}
           <Button
             variant="ghost"
             size="icon-sm"

frontend/src/components/Flow/FlowDock.tsx

@@ -3,7 +3,9 @@ import {
   AlertCircle,
   Loader2,
   Maximize2,
+  Pause,
   Play,
+  PlayCircle,
   Plus,
   ZoomIn,
   ZoomOut,
@@ -24,6 +26,8 @@ import {
   TooltipTrigger,
 } from "@/components/ui/tooltip"
 import { slideUp, transitions } from "@/lib/motion"
+import { cn } from "@/lib/utils"
+import { LogsPanel } from "./LogsPanel"
 
 /**
  * What "fit" means on this canvas: the view a flow opens with, and the one the
@@ -38,16 +42,24 @@ export const FIT_VIEW = { padding: 0.25, maxZoom: 1.2 }
  * affordance on this view; everything else stays quiet.
  */
 export function FlowDock({
+  flow,
   issues,
   running,
+  enabled,
+  paused,
   onAddNode,
   onRun,
+  onTogglePause,
   onFocusNode,
 }: {
+  flow: string
   issues: ValidationIssue[]
   running: boolean
+  enabled: boolean
+  paused: boolean
   onAddNode: () => void
   onRun: () => void
+  onTogglePause: () => void
   onFocusNode: (nodeId: string) => void
 }) {
   const { zoomIn, zoomOut, fitView } = useReactFlow()
@@ -152,21 +164,58 @@ export function FlowDock({
 
       <Separator orientation="vertical" className="mx-0.5 !h-5" />
 
-      <Button
-        variant="brand"
-        size="sm"
-        className="h-11 gap-1.5 md:h-8"
-        onClick={onRun}
-        disabled={running}
-        data-testid="run-flow"
-      >
-        {running ? (
-          <Loader2 className="size-4 animate-spin" />
-        ) : (
-          <Play className="size-4" />
-        )}
-        Run
-      </Button>
+      <LogsPanel flow={flow} />
+
+      <Tooltip>
+        <TooltipTrigger asChild>
+          <Button
+            variant="ghost"
+            size="icon"
+            className={cn(
+              "size-11 text-muted-foreground md:size-8",
+              paused && "text-primary",
+            )}
+            onClick={onTogglePause}
+            disabled={!enabled}
+            aria-label={paused ? "Resume flow" : "Pause flow"}
+            data-testid={paused ? "resume-flow" : "pause-flow"}
+          >
+            {paused ? <PlayCircle /> : <Pause />}
+          </Button>
+        </TooltipTrigger>
+        <TooltipContent>
+          {!enabled
+            ? "This flow is stopped"
+            : paused
+              ? "Let the flow carry on"
+              : "Hold the nodes; values still arrive"}
+        </TooltipContent>
+      </Tooltip>
+
+      <Tooltip>
+        <TooltipTrigger asChild>
+          <span>
+            <Button
+              variant="brand"
+              size="sm"
+              className="h-11 gap-1.5 md:h-8"
+              onClick={onRun}
+              disabled={running || !enabled}
+              data-testid="run-flow"
+            >
+              {running ? (
+                <Loader2 className="size-4 animate-spin" />
+              ) : (
+                <Play className="size-4" />
+              )}
+              Run
+            </Button>
+          </span>
+        </TooltipTrigger>
+        <TooltipContent>
+          {enabled ? "Run every node once" : "Start the flow to run it"}
+        </TooltipContent>
+      </Tooltip>
     </motion.div>
   )
 }

frontend/src/components/Flow/FlowEditor.tsx

@@ -49,7 +49,7 @@ import { FlowTabs } from "./FlowTabs"
 import { LiveEdge } from "./LiveEdge"
 import { NodePanel } from "./NodePanel"
 import "./flow.css"
-import { liveStore } from "./liveStore"
+import { liveStore, useFlowPaused } from "./liveStore"
 import {
   flowKeys,
   flowQueryOptions,
@@ -230,6 +230,7 @@ function FlowEditorInner({
   const [editorExpanded, setEditorExpanded] = useState(false)
 
   const issues = detail.issues ?? []
+  const paused = useFlowPaused(flowName)
 
   // Keep the latest document in a ref so autosave never captures a stale copy.
   const latest = useRef<FlowDef_Input>(detail.definition)
@@ -389,6 +390,27 @@ function FlowEditorInner({
       showErrorToast("The flow could not run. Check the node errors."),
   })
 
+  const enableMutation = useMutation({
+    mutationFn: (next: boolean) =>
+      next
+        ? FlowsService.startFlow({ name: flowName })
+        : FlowsService.stopFlow({ name: flowName }),
+    onSuccess: (detail) => {
+      queryClient.setQueryData(flowKeys.detail(flowName), detail)
+      queryClient.invalidateQueries({ queryKey: flowKeys.all, exact: true })
+    },
+    onError: () => showErrorToast("The flow could not be started or stopped."),
+  })
+
+  const pauseMutation = useMutation({
+    mutationFn: (next: boolean) =>
+      next
+        ? FlowsService.pauseFlow({ name: flowName })
+        : FlowsService.resumeFlow({ name: flowName }),
+    // The engine answers with a flow_paused event, which is what the dock reads.
+    onError: () => showErrorToast("The flow could not be paused."),
+  })
+
   const renameMutation = useMutation({
     mutationFn: (newName: string) =>
       FlowsService.renameFlow({
@@ -659,6 +681,7 @@ function FlowEditorInner({
             message: (edge.data as { message: string }).message,
             from: label(edge.source),
             to: label(edge.target),
+            sourceId: edge.source,
             x: event.clientX,
             y: event.clientY,
           })
@@ -710,14 +733,18 @@ function FlowEditorInner({
         {panelOpen ? null : (
           <FlowDock
             key="flow-dock"
+            flow={flowName}
             issues={issues}
             running={runMutation.isPending}
+            enabled={detail.enabled ?? true}
+            paused={paused}
             onAddNode={() => setPaletteOpen(true)}
             onRun={async () => {
               // Running executes what is stored, so the queued edit goes first.
               await flush()
               runMutation.mutate()
             }}
+            onTogglePause={() => pauseMutation.mutate(!paused)}
             onFocusNode={focusNode}
           />
         )}
@@ -752,6 +779,9 @@ function FlowEditorInner({
           renameMutation.mutate(newName)
         }}
         onDelete={() => deleteMutation.mutate()}
+        enabled={detail.enabled ?? true}
+        toggling={enableMutation.isPending}
+        onToggleEnabled={(next) => enableMutation.mutate(next)}
         hasDraft={detail.has_draft ?? false}
         discarding={discard.isPending}
         onDiscardDraft={() => {

frontend/src/components/Flow/FlowPanel.tsx

@@ -11,6 +11,7 @@ import {
   DialogTitle,
 } from "@/components/ui/dialog"
 import { Input } from "@/components/ui/input"
+import { Switch } from "@/components/ui/switch"
 import { PANEL_SECTION, SidePanel } from "./SidePanel"
 
 const NAME_PATTERN = /^[a-z][a-z0-9_]*$/
@@ -32,6 +33,9 @@ export function FlowPanel({
   hasDraft,
   discarding,
   onDiscardDraft,
+  enabled,
+  toggling,
+  onToggleEnabled,
   onClose,
 }: {
   open: boolean
@@ -44,6 +48,9 @@ export function FlowPanel({
   hasDraft: boolean
   discarding: boolean
   onDiscardDraft: () => void
+  enabled: boolean
+  toggling: boolean
+  onToggleEnabled: (next: boolean) => void
   onClose: () => void
 }) {
   const [name, setName] = useState(definition.name)
@@ -85,6 +92,24 @@ export function FlowPanel({
         }
       >
         <div className="grid gap-5 p-4">
+          <div className="grid gap-2">
+            <span className={PANEL_SECTION}>Running</span>
+            <div className="flex items-center justify-between gap-3">
+              <p className="text-sm text-muted-foreground">
+                {enabled
+                  ? "The engine runs this flow: subscriptions, schedules and webhooks are live."
+                  : "Stopped. Nothing of this flow is subscribed, scheduled or reachable."}
+              </p>
+              <Switch
+                checked={enabled}
+                disabled={toggling}
+                onCheckedChange={onToggleEnabled}
+                aria-label="Run this flow"
+                data-testid="flow-enabled"
+              />
+            </div>
+          </div>
+
           <div className="grid gap-2">
             <span className={PANEL_SECTION}>Name</span>
             <div className="flex items-center gap-1.5">

frontend/src/components/Flow/LogsPanel.tsx

@@ -0,0 +1,120 @@
+import { Terminal } from "lucide-react"
+import { useEffect, useRef } from "react"
+
+import { Button } from "@/components/ui/button"
+import {
+  Popover,
+  PopoverContent,
+  PopoverTrigger,
+} from "@/components/ui/popover"
+import { ScrollArea } from "@/components/ui/scroll-area"
+import {
+  Tooltip,
+  TooltipContent,
+  TooltipTrigger,
+} from "@/components/ui/tooltip"
+import { cn } from "@/lib/utils"
+import { liveStore, useLiveLogs } from "./liveStore"
+
+function shortTime(ts: number): string {
+  return new Date(ts * 1000).toLocaleTimeString(undefined, {
+    hour12: false,
+    hour: "2-digit",
+    minute: "2-digit",
+    second: "2-digit",
+  })
+}
+
+/** Drop the flow prefix: every line in here belongs to the open flow. */
+function nodeLabel(nodeId: string, flow: string): string {
+  return nodeId.startsWith(`${flow}.`) ? nodeId.slice(flow.length + 1) : nodeId
+}
+
+/**
+ * What the nodes of this flow printed, and the tracebacks of the ones that
+ * failed — the detail the one-line error bubble on a node has no room for.
+ */
+export function LogsPanel({ flow }: { flow: string }) {
+  const lines = useLiveLogs().filter((line) => line.flow === flow)
+  const bottom = useRef<HTMLLIElement | null>(null)
+
+  // Follow the tail, which is where a running flow puts what just happened.
+  // biome-ignore lint/correctness/useExhaustiveDependencies: a new line is what scrolls.
+  useEffect(() => {
+    bottom.current?.scrollIntoView({ block: "end" })
+  }, [lines.length])
+
+  return (
+    <Popover>
+      <Tooltip>
+        <TooltipTrigger asChild>
+          <PopoverTrigger asChild>
+            <Button
+              variant="ghost"
+              size="icon"
+              className="size-11 text-muted-foreground md:size-8"
+              aria-label="Logs"
+              data-testid="flow-logs"
+            >
+              <Terminal />
+            </Button>
+          </PopoverTrigger>
+        </TooltipTrigger>
+        <TooltipContent>What this flow printed</TooltipContent>
+      </Tooltip>
+
+      <PopoverContent align="center" className="w-[28rem] p-0">
+        <div className="flex items-center justify-between border-b border-border px-3 py-2">
+          <p className="text-xs font-medium uppercase tracking-[0.5px] text-muted-foreground">
+            Logs
+          </p>
+          <Button
+            variant="ghost"
+            size="sm"
+            className="h-6 px-2 text-xs text-muted-foreground"
+            onClick={() => liveStore.clearLogs()}
+            disabled={lines.length === 0}
+          >
+            Clear
+          </Button>
+        </div>
+
+        {lines.length === 0 ? (
+          <p className="px-3 py-6 text-center text-sm text-muted-foreground">
+            Nothing yet. Anything a node prints shows up here.
+          </p>
+        ) : (
+          <ScrollArea className="h-72">
+            <ul className="grid gap-1.5 p-3 font-mono text-xs">
+              {lines.map((line, index) => (
+                <li
+                  // Lines are append-only and repeat freely, so position is the
+                  // only stable identity they have.
+                  key={`${line.ts}-${line.node}-${index}`}
+                  className="grid grid-cols-[auto_1fr] gap-2"
+                >
+                  <span className="text-muted-foreground">
+                    {shortTime(line.ts)}{" "}
+                    <span className="text-foreground/70">
+                      {nodeLabel(line.node, flow)}
+                    </span>
+                  </span>
+                  <span
+                    className={cn(
+                      "whitespace-pre-wrap break-words",
+                      line.level === "error" && "text-destructive",
+                    )}
+                  >
+                    {line.text.replace(/\n+$/, "")}
+                    {line.truncated ? "\n… truncated" : ""}
+                  </span>
+                </li>
+              ))}
+              <li ref={bottom} aria-hidden />
+            </ul>
+          </ScrollArea>
+        )}
+      </PopoverContent>
+    </Popover>
+  )
+}

frontend/src/components/Flow/liveStore.ts

@@ -13,14 +13,28 @@ export type LiveStatus = {
   status: "active" | "error" | "running" | "success"
   error?: string | null
 }
+/** One node execution's output, as the log panel shows it. */
+export type LogLine = {
+  flow: string
+  node: string
+  text: string
+  level: "info" | "error"
+  truncated?: boolean
+  ts: number
+}
 
 type Listener = () => void
 
+/** Enough to see what a flow has been doing, not a log store. */
+const LOG_LIMIT = 500
+
 const values = new Map<string, LiveValue>()
 const statuses = new Map<string, LiveStatus>()
 // How many times a node has emitted. The number itself means nothing; a change
 // is what restarts the pulse.
 const emits = new Map<string, number>()
+let logLines: LogLine[] = []
+const paused = new Set<string>()
 const listeners = new Map<string, Set<Listener>>()
 
 let connected = false
@@ -79,6 +93,33 @@ export const liveStore = {
     emits.set(nodeId, (emits.get(nodeId) ?? 0) + 1)
     notify(`emit:${nodeId}`)
   },
+  appendLog(line: LogLine) {
+    // A new array each time, so the hook's snapshot comparison sees the change.
+    logLines = [...logLines, line].slice(-LOG_LIMIT)
+    notify("logs")
+  },
+  setLogs(lines: LogLine[]) {
+    logLines = lines.slice(-LOG_LIMIT)
+    notify("logs")
+  },
+  clearLogs() {
+    logLines = []
+    notify("logs")
+  },
+  setPaused(flow: string, isPaused: boolean) {
+    if (isPaused) paused.add(flow)
+    else paused.delete(flow)
+    notify(`paused:${flow}`)
+  },
+  setPausedFlows(flows: string[]) {
+    const next = new Set(flows)
+    for (const flow of new Set([...paused, ...next])) {
+      if (paused.has(flow) === next.has(flow)) continue
+      if (next.has(flow)) paused.add(flow)
+      else paused.delete(flow)
+      notify(`paused:${flow}`)
+    }
+  },
   setConnected(next: boolean) {
     if (connected === next) return
     connected = next
@@ -94,6 +135,10 @@ export const liveStore = {
     statuses.clear()
     for (const key of emits.keys()) notify(`emit:${key}`)
     emits.clear()
+    logLines = []
+    notify("logs")
+    for (const flow of paused) notify(`paused:${flow}`)
+    paused.clear()
   },
 }
 
@@ -119,6 +164,21 @@ export function useNodeEmits(nodeId: string): number {
   )
 }
 
+/** Every captured line, newest last. Filtered by the panel that shows it. */
+export function useLiveLogs(): LogLine[] {
+  return useSyncExternalStore(
+    (listener) => subscribeKey("logs", listener),
+    () => logLines,
+  )
+}
+
+export function useFlowPaused(flow: string): boolean {
+  return useSyncExternalStore(
+    (listener) => subscribeKey(`paused:${flow}`, listener),
+    () => paused.has(flow),
+  )
+}
+
 export function useLiveConnection(): boolean {
   return useSyncExternalStore(
     (listener) => {

frontend/src/components/Flow/useFlowSocket.ts

@@ -2,7 +2,7 @@ import { useQueryClient } from "@tanstack/react-query"
 import { useEffect, useRef } from "react"
 
 import { OpenAPI } from "@/client"
-import { liveStore } from "./liveStore"
+import { type LogLine, liveStore } from "./liveStore"
 import { flowKeys } from "./queries"
 
 const RECONNECT_MIN = 1000
@@ -13,14 +13,19 @@ type FlowEvent =
       type: "snapshot"
       values: Record<string, { value: unknown; ts: number | null }>
       nodes: { id: string; status: string; error?: string | null }[]
+      paused?: string[]
+      logs?: LogLine[]
     }
   | { type: "message_value"; name: string; value: unknown; ts: number }
   | { type: "node_executed"; node: string; outputs: number }
   | { type: "node_error"; node: string; error: string }
   | { type: "node_status"; node: string; status: string; error?: string | null }
+  | ({ type: "node_log" } & LogLine)
+  | { type: "flow_paused"; flow: string; paused: boolean }
   | {
       type: "pipeline_rebuilt"
       nodes: { id: string; status: string; error?: string | null }[]
+      paused?: string[]
     }
 
 function socketUrl(): string {
@@ -65,6 +70,8 @@ export function useFlowSocket(onAuthFailure?: () => void): void {
           case "snapshot":
             liveStore.setValues(message.values)
             liveStore.setStatuses(message.nodes)
+            liveStore.setPausedFlows(message.paused ?? [])
+            liveStore.setLogs(message.logs ?? [])
             break
           case "message_value":
             liveStore.setValue(message.name, {
@@ -88,10 +95,17 @@ export function useFlowSocket(onAuthFailure?: () => void): void {
               error: message.error,
             })
             break
+          case "node_log":
+            liveStore.appendLog(message)
+            break
+          case "flow_paused":
+            liveStore.setPaused(message.flow, message.paused)
+            break
           case "pipeline_rebuilt":
             liveStore.setStatuses(message.nodes)
-            // Someone published, here or in another tab: the draft markers on
-            // the flow chips are stale until the list is fetched again.
+            liveStore.setPausedFlows(message.paused ?? [])
+            // Someone published or started a flow, here or in another tab: the
+            // markers on the flow chips are stale until the list is refetched.
             queryClient.invalidateQueries({ queryKey: flowKeys.all })
             break
         }

frontend/src/routes/_layout/index.tsx

@@ -1,6 +1,15 @@
-import { createFileRoute } from "@tanstack/react-router"
+import { useMutation, useQuery, useQueryClient } from "@tanstack/react-query"
+import { createFileRoute, Link } from "@tanstack/react-router"
+import { AlertCircle, Workflow } from "lucide-react"
 
+import { type FlowSummary, FlowsService } from "@/client"
+import { flowKeys, flowsQueryOptions } from "@/components/Flow/queries"
+import { Badge } from "@/components/ui/badge"
+import { Card } from "@/components/ui/card"
+import { Skeleton } from "@/components/ui/skeleton"
+import { Switch } from "@/components/ui/switch"
 import useAuth from "@/hooks/useAuth"
+import useCustomToast from "@/hooks/useCustomToast"
 
 export const Route = createFileRoute("/_layout/")({
   component: Dashboard,
@@ -13,19 +22,137 @@ export const Route = createFileRoute("/_layout/")({
   }),
 })
 
-function Dashboard() {
-  const { user: currentUser } = useAuth()
+/** Another tab can stop a flow, and the engine can fail one on its own. */
+const REFRESH_INTERVAL = 10_000
+
+function Tile({ label, value }: { label: string; value: number }) {
+  return (
+    <Card className="gap-1 py-4">
+      <div className="px-5">
+        <p className="text-2xl font-semibold tabular-nums">{value}</p>
+        <p className="text-sm text-muted-foreground">{label}</p>
+      </div>
+    </Card>
+  )
+}
+
+function FlowRow({ flow }: { flow: FlowSummary }) {
+  const queryClient = useQueryClient()
+  const { showErrorToast } = useCustomToast()
+  const enabled = flow.enabled ?? true
+
+  const toggle = useMutation({
+    mutationFn: (next: boolean) =>
+      next
+        ? FlowsService.startFlow({ name: flow.name })
+        : FlowsService.stopFlow({ name: flow.name }),
+    onSuccess: () => {
+      queryClient.invalidateQueries({ queryKey: flowKeys.all })
+      queryClient.invalidateQueries({ queryKey: flowKeys.detail(flow.name) })
+    },
+    onError: () => showErrorToast("The flow could not be started or stopped."),
+  })
 
   return (
-    <div>
-      <div>
-        <h1 className="text-2xl truncate max-w-sm">
-          Hi, {currentUser?.full_name || currentUser?.email} 👋
-        </h1>
-        <p className="text-muted-foreground">
-          Welcome back, nice to see you again!!!
+    <div
+      className="flex items-center gap-3 border-b border-border px-5 py-3 last:border-b-0"
+      data-testid="dashboard-flow-row"
+    >
+      <Link
+        to="/flows/$flowName"
+        params={{ flowName: flow.name }}
+        className="min-w-0 flex-1"
+      >
+        <p className="truncate font-medium">{flow.title || flow.name}</p>
+        <p className="truncate text-sm text-muted-foreground">
+          {flow.node_count === 1 ? "1 node" : `${flow.node_count} nodes`}
+          {flow.has_draft ? " · unpublished changes" : ""}
         </p>
+      </Link>
+
+      {(flow.error_count ?? 0) > 0 ? (
+        <Badge variant="destructive" className="gap-1">
+          <AlertCircle />
+          {flow.error_count}
+        </Badge>
+      ) : null}
+
+      <Badge
+        variant="outline"
+        className={
+          enabled
+            ? flow.paused
+              ? "border-transparent bg-primary/15 text-primary"
+              : "border-transparent bg-status-success/15 text-status-success"
+            : "text-muted-foreground"
+        }
+      >
+        {enabled ? (flow.paused ? "Paused" : "Running") : "Stopped"}
+      </Badge>
+
+      <Switch
+        checked={enabled}
+        disabled={toggle.isPending}
+        onCheckedChange={(next) => toggle.mutate(next)}
+        aria-label={`Run ${flow.title || flow.name}`}
+        data-testid="flow-enabled-switch"
+      />
+    </div>
+  )
+}
+
+function Dashboard() {
+  const { user: currentUser } = useAuth()
+  const { data, isPending } = useQuery({
+    ...flowsQueryOptions(),
+    refetchInterval: REFRESH_INTERVAL,
+  })
+
+  const flows = data?.data ?? []
+  const running = flows.filter((flow) => flow.enabled ?? true).length
+  const failing = flows.filter((flow) => (flow.error_count ?? 0) > 0).length
+
+  return (
+    <div className="grid gap-6">
+      <div>
+        <h1 className="max-w-sm truncate text-2xl">
+          Hi, {currentUser?.full_name || currentUser?.email} 👋
+        </h1>
+        <p className="text-muted-foreground">
+          {flows.length === 0
+            ? "No flows yet."
+            : `${running} of ${flows.length} flows are running.`}
+        </p>
       </div>
+
+      <div className="grid gap-3 sm:grid-cols-3">
+        <Tile label="Flows" value={flows.length} />
+        <Tile label="Running" value={running} />
+        <Tile label="With errors" value={failing} />
+      </div>
+
+      <Card className="gap-0 py-0">
+        {isPending ? (
+          <div className="grid gap-3 p-5">
+            <Skeleton className="h-5 w-40" />
+            <Skeleton className="h-5 w-28" />
+          </div>
+        ) : flows.length === 0 ? (
+          <div className="flex flex-col items-center gap-3 px-5 py-10 text-center">
+            <span className="flex size-12 items-center justify-center rounded-full bg-muted text-muted-foreground">
+              <Workflow className="size-5" />
+            </span>
+            <p className="text-sm text-muted-foreground">
+              Flows you build show up here, with what they are doing.
+            </p>
+            <Link to="/flows" className="text-sm font-medium underline">
+              Go to flows
+            </Link>
+          </div>
+        ) : (
+          flows.map((flow) => <FlowRow key={flow.name} flow={flow} />)
+        )}
+      </Card>
     </div>
   )
 }

frontend/tests/drafts.spec.ts

@@ -14,7 +14,11 @@ test.describe.configure({ mode: "serial" })
 
 const apiUrl = process.env.VITE_API_URL || "http://api.localhost"
 
-async function api(page: Page, path: string, init: Record<string, unknown> = {}) {
+async function api(
+  page: Page,
+  path: string,
+  init: Record<string, unknown> = {},
+) {
   const token = await page.evaluate(() => localStorage.getItem("access_token"))
   return page.request.fetch(`${apiUrl}/api/v1${path}`, {
     ...init,

frontend/tests/runtime.spec.ts

@@ -0,0 +1,128 @@
+import { expect, type Page, test } from "@playwright/test"
+
+/**
+ * Flows can be taken off the engine and put back, and what a node prints — or
+ * the traceback of one that fails — is readable without leaving the canvas.
+ */
+
+const flowName = `test_runtime_${Date.now().toString(36)}`
+
+test.use({ storageState: "playwright/.auth/user.json" })
+
+test.describe.configure({ mode: "serial" })
+
+const apiUrl = process.env.VITE_API_URL || "http://api.localhost"
+
+const PRINTING_NODE = `def process(params):
+    print("sensor read 21.5 degrees")
+    return {"reading": 21.5}
+`
+const BROKEN_NODE = `def process(reading, params):
+    raise RuntimeError("downstream blew up")
+`
+
+async function api(
+  page: Page,
+  path: string,
+  init: Record<string, unknown> = {},
+) {
+  const token = await page.evaluate(() => localStorage.getItem("access_token"))
+  return page.request.fetch(`${apiUrl}/api/v1${path}`, {
+    ...init,
+    headers: { Authorization: `Bearer ${token}` },
+  })
+}
+
+test.beforeAll(async ({ browser }) => {
+  const page = await browser.newPage({
+    storageState: "playwright/.auth/user.json",
+  })
+  await page.goto("/")
+  await api(page, `/flows/${flowName}`, {
+    method: "PUT",
+    data: {
+      name: flowName,
+      title: "Runtime",
+      nodes: [
+        {
+          id: "sensor",
+          type: "python",
+          position: { x: 0, y: 0 },
+          provides: [{ name: "reading", dtype: "float" }],
+        },
+        {
+          id: "logger",
+          type: "python",
+          position: { x: 260, y: 0 },
+          requires: [{ name: "reading", dtype: "float" }],
+        },
+      ],
+    },
+  })
+  await api(page, `/flows/${flowName}/nodes/sensor/source`, {
+    method: "PUT",
+    data: { code: PRINTING_NODE },
+  })
+  await api(page, `/flows/${flowName}/nodes/logger/source`, {
+    method: "PUT",
+    data: { code: BROKEN_NODE },
+  })
+  const detail = await (await api(page, `/flows/${flowName}`)).json()
+  await api(page, `/flows/${flowName}/publish`, {
+    method: "POST",
+    data: { version: detail.definition.version },
+  })
+  await page.close()
+})
+
+test("the dashboard lists flows and can stop one", async ({ page }) => {
+  await page.goto("/")
+  const row = page
+    .getByTestId("dashboard-flow-row")
+    .filter({ hasText: "Runtime" })
+  await expect(row).toBeVisible()
+  await expect(row).toContainText("Running")
+
+  await row.getByTestId("flow-enabled-switch").click()
+  await expect(row).toContainText("Stopped")
+
+  // A stopped flow is not something the engine will run.
+  const refused = await api(page, `/flows/${flowName}/run`, {
+    method: "POST",
+    data: { inputs: {} },
+  })
+  expect(refused.status()).toBe(409)
+
+  await row.getByTestId("flow-enabled-switch").click()
+  await expect(row).toContainText("Running")
+})
+
+test("the logs panel shows what a node printed and why one failed", async ({
+  page,
+}) => {
+  await page.goto(`/flows/${flowName}`)
+  await page.waitForSelector(".react-flow__node")
+
+  await page.getByTestId("run-flow").click()
+  await page.getByTestId("flow-logs").click()
+
+  const panel = page.locator('[data-slot="popover-content"]')
+  await expect(panel).toContainText("sensor read 21.5 degrees")
+  await expect(panel).toContainText("RuntimeError")
+})
+
+test("a flow can be paused and let go again", async ({ page }) => {
+  await page.goto(`/flows/${flowName}`)
+  await page.waitForSelector(".react-flow__node")
+
+  await page.getByTestId("pause-flow").click()
+  await expect(page.getByTestId("resume-flow")).toBeVisible()
+  expect((await (await api(page, `/flows/${flowName}`)).json()).paused).toBe(
+    true,
+  )
+
+  await page.getByTestId("resume-flow").click()
+  await expect(page.getByTestId("pause-flow")).toBeVisible()
+
+  await api(page, `/flows/${flowName}`, { method: "DELETE" })
+})

frontend/tests/utils/user.ts

@@ -7,7 +7,7 @@ export async function logInUser(page: Page, email: string, password: string) {
   await page.getByTestId("password-input").fill(password)
   await page.getByRole("button", { name: "Log In" }).click()
   await page.waitForURL("/")
-  await expect(
-    page.getByText("Welcome back, nice to see you again!"),
-  ).toBeVisible()
+  // The greeting is the one part of the dashboard that is there whether or not
+  // any flows are.
+  await expect(page.getByRole("heading", { name: /^Hi, / })).toBeVisible()
 }