Survive a busy engine: retry, idempotent submit, resilient waiting

A driver script died of one slow answer: httpx.ReadTimeout out of
RunHandle.refresh() with a 30 s read timeout and no retry anywhere, which
cost a sweep 78 of its 84 runs.

- Split the timeout (5 s connect, 120 s read): a wrong URL fails at once,
  and a busy engine gets longer than the slowest thing it does on purpose
  (a 60 s compile, a 15 s rebuild wait).
- Retry idempotent calls three times on a transport error or 502/503/504.
  503 is the engine's own "ask again" — it is what RebuildBusy answers.
- Submit carries a key the engine stores with the run, so a retry after a
  timeout returns that run instead of starting a second. A sweep keys every
  entry, so a half-created one recreates only what is missing.
- wait() and --follow tolerate five failed polls in a row; a 404 still stops
  at once, because that is an answer rather than a gap.
- CLI says "engine not answering" and names the run still on the engine,
  instead of printing a traceback.
- runs: clamp the params column to 80 characters; events() takes the
  flow/since/until the endpoint already had; RunHandle.failures answers
  "what killed this run" from the run's own node rows.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/fluksio/alembic/versions/f2c6a8d15e93_run_idempotency_key.py

@@ -0,0 +1,40 @@
+"""run.idempotency_key
+
+Submitting is a POST, so a client that retries one after a timeout cannot know
+whether the first attempt landed. A key the caller mints per submission makes
+the answer a lookup: the same key returns the run it already created.
+
+Revision ID: f2c6a8d15e93
+Revises: d1f7a3c8b204
+Create Date: 2026-08-26
+
+"""
+
+import sqlalchemy as sa
+import sqlmodel.sql.sqltypes
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision = "f2c6a8d15e93"
+down_revision = "d1f7a3c8b204"
+branch_labels = None
+depends_on = None
+
+
+def upgrade():
+    op.add_column(
+        "run",
+        sa.Column(
+            "idempotency_key",
+            sqlmodel.sql.sqltypes.AutoString(length=64),
+            nullable=True,
+        ),
+    )
+    # Nullable and unique: every run submitted without a key stays NULL, and
+    # both SQLite and Postgres allow as many of those as there are runs.
+    op.create_index("ix_run_idempotency_key", "run", ["idempotency_key"], unique=True)
+
+
+def downgrade():
+    op.drop_index("ix_run_idempotency_key", table_name="run")
+    op.drop_column("run", "idempotency_key")

backend/fluksio/api/routes/runs.py

@@ -50,11 +50,17 @@ class RunCreate(BaseModel):
     no_cache: bool = False
     #: Who is asking. The dashboard leaves it, and is the "api" default.
     cause: RunCause = "api"
+    #: A key the caller minted for this submission. Sending it again returns
+    #: the run it already made, so a retry after a timeout cannot double-submit.
+    idempotency_key: str | None = Field(default=None, max_length=64)
 
 
 class SweepEntry(BaseModel):
     params: dict[str, Any] = Field(default_factory=dict)
     seed: int | None = None
+    #: One per entry, so retrying a half-created sweep recreates only the runs
+    #: whose rows never landed.
+    idempotency_key: str | None = Field(default=None, max_length=64)
 
 
 class SweepCreate(BaseModel):
@@ -184,6 +190,7 @@ async def create_run(
             actor=user.email,
             draft=body.draft,
             no_cache=body.no_cache,
+            idempotency_key=body.idempotency_key,
         )
     except FlowNotFound as exc:
         raise HTTPException(status_code=404, detail=str(exc)) from exc
@@ -221,6 +228,7 @@ async def create_sweep(
                 actor=user.email,
                 draft=body.draft,
                 no_cache=body.no_cache,
+                idempotency_key=entry.idempotency_key,
             )
             for entry in body.runs
         ]

backend/fluksio/flow/runs.py

@@ -42,6 +42,7 @@ from typing import Any
 
 from sqlalchemy import update
 from sqlalchemy.dialects.sqlite import insert as upsert
+from sqlalchemy.exc import IntegrityError
 from sqlmodel import Session, col, select
 
 from fluksio.core.db import engine as db_engine
@@ -549,8 +550,16 @@ class RunService:
         actor: str = "",
         draft: bool = False,
         no_cache: bool = False,
+        idempotency_key: str | None = None,
     ) -> Run:
         """Journal a run and wake an engine up for it. Never blocks on it."""
+        # Before anything else, including reading the flow: a caller retrying a
+        # submit it never got an answer for is owed the run that answer was
+        # about, whatever the flow says now.
+        if idempotency_key:
+            existing = self._by_key(idempotency_key)
+            if existing is not None:
+                return existing
         flow = self.controller.store.read_flow(flow_name, draft=draft)
         issues = batch_issues(flow)
         if issues:
@@ -580,15 +589,33 @@ class RunService:
             labels=required_labels(flow),
             created_at=datetime.now(UTC),
             actor=actor,
+            idempotency_key=idempotency_key,
         )
         with Session(db_engine) as session:
             session.add(run)
-            session.commit()
+            try:
+                session.commit()
+            except IntegrityError:
+                # Two retries of one submit raced here. The unique index is
+                # what decided which of them is the run; this one reads it
+                # back rather than queueing a second execution of it.
+                session.rollback()
+                existing = self._by_key(idempotency_key) if idempotency_key else None
+                if existing is None:
+                    raise
+                return existing
             session.refresh(run)
 
         self.queue.add(WorkItem(kind="run", node="", flow=flow.name, run_id=run.id))
         return run
 
+    def _by_key(self, idempotency_key: str) -> Run | None:
+        """The run a key already made, if it made one."""
+        with Session(db_engine) as session:
+            return session.exec(
+                select(Run).where(col(Run.idempotency_key) == idempotency_key)
+            ).first()
+
     def cancel(self, run_id: str) -> bool:
         """Stop a run: kill what it is executing, schedule nothing further."""
         with self._lock:

backend/fluksio/models.py

@@ -340,6 +340,12 @@ class Run(SQLModel, table=True):
     group_id: str | None = Field(default=None, index=True, max_length=64)
     #: The run this one was made from, on a retry.
     parent_id: str | None = Field(default=None, max_length=64)
+    #: A key the caller made up for one submission, so retrying a submit that
+    #: may already have landed returns that run instead of starting a second.
+    #: Unique where it is set; null for anything submitted without one.
+    idempotency_key: str | None = Field(
+        default=None, unique=True, index=True, max_length=64
+    )
     #: Where it was asked for: api, cli, sdk, hook or sweep.
     cause: str = Field(default="api", max_length=32)
     #: Re-execute every node, whatever the stage cache holds for it.

backend/fluksio/sdk/cli.py

@@ -20,9 +20,12 @@ from contextlib import contextmanager
 from pathlib import Path
 from typing import Any
 
+import httpx
+
 from fluksio.sdk import FLOWS, Flow, SyncError
 from fluksio.sdk.client import (
     GLOBAL_DATA_DIR,
+    WAIT_TOLERANCE,
     ApiError,
     Client,
     RunHandle,
@@ -56,6 +59,14 @@ def _fail(message: str) -> int:
     return 1
 
 
+def _unreachable(exc: Exception, note: str = "") -> int:
+    """The engine did not answer. Say so as a sentence, not a traceback."""
+    return _fail(
+        f"engine not answering ({type(exc).__name__}: {exc})"
+        + (f". {note}" if note else "")
+    )
+
+
 def _status(text: str, width: int = 0) -> str:
     """A status, coloured when a terminal is reading it.
 
@@ -267,6 +278,8 @@ def cmd_sync(args: argparse.Namespace) -> int:
         )
     except (SyncError, ApiError) as exc:
         return _fail(str(exc))
+    except httpx.HTTPError as exc:
+        return _unreachable(exc, "Nothing was published; sync again when it is back.")
 
     for report in reports:
         if report.unchanged:
@@ -402,9 +415,23 @@ def _follow(client: Client, handle: RunHandle, poll: float = 1.0) -> None:
     before the numbers, so the last batch is never the one that gets missed.
     """
     seen: set[tuple[str, int]] = set()
+    failures = 0
     while True:
-        done = handle.refresh().done
-        for point in client.metrics(handle.id):
+        try:
+            done = handle.refresh().done
+            points = client.metrics(handle.id)
+        except (httpx.HTTPError, ApiError) as exc:
+            # Following an eight-hour run must not end because one poll of it
+            # did. The run is still going; only this side lost sight of it.
+            if isinstance(exc, ApiError) and exc.status < 500:
+                raise
+            failures += 1
+            if failures >= WAIT_TOLERANCE:
+                raise
+            time.sleep(poll)
+            continue
+        failures = 0
+        for point in points:
             mark = (str(point.get("name", "")), int(point.get("step", -1)))
             if mark in seen:
                 continue
@@ -419,7 +446,7 @@ def _cancel(client: Client, handle: RunHandle) -> int:
     """Ctrl-C means stop the run, not just stop watching it."""
     try:
         client.cancel(handle.id)
-    except (SyncError, ApiError) as exc:
+    except (SyncError, ApiError, httpx.HTTPError) as exc:
         return _fail(f"could not cancel {handle.id}: {exc}")
     _say(f"{handle.id}  {_status('cancelled')}")
     return 130
@@ -429,7 +456,7 @@ def _cached_note(client: Client, handle: RunHandle) -> str:
     """How much of the run earlier ones had already answered."""
     try:
         nodes = client.run(handle.id).get("nodes") or []
-    except (SyncError, ApiError):
+    except (SyncError, ApiError, httpx.HTTPError):
         return ""
     cached = sum(1 for node in nodes if node.get("status") == "cached")
     return f"  ({cached}/{len(nodes)} {_status('cached')})" if cached else ""
@@ -439,6 +466,7 @@ def cmd_run(args: argparse.Namespace, rest: list[str]) -> int:
     # An in-process engine lives exactly as long as this command, so a run
     # nobody waits for would be thrown away with the queue holding it.
     wait = args.wait or args.follow or args.local
+    handle: RunHandle | None = None
     try:
         with _client_for(args) as client:
             if not args.no_sync:
@@ -482,6 +510,12 @@ def cmd_run(args: argparse.Namespace, rest: list[str]) -> int:
             return 0 if handle.status == "ok" else 1
     except (SyncError, ApiError) as exc:
         return _fail(str(exc))
+    except httpx.HTTPError as exc:
+        # The run is the engine's, not this command's: it carries on, and its
+        # id is how to find it again.
+        return _unreachable(
+            exc, f"Run {handle.id} is still on the engine." if handle else ""
+        )
 
 
 # ---------------------------------------------------------------------------
@@ -651,6 +685,14 @@ def cmd_status(args: argparse.Namespace) -> int:
         return 130
     except (SyncError, ApiError) as exc:
         return _fail(str(exc))
+    except httpx.HTTPError as exc:
+        return _unreachable(exc)
+
+
+#: How much of a run's inputs the list shows. A flow taking a few kB of JSON
+#: would otherwise make the table unreadable; `client.runs()` is where the
+#: whole value is read.
+PARAMS_WIDTH = 80
 
 
 def cmd_runs(args: argparse.Namespace) -> int:
@@ -659,11 +701,16 @@ def cmd_runs(args: argparse.Namespace) -> int:
             rows = client.runs(flow=args.flow, limit=args.limit)
     except (SyncError, ApiError) as exc:
         return _fail(str(exc))
+    except httpx.HTTPError as exc:
+        return _unreachable(exc)
     for row in rows:
         commit = (row.get("origin_commit") or "")[:7]
+        params = json.dumps(row["params"])
+        if len(params) > PARAMS_WIDTH:
+            params = params[: PARAMS_WIDTH - 3] + "..."
         _say(
             f"{row['id']}  {_status(row['status'], 9)} {row['flow']:<16} "
-            f"{row['duration_ms'] / 1000:7.1f}s  {commit:<8} {json.dumps(row['params'])}"
+            f"{row['duration_ms'] / 1000:7.1f}s  {commit:<8} {params}"
         )
     return 0
 
@@ -725,6 +772,8 @@ def cmd_sweep(args: argparse.Namespace) -> int:
             return 1 if failed else 0
     except (SyncError, ApiError) as exc:
         return _fail(str(exc))
+    except httpx.HTTPError as exc:
+        return _unreachable(exc, "The runs are still on the engine; `fluksio runs`.")
 
 
 # ---------------------------------------------------------------------------

backend/fluksio/sdk/client.py

@@ -11,6 +11,7 @@ import json
 import os
 import subprocess
 import time
+import uuid
 from collections.abc import Iterable
 from pathlib import Path
 from typing import Any
@@ -34,6 +35,27 @@ API = "/api/v1"
 #: A run is over when it reaches one of these.
 DONE = frozenset({"ok", "error", "cancelled", "abandoned"})
 
+#: How long to wait for an answer. Split, because the two halves mean
+#: different things: an engine that is not there refuses the connection at
+#: once, so waiting 30 s for it only ever delays a typo in the URL, while an
+#: engine that *is* there can be busy — a rebuild waits 15 s before it answers
+#: 503, and compiling a node is given 60 s — so the read has to outlast the
+#: slowest thing the engine does on purpose.
+CONNECT_TIMEOUT = 5.0
+READ_TIMEOUT = 120.0
+
+#: How many times an idempotent call is tried again before it gives up, and
+#: what a busy engine answers with. A read timeout used to kill a driver
+#: script outright, which is a poor trade for a request that costs nothing to
+#: repeat. 503 is the engine's own "ask again": it is what a rebuild answers.
+RETRIES = 3
+RETRY_STATUS = frozenset({502, 503, 504})
+
+#: How many refreshes in a row may fail before waiting gives up. Each one is
+#: already several attempts, so this is minutes of a missing engine, not a
+#: blip.
+WAIT_TOLERANCE = 5
+
 
 #: What a project-local installation is called, beside `.venv` and `.git`.
 DATA_DIR_NAME = ".fluksio"
@@ -124,13 +146,19 @@ class Client:
     """An authenticated engine, addressed over its HTTP API."""
 
     def __init__(
-        self, url: str = "", token: str = "", http: Any = None, timeout: float = 30.0
+        self,
+        url: str = "",
+        token: str = "",
+        http: Any = None,
+        timeout: Any = None,
+        retries: int = RETRIES,
     ) -> None:
         stored = _stored()
         self.url = url or os.environ.get("FLUKSIO_URL") or stored.get("url") or ""
         self.token = (
             token or os.environ.get("FLUKSIO_TOKEN") or stored.get("token") or ""
         )
+        self.retries = retries
         if http is None:
             if not self.url:
                 raise SyncError(
@@ -139,6 +167,10 @@ class Client:
                 )
             import httpx
 
+            if timeout is None:
+                timeout = httpx.Timeout(
+                    30.0, connect=CONNECT_TIMEOUT, read=READ_TIMEOUT
+                )
             http = httpx.Client(base_url=self.url, timeout=timeout)
         self.http = http
         if self.token:
@@ -146,8 +178,38 @@ class Client:
 
     # -- plumbing ----------------------------------------------------------
 
-    def _call(self, method: str, path: str, **kwargs: Any) -> Any:
-        response = self.http.request(method, f"{API}{path}", **kwargs)
+    def _request(
+        self, method: str, path: str, idempotent: bool = False, **kwargs: Any
+    ) -> Any:
+        """One request, tried again while it is safe and worth it.
+
+        Only a call that may be repeated without meaning something different
+        is retried — every GET, and the writes that carry a key or are already
+        a no-op the second time. The last attempt is made outside the loop, so
+        whatever it raises is what the caller sees.
+        """
+        import httpx
+
+        attempts = self.retries + 1 if idempotent else 1
+        for attempt in range(attempts - 1):
+            try:
+                response = self.http.request(method, f"{API}{path}", **kwargs)
+                if response.status_code not in RETRY_STATUS:
+                    return response
+            except httpx.TransportError:
+                pass
+            time.sleep(2**attempt)
+        return self.http.request(method, f"{API}{path}", **kwargs)
+
+    def _call(
+        self, method: str, path: str, idempotent: bool | None = None, **kwargs: Any
+    ) -> Any:
+        response = self._request(
+            method,
+            path,
+            idempotent=(method == "GET") if idempotent is None else idempotent,
+            **kwargs,
+        )
         if response.status_code == 409:
             raise Conflict(_detail(response))
         if response.status_code >= 400:
@@ -192,7 +254,9 @@ class Client:
 
     def refresh_modules(self) -> None:
         """Retire the engine's workers, so the next run imports the code as it is."""
-        self._call("POST", "/modules/refresh")
+        # Retiring workers twice is retiring workers, and this is the call most
+        # likely to meet a rebuild's 503.
+        self._call("POST", "/modules/refresh", idempotent=True)
 
     def flows(self) -> list[dict[str, Any]]:
         """Every flow with its node and error counts, as the home screen lists."""
@@ -211,11 +275,29 @@ class Client:
         result: dict[str, Any] = self._call("GET", "/cloud/status")
         return result
 
-    def events(self, kind: str = "failure", limit: int = 10) -> list[dict[str, Any]]:
-        """What went wrong, or who changed what. Newest first."""
-        result = self._call(
-            "GET", "/observability/events", params={"kind": kind, "limit": limit}
-        )
+    def events(
+        self,
+        kind: str = "failure",
+        limit: int = 10,
+        flow: str = "",
+        since: Any = None,
+        until: Any = None,
+    ) -> list[dict[str, Any]]:
+        """What went wrong, or who changed what. Newest first.
+
+        Engine-wide, and narrowed by flow or by time. What *one run* did is a
+        question about that run: :attr:`RunHandle.failures` answers it from the
+        run's own node rows, which carry the traceback anyway.
+        """
+        query: dict[str, Any] = {"kind": kind, "limit": limit}
+        if flow:
+            query["flow"] = flow
+        for name, value in (("since", since), ("until", until)):
+            if value is not None:
+                query[name] = (
+                    value.isoformat() if hasattr(value, "isoformat") else value
+                )
+        result = self._call("GET", "/observability/events", params=query)
         return list(result or [])
 
     # -- runs --------------------------------------------------------------
@@ -229,15 +311,22 @@ class Client:
         cause: str = "sdk",
     ) -> RunHandle:
         """Queue a run. ``cause`` is what the history records it as coming
-        from — a script is the default, `fluksio run` says so itself."""
+        from — a script is the default, `fluksio run` says so itself.
+
+        Safe to retry: the key is minted here, once per call, so an attempt
+        that timed out on the way back is answered with the run it made rather
+        than starting a second one.
+        """
         row = self._call(
             "POST",
             f"/runs/flows/{flow}",
+            idempotent=True,
             json={
                 "params": params or {},
                 "seed": seed,
                 "no_cache": no_cache,
                 "cause": cause,
+                "idempotency_key": uuid.uuid4().hex,
             },
         )
         return RunHandle(self, row["id"], row)
@@ -248,11 +337,17 @@ class Client:
         entries: list[dict[str, Any]],
         no_cache: bool = False,
     ) -> list[RunHandle]:
-        """Many runs of one flow at once. The caller decides what varies."""
+        """Many runs of one flow at once. The caller decides what varies.
+
+        A key per entry rather than one for the sweep: a retry then recreates
+        only the runs whose rows never landed.
+        """
+        keyed = [{**entry, "idempotency_key": uuid.uuid4().hex} for entry in entries]
         rows = self._call(
             "POST",
             f"/runs/flows/{flow}/sweep",
-            json={"runs": entries, "no_cache": no_cache},
+            idempotent=True,
+            json={"runs": keyed, "no_cache": no_cache},
         )
         return [RunHandle(self, row["id"], row) for row in rows]
 
@@ -280,10 +375,11 @@ class Client:
         )
 
     def cancel(self, run_id: str) -> Any:
-        return self._call("POST", f"/runs/{run_id}/cancel")
+        # Cancelling a cancelled run is cancelled.
+        return self._call("POST", f"/runs/{run_id}/cancel", idempotent=True)
 
     def download(self, digest: str) -> bytes:
-        response = self.http.request("GET", f"{API}/artifacts/{digest}")
+        response = self._request("GET", f"/artifacts/{digest}", idempotent=True)
         if response.status_code >= 400:
             raise ApiError(response.status_code, _detail(response))
         return bytes(response.content)
@@ -328,13 +424,45 @@ class RunHandle:
         rows = self._row.get("artifacts")
         return rows if isinstance(rows, list) else []
 
-    def wait(self, timeout: float = 0.0, poll: float = 1.0) -> RunHandle:
-        """Block until the run is over, or ``timeout`` seconds have passed."""
-        deadline = time.monotonic() + timeout if timeout else 0.0
-        while True:
+    @property
+    def failures(self) -> list[dict[str, Any]]:
+        """The nodes that failed, each with its error and its logs."""
+        if "nodes" not in self._row:
             self.refresh()
-            if self.done:
-                return self
+        rows = self._row.get("nodes")
+        rows = rows if isinstance(rows, list) else []
+        return [row for row in rows if row.get("status") == "error"]
+
+    def wait(self, timeout: float = 0.0, poll: float = 1.0) -> RunHandle:
+        """Block until the run is over, or ``timeout`` seconds have passed.
+
+        A run outlives the engine being briefly unreachable, so waiting for one
+        does too: a few failed refreshes in a row are a busy engine, not a
+        finished run. What is not tolerated is the engine answering — a 404
+        means the run is gone, and repeating the question will not bring it
+        back.
+        """
+        import httpx
+
+        deadline = time.monotonic() + timeout if timeout else 0.0
+        failures = 0
+        while True:
+            try:
+                self.refresh()
+            except ApiError as exc:
+                if exc.status < 500:
+                    raise
+                failures += 1
+                if failures >= WAIT_TOLERANCE:
+                    raise
+            except httpx.HTTPError:
+                failures += 1
+                if failures >= WAIT_TOLERANCE:
+                    raise
+            else:
+                failures = 0
+                if self.done:
+                    return self
             if deadline and time.monotonic() > deadline:
                 raise TimeoutError(f"run {self.id} is still {self.status}")
             time.sleep(poll)

backend/pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "fluksio"
-version = "0.1.3"
+version = "0.1.4"
 description = "Node-based automation engine: flows, dashboards, batch runs"
 readme = "README.md"
 license = "AGPL-3.0-or-later"

backend/tests/api/routes/test_runs.py

@@ -19,6 +19,7 @@ from fluksio.flow.runs import (
     OUTPUT_CAP,
     RunCache,
     RunRejected,
+    RunService,
     _cacheable,
     new_run_id,
     resolve_references,
@@ -349,6 +350,59 @@ def test_a_run_records_which_caller_asked_for_it(
     assert refused.status_code == 422
 
 
+class _Unusable:
+    """Anything reaching this is something a deduplicated submit should not do."""
+
+    def __getattr__(self, name):
+        raise AssertionError(f"a repeated submit must not reach {name}")
+
+
+def test_a_repeated_submit_returns_the_run_it_already_made():
+    """The key is the answer to "did my first attempt land?".
+
+    Answered before the flow is even read: a caller retrying a submit it never
+    got a reply to is owed that run, whatever has been published since.
+    """
+    service = RunService(controller=_Unusable(), queue=_Unusable())
+    with Session(db_engine) as session:
+        session.add(
+            Run(
+                id="dedup-1",
+                flow="study",
+                status="running",
+                idempotency_key="key-abc",
+                created_at=datetime.now(UTC),
+            )
+        )
+        session.commit()
+
+    again = service.submit("study", {"lr": 0.1}, idempotency_key="key-abc")
+
+    assert again.id == "dedup-1"
+
+
+def test_a_key_nobody_used_submits_normally(
+    client, superuser_token_headers, monkeypatch
+):
+    """The route carries the key through; without one nothing changes."""
+    seen: dict[str, object] = {}
+
+    class Recorder:
+        def submit(self, name, **kwargs):
+            seen.update(kwargs)
+            return Run(id="keyed-1", flow=name, created_at=datetime.now(UTC))
+
+    monkeypatch.setattr(client.app.state, "run_service", Recorder())
+    answer = client.post(
+        f"{settings.API_V1_STR}/runs/flows/demo",
+        headers=superuser_token_headers,
+        json={"idempotency_key": "key-xyz"},
+    )
+
+    assert answer.status_code == 202
+    assert seen["idempotency_key"] == "key-xyz"
+
+
 def test_overview_is_not_read_as_a_run_id(client, superuser_token_headers):
     """`/overview` is declared before `/{run_id}`, which would swallow it."""
     answer = client.get(

backend/tests/sdk/test_client.py

@@ -0,0 +1,132 @@
+"""What the client does when the engine is slow, busy, or briefly gone."""
+
+import httpx
+import pytest
+
+from fluksio.sdk.client import WAIT_TOLERANCE, ApiError, Client, RunHandle
+
+
+def a_client(handler, monkeypatch, **kwargs):
+    """A client whose transport is a function, and whose backoff costs nothing."""
+    monkeypatch.setattr("fluksio.sdk.client.time.sleep", lambda _seconds: None)
+    transport = httpx.MockTransport(handler)
+    http = httpx.Client(transport=transport, base_url="http://engine")
+    return Client(url="http://engine", token="t", http=http, **kwargs)
+
+
+def test_idempotent_get_is_tried_again(monkeypatch):
+    calls = []
+
+    def handler(request):
+        calls.append(request)
+        if len(calls) < 3:
+            raise httpx.ReadTimeout("too slow", request=request)
+        return httpx.Response(200, json={"id": "r1", "status": "ok"})
+
+    client = a_client(handler, monkeypatch)
+    assert client.run("r1")["status"] == "ok"
+    assert len(calls) == 3
+
+
+def test_a_busy_engine_is_asked_again(monkeypatch):
+    codes = iter([503, 503, 200])
+
+    def handler(request):
+        code = next(codes)
+        return httpx.Response(code, json={"id": "r1"} if code == 200 else {})
+
+    client = a_client(handler, monkeypatch)
+    assert client.run("r1")["id"] == "r1"
+
+
+def test_giving_up_raises_what_it_last_saw(monkeypatch):
+    def handler(request):
+        raise httpx.ReadTimeout("too slow", request=request)
+
+    client = a_client(handler, monkeypatch, retries=2)
+    with pytest.raises(httpx.ReadTimeout):
+        client.run("r1")
+
+
+def test_a_write_is_not_repeated(monkeypatch):
+    calls = []
+
+    def handler(request):
+        calls.append(request)
+        raise httpx.ReadTimeout("too slow", request=request)
+
+    client = a_client(handler, monkeypatch)
+    with pytest.raises(httpx.ReadTimeout):
+        client.put_source("train", "fit", "code")
+    assert len(calls) == 1, "a source write means something different twice"
+
+
+def test_submit_carries_one_key_across_its_retries(monkeypatch):
+    bodies = []
+
+    def handler(request):
+        bodies.append(httpx.Response(200, content=request.content).json())
+        if len(bodies) < 3:
+            raise httpx.ConnectError("no route", request=request)
+        return httpx.Response(202, json={"id": "r1", "status": "queued"})
+
+    client = a_client(handler, monkeypatch)
+    assert client.submit("train", {"lr": 0.1}).id == "r1"
+    keys = {body["idempotency_key"] for body in bodies}
+    assert len(keys) == 1, "a retry must not read as a second run"
+    assert len(next(iter(keys))) == 32
+
+
+def test_a_sweep_keys_every_entry(monkeypatch):
+    seen = {}
+
+    def handler(request):
+        body = httpx.Response(200, content=request.content).json()
+        seen["runs"] = body["runs"]
+        return httpx.Response(202, json=[{"id": "r1"}, {"id": "r2"}])
+
+    client = a_client(handler, monkeypatch)
+    client.sweep("train", [{"params": {"lr": 0.1}}, {"params": {"lr": 0.2}}])
+    keys = [entry["idempotency_key"] for entry in seen["runs"]]
+    assert len(set(keys)) == 2
+
+
+def test_waiting_survives_a_few_bad_answers(monkeypatch):
+    answers = iter(
+        [503] * (WAIT_TOLERANCE - 1) + [200]  # then the run is finished
+    )
+
+    def handler(request):
+        code = next(answers)
+        if code != 200:
+            return httpx.Response(code, json={})
+        return httpx.Response(200, json={"id": "r1", "status": "ok"})
+
+    client = a_client(handler, monkeypatch, retries=0)
+    handle = RunHandle(client, "r1", {"status": "running"})
+    assert handle.wait(poll=0).status == "ok"
+
+
+def test_waiting_gives_up_eventually(monkeypatch):
+    def handler(request):
+        return httpx.Response(503, json={})
+
+    client = a_client(handler, monkeypatch, retries=0)
+    handle = RunHandle(client, "r1", {"status": "running"})
+    with pytest.raises(ApiError):
+        handle.wait(poll=0)
+
+
+def test_a_run_that_is_gone_stops_the_wait_at_once(monkeypatch):
+    calls = []
+
+    def handler(request):
+        calls.append(request)
+        return httpx.Response(404, json={"detail": "no such run"})
+
+    client = a_client(handler, monkeypatch, retries=0)
+    handle = RunHandle(client, "r1", {"status": "running"})
+    with pytest.raises(ApiError) as caught:
+        handle.wait(poll=0)
+    assert caught.value.status == 404
+    assert len(calls) == 1, "a 404 is an answer, not a blip"