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feat(backend-common): drain and exit the worker when its engine is lost - #17727
The shared backend Worker can now shut a worker down when its engine is lost, so its supervisor restarts it. Today a worker stays registered until its own process exits, so a sidecar whose engine died keeps receiving requests it cannot serve.
New optional LLMEngine hook, engine_lost(). It resolves, with an error, once the engine can no longer serve. By default it never resolves, so SGLang, TensorRT-LLM, LPX and Python engines are unchanged.
The Worker treats it like SIGTERM. It cancels its shutdown token and runs the existing path under the existing shutdown deadline: withdraw from discovery, grace period, prefill drain, cleanup(). It then returns the engine's error, so the process exits with code 1.
In Kubernetes the sidecar container is restarted, and the vLLM container keeps running.
The restarted sidecar connects as at startup.
A signal shutdown that has already begun keeps its own result. The Worker stops watching the hook once shutdown starts.
Stack: this PR → #17728 (vLLM), with the SGLang and TensorRT-LLM versions stacked alongside it. Design: #17643.
Where should the reviewer start?
run_with_shutdown in lib/backend-common/src/worker.rs, then the hook's doc in engine.rs.
Validation
New unit test:lost_engine_withdraws_the_worker_and_returns_its_error in worker.rs.
It runs the real Worker with in-process discovery and a fake engine whose engine_lost() resolves while it is serving.
It checks that the endpoint leaves discovery, that cleanup() runs once, that the shutdown token is cancelled, and that the Worker returns the engine's EngineShutdown error.
cargo fmt --check and cargo clippy --all-targets -D warnings are clean for backend-common and the sidecar crates.
cargo test for backend-common, the four sidecars, sidecar-common and the testkit: 666 passed, 0 failed, 9 ignored (the existing ignored live-server tests). The testkit process tests ran with --test-threads=8.
Add optional availability(), recover() and on_recovered() engine hooks. The worker withdraws its endpoint while the engine is NotReady, also withdraws the model card while it is Lost, calls recover(), re-registers fresh metadata with the caller's taints, and calls on_recovered() before the endpoint is routable again. Operator pause survives recovery. Pause, resume, taint, recovery and shutdown discovery writes share one lock, record an unknown state before each write so a dropped HTTP request cannot leave a stale record, and skip endpoint writes that discovery already matches. A prefill worker whose engine is lost skips the shutdown drain.
Signed-off-by: Julien Darve <jdarve@NVIDIA.com>
The engine API adds availability and recovery hooks. Workers synchronize endpoint and model-card registration with engine availability, shutdown, and operator pause state. Recovery refreshes model metadata and restores saved taints. Lifecycle tests cover availability transitions, recovery, and pause and resume behavior.
Changes
Engine lifecycle
Layer / File(s)
Summary
Availability and recovery contract lib/backend-common/src/engine.rs, lib/backend-common/src/lib.rs, lib/backend-common/AGENTS.md
The public API adds EngineAvailability and three LLMEngine lifecycle hooks. The documentation describes their behavior. The engine re-export list is reformatted without changing its exported names.
Worker registration and recovery lib/backend-common/src/worker.rs
The worker tracks discovery registration and availability state. It withdraws endpoints and model cards as required, recovers model metadata, restores saved taints, and invokes on_recovered() before the endpoint becomes routable.
Controls, taints, and lifecycle validation lib/backend-common/src/worker.rs
Shutdown, taint updates, and pause and resume controls use shared registration state. Tests cover availability transitions, refreshed metadata, retained taints, and paused workers.
Engine-loss withdrawal and recovery are opt-in, and existing engines are unchanged. One narrow race remains: if the runtime shuts down while an engine is being recovered, the worker may keep running or re-register its model card until recovery finishes. Fixing this before merge would make shutdown behavior during recovery more reliable.
Pre-merge checks: 4 · 1
Failed checks: 1 warning
Check name
Status
Explanation
Resolution
Docstring Coverage
Docstring coverage is 69.23% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…
Write docstrings for the functions missing them to satisfy the coverage threshold.
Passed checks: 4 passed
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Linked Issues check
Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check
Check skipped because no linked issues were found for this pull request.
Title check
The title clearly describes the worker behavior for a lost engine. It does not mention the recovery-related changes, but it accurately covers a major part of the changeset.
Description check
The description is detailed and on topic. It explains the worker behavior, implementation scope, testing, related issues, and reviewer starting points. It uses a "Summary" heading instead of the templ…
Full details: Description check
Explanation
The description is detailed and on topic. It explains the worker behavior, implementation scope, testing, related issues, and reviewer starting points. It uses a "Summary" heading instead of the template's "Overview" and does not include explicit "Key Changes" or "Issues and Ongoing Work" headings. The pull request checklist is also missing, but the required technical information is mostly present.
Full details: Docstring Coverage
Explanation
Docstring coverage is 69.23% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 13 functions across 3 files. (1 skipped: 1 unsupported.)
Resolution
Write docstrings for the functions missing them to satisfy the coverage threshold.
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Reviewing files that changed from the base of the PR and between eb9ab16 and 57c102a.
Files selected for processing: 4
lib/backend-common/AGENTS.md
lib/backend-common/src/engine.rs
lib/backend-common/src/lib.rs
lib/backend-common/src/worker.rs
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @lib/backend-common/src/worker.rs:
- Around line 1438-1442: Update recover_engine so only self.engine.recover() is
raced against serve_fut and shutdown; keep the locked discovery writes before
and after recovery outside that race. If serve_fut completes during recovery,
return its result through the existing shutdown path.
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Reviewing files that changed from the base of the PR and between eb9ab16 and 57c102a.
Files selected for processing: 4
lib/backend-common/AGENTS.md
lib/backend-common/src/engine.rs
lib/backend-common/src/lib.rs
lib/backend-common/src/worker.rs
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The serve loop waited on LLMEngine::recover() without watching its
endpoint, so a runtime shutdown during recovery left the worker waiting
and could re-register the model card after shutdown started. Race
recover() against the endpoint and the shutdown token; the discovery
writes before and after it still run to completion.
Signed-off-by: Julien Darve <jdarve@NVIDIA.com>
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Reviewed c10c879..e929de3 (4 files, lib/backend-common) with Codex, Gemini and Claude Code's /code-review. All three reviews counted. 4 blockers (P1) and 6 P2 / 2 P3 findings. Not checked: no builds, tests, clippy or lint were run by any reviewer; no changed file was left unread.
EngineAvailability::NotReady is documented as alive but not accepting work, e.g. while paused or asleep. After a RegisterAfter control (resume_generation, wake_up, resume_memory_occupation) the callback wakes the engine, then wrap_engine_control_callback sets is_paused = false and calls sync_endpoint. But registration.engine is still NotReady (it is updated only asynchronously by the serve loop), so is_routable() is false: the endpoint stays unregistered and /health stays NotReady while the control call returns 200 OK. A later registration failure in the serve loop is only logged (line 1414-1416), not returned to the caller.
registration.engine is updated only inside the serve loop, which has exited once shutdown begins. If the engine goes Lost during shutdown or the drain itself (e.g. a sidecar's engine container exits on pod SIGTERM), drain_until_idle_or_deadline never rechecks and polls is_quiescent() for the full DYN_PREFILL_DRAIN_TIMEOUT_S budget when it returns Ok(None) or Err(_).
A recovered worker has card = Some(true) re-attached by register_recovered_engine. If shutdown relies on lease or endpoint teardown to clear the card, confirm the new card cannot outlive its endpoint.
Engine recovery now records the new model card on the admission gate
created at startup (`WorkerAdmissionGate::get`), since #17481 made
`get_or_create` fallible.
Signed-off-by: Julien Darve <jdarve@NVIDIA.com>
A discovery write that failed when the engine's availability changed was
only logged, so one transient error could leave a healthy worker out of
discovery until its next change. The serve loop now retries the sync
every second until it succeeds.
Also correct the registration docs, which said these writes are never
cancelled, and the comment on skipping the drain for a lost engine.
Signed-off-by: Julien Darve <jdarve@NVIDIA.com>
Replace the availability, recover and on_recovered hooks with one optional
LLMEngine hook, engine_lost(). When it resolves, the Worker shuts down as on
SIGTERM (withdraw from discovery, grace period, drain, cleanup) under the
same shutdown deadline, and returns the engine's error so the process exits
and its supervisor restarts it. Engines that don't implement it never
resolve it, so their behavior is unchanged.
Signed-off-by: Julien Darve <jdarve@NVIDIA.com>
JulienDarve
changed the title
feat(backend-common): withdraw and recover workers on engine loss
feat(backend-common): drain and exit the worker when its engine is lost
Nov 3, 2026
Replace the in-process reconnect with #17727's engine_lost() hook. After
start(), the vLLM sidecar watches vllm.Inference health; NOT_SERVING, the end
of the stream, a stream error, or failing to open the watch resolves the hook,
so the worker withdraws, drains and exits with an error, and its supervisor
restarts it. On vLLM v0.33.0, vllm.Inference changes only on engine failure
or shutdown, so a pause or sleep does not restart the sidecar.
The previous launcher, manifest, keepalive and testkit changes are removed;
the vLLM Mocker fixture only gains a way to set vllm.Inference NOT_SERVING.
Signed-off-by: Julien Darve <jdarve@NVIDIA.com>
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Summary
The shared backend
Workercan now shut a worker down when its engine is lost, so its supervisor restarts it. Today a worker stays registered until its own process exits, so a sidecar whose engine died keeps receiving requests it cannot serve.LLMEnginehook,engine_lost(). It resolves, with an error, once the engine can no longer serve. By default it never resolves, so SGLang, TensorRT-LLM, LPX and Python engines are unchanged.Workertreats it like SIGTERM. It cancels its shutdown token and runs the existing path under the existing shutdown deadline: withdraw from discovery, grace period, prefill drain,cleanup(). It then returns the engine's error, so the process exits with code 1.Workerstops watching the hook once shutdown starts.Stack: this PR → #17728 (vLLM), with the SGLang and TensorRT-LLM versions stacked alongside it. Design: #17643.
Where should the reviewer start?
run_with_shutdowninlib/backend-common/src/worker.rs, then the hook's doc inengine.rs.Validation
lost_engine_withdraws_the_worker_and_returns_its_errorinworker.rs.Workerwith in-process discovery and a fake engine whoseengine_lost()resolves while it is serving.cleanup()runs once, that the shutdown token is cancelled, and that theWorkerreturns the engine'sEngineShutdownerror.lib/backend-commonis identical to this PR's head a68fccb):cargo fmt --checkandcargo clippy --all-targets -D warningsare clean forbackend-commonand the sidecar crates.cargo testforbackend-common, the four sidecars, sidecar-common and the testkit: 666 passed, 0 failed, 9 ignored (the existing ignored live-server tests). The testkit process tests ran with--test-threads=8.select!branch removed, the new unit test fails, and so does feat(sidecar): restart the vLLM sidecar when vLLM stops serving #17728's vLLM process test (the worker is never withdrawn).Related Issues
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