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fix(pd): re-read a recovered PD worker's KV engine id before it is promoted - #2492
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…omoted A vLLM prefill using MooncakeConnector that restarted on the same address came back as a new engine process with a new KV transfer engine id, but the gateway kept minting the decode's kv_transfer_params with the id it discovered at registration, stored in the immutable worker spec. The decode had that id cached against the old process's side-channel port, pulled from a dead address and waited out the engine's 480 s transfer timeout on every request (#2491). NIXL was unaffected because the prefill returns per-request params that the gateway relays. The engine id in force now lives on the worker: seeded from the spec and replaced when a probe that succeeds after a Failed or NotReady status re-reads it over gRPC, before the worker is promoted. Both PD routers mint from that id through `connector_mode_for_worker`. Signed-off-by: Alex McC <319643551+hello-alexmcc@users.noreply.github.com>
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WalkthroughWorkers now track KV engine IDs separately from static metadata. Recovered gRPC workers rediscover and refresh the ID before becoming ready. KV transfer routing reads the current ID. ChangesKV engine ID recovery flow
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Severity of issue fixed: Medium Merge Risk: ⚪ Minimal · up to Recovered workers refresh their KV engine ID before returning to service, so PD routing uses the restarted worker’s current engine metadata. No current merge-blocking risk is identified. Sequence Diagram(s)sequenceDiagram
participant WorkerManager
participant discover_grpc_kv_engine_id
participant Worker
participant KVTransferRouter
WorkerManager->>discover_grpc_kv_engine_id: discover current kv_engine_id
discover_grpc_kv_engine_id-->>WorkerManager: return metadata or timeout
WorkerManager->>Worker: refresh_kv_engine_id
KVTransferRouter->>Worker: read current kv_engine_id
Worker-->>KVTransferRouter: return refreshed engine ID
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
✨ Finishing Touches 💡 1📝 Generate docstrings 💡
🧪 Generate unit tests (beta)
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| { | ||
| return; | ||
| } | ||
| match discover_grpc_kv_engine_id(worker.url(), spec.runtime_type.as_str()).await { |
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🔴 Important: this gRPC round-trip has no timeout, and it runs inside the probe future — a hang here permanently strands the worker it is trying to recover.
discover_grpc_kv_engine_id → fetch_grpc_metadata does GrpcClient::connect + get_model_info() + get_server_info() with no deadline on any of them (METADATA_TIMEOUT in discover_metadata.rs is only applied to the HTTP paths). Contrast the probe it now piggybacks on: BasicWorker::grpc_health_check wraps its RPC in time::timeout(health_config.timeout_secs, ...) precisely so a wedged engine cannot stall the loop.
Failure scenario: a restarted vLLM engine accepts the TCP connection and answers health_check, but GetServerInfo/GetModelInfo never returns (engine mid-init, wedged scheduler thread). The probe future never resolves, so apply_probe_completion never runs; worker_id stays in in_flight forever, which (a) excludes it from re-queue at manager.rs:445 so it is never probed again and never promoted out of Failed, and (b) permanently burns one of MAX_CONCURRENT_HEALTH_PROBES slots (manager.rs:438) — enough wedged PD workers and the whole health-check loop starves. That is a strictly worse outcome than the stale-engine-id bug this PR fixes.
Wrapping the call in tokio::time::timeout (the health config's timeout_secs, or REQUEST_TIMEOUT) and treating expiry as the existing Err branch — keep the previous id and warn — makes the worst case "stale id, as before" instead of "worker never recovers".
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Agreed, and fixed: the re-read is wrapped in tokio::time::timeout with the probe's timeout_secs, and an expiry takes the same path as a failed read (previous id kept, warning logged). Regression test added: a loopback server that accepts and never answers; the helper returns within the timeout with the old id intact.
| /// came back on the same address (a restart), and without | ||
| /// `--remove-unhealthy-workers` this is the only way it rejoins | ||
| /// - Failed stays Failed on failure; Draining never transitions here | ||
| /// A PD worker that answers its probe again after failing may be a new engine |
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🟡 Nit: the new function was inserted between compute_next_status's doc comment and its signature, so that doc block now documents refresh_kv_engine_id_after_recovery instead. Rustdoc for the state machine ("Apply the state machine to a probe outcome… Pending → Ready on success_threshold…", lines 711-723) is now attached to the engine-id refresh, and compute_next_status at line 754 is left undocumented. Moving the new function (and its own doc comment) above line 711 or below compute_next_status restores both.
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Moved above compute_next_status's doc block; both functions carry their own docs again.
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Inline comments:
In `@model_gateway/src/workflow/steps/local/discover_metadata.rs`:
- Line 266: The discover_grpc_kv_engine_id flow must bound fetch_grpc_metadata
with a timeout covering connection and metadata RPCs; on timeout, propagate the
error through the existing warning path so the previous engine ID is retained.
Add a regression test for a healthy worker whose metadata RPC never completes,
verifying the probe times out without blocking subsequent health probes.
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📒 Files selected for processing (6)
model_gateway/src/routers/common/kv_transfer.rsmodel_gateway/src/worker/builder.rsmodel_gateway/src/worker/manager.rsmodel_gateway/src/worker/worker.rsmodel_gateway/src/workflow/steps/local/discover_metadata.rsmodel_gateway/src/workflow/steps/local/mod.rs
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| && matches!( | ||
| launched_status, | ||
| WorkerStatus::Failed | WorkerStatus::NotReady | ||
| ) |
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🟡 Nit: Pending is excluded, which leaves one variant of the same bug uncovered. Registration tolerates a failed metadata read — discover_metadata.rs:412 does unwrap_or_else(|e| { warn!(...); (HashMap::new(), None) }), and take_kv_transfer_metadata has no requirement that a PD worker carry an id — so a PD worker registered while its engine was still coming up lands in Pending with kv_engine_id: None. Its promotion is Pending → Ready, never Failed/NotReady, so this refresh never runs and the worker mints handoffs with no engine id for the rest of its life. Adding WorkerStatus::Pending to the matches! would close it (at the cost of one metadata read per pending probe on PD workers).
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Taken: a Pending PD worker whose engine id is still None also re-reads on a successful probe (a Pending worker that already has an id does not, since it is the same process registration read). Note this still needs kv_connector to be known; a registration whose metadata read failed entirely has no connector either and stays Passthrough, which is the pre-existing behaviour.
| /// The KV transfer engine id in force, seeded from the spec and replaced | ||
| /// when a recovered engine reports a new one (see | ||
| /// [`Worker::refresh_kv_engine_id`]). Not shared across same-URL | ||
| /// replacements: a replacement is built from a fresh discovery. |
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🟣 Pre-existing: "a replacement is built from a fresh discovery" holds for the K8s remove+add path and for re-registration, but not for UpdateWorkerPropertiesStep. That step rebuilds the worker from the old spec's fields (update_worker_properties.rs:88-150) and the update workflow runs find_worker_to_update → update_worker_properties with no discovery step in between (steps/local/mod.rs:158-181), so nothing re-reads the engine.
The pre-existing part: that builder never sets kv_connector, kv_role, or kv_engine_id, and take_kv_transfer_metadata strips those keys out of spec.labels at creation — so a PUT that only touches labels/priority on a PD worker silently drops its whole KV transfer config today, not just the id. Not introduced here and not this PR's job to fix, but it does mean a refreshed id is thrown away by any worker-properties update, so the field doc's claim reads stronger than the code supports. Worth softening the wording, and worth a follow-up issue for the connector loss.
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Verified and agreed: the properties rebuild carries none of kv_connector, kv_role, kv_engine_id, bootstrap host/port or pairing_protocol, so a label or priority PUT on a PD worker drops its whole transfer config today. Softened the field doc to say a rebuilt worker starts from its spec's unrefreshed id, and filed the connector loss as a follow-up issue.
…ker that registered without one The metadata re-read ran inside the probe future with no deadline, so an engine that answers its health check but never its metadata RPC would have held the probe slot forever, which is worse than the stale id it replaces. The read is now bounded by the probe timeout and an expiry keeps the previous id with a warning; a loopback server that accepts and never answers pins that. A PD worker registered before its engine reported an id promotes from Pending, so that case re-reads too. The helper no longer sits between `compute_next_status` and its doc comment. Signed-off-by: Alex McC <319643551+hello-alexmcc@users.noreply.github.com>
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| match read { | ||
| Ok(Ok(discovered)) => { | ||
| let previous = worker.kv_engine_id(); | ||
| if worker.refresh_kv_engine_id(discovered.clone()) { |
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🔴 Important: a partial metadata read is Ok(None) here, and refresh_kv_engine_id(None) clears a known-good id — the opposite of the "a read that fails keeps the previous one" contract the doc comment above states.
discover_grpc_kv_engine_id (discover_metadata.rs:262-268) only returns Err when connect or get_model_info fails. get_server_info — the RPC that actually carries kv_engine_id for vLLM (ServerInfo::Vllm(info) => flat_labels(info)) — is tolerated inside fetch_grpc_metadata: Err(e) => warn!("Failed to fetch gRPC server info: {}", e). The function then succeeds with a label map that has no kv_engine_id, so discovered is None and the ArcSwapOption is stored as None.
Failure scenario: a restarted vLLM prefill answers health_check and GetModelInfo, but GetServerInfo transiently fails (engine still assembling its server-args snapshot — exactly the window a recovery probe lands in). The worker is promoted with kv_engine_id: None. connector_mode_for_worker then yields Mooncake { engine_id: None }, and pd_router.rs:1109-1117 takes the legacy branch: "Mooncake without a discovered kv_engine_id cannot be minted for; decode recomputes the prompt locally" — silently, for every request, until the worker next cycles through Failed. So a flaky side-RPC converts the bug this PR fixes (stale id → hang) into a permanent silent loss of PD.
Two smaller consequences of the same None write: it also discards an operator-supplied spec.kv_engine_id (the smg.ai/kv-engine-id annotation), which take_kv_transfer_metadata deliberately prefers over the discovered label at registration (create_worker.rs:307-319) — recovery silently inverts that precedence; and the Pending arm added in this push re-reads on every probe while the id is None, so a worker cleared this way keeps paying a full gRPC metadata round-trip per probe.
Only writing a discovered id when there is one keeps the worst case "stale id, as before":
| if worker.refresh_kv_engine_id(discovered.clone()) { | |
| Ok(Ok(Some(discovered))) => { | |
| let previous = worker.kv_engine_id(); | |
| if worker.refresh_kv_engine_id(Some(discovered.clone())) { |
(with a matching Ok(Ok(None)) => arm warning that the engine reported no id and the previous one is kept).
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Fixed as suggested: only Ok(Some(id)) is written; Ok(None) warns and keeps the previous id, so a partial read can neither clear a known id nor put the worker on the legacy path. On the precedence point: an id the engine actually reports wins over the spec's on recovery, since it is the one the handoff must target; an operator-pinned id that matches the engine's is unchanged, and one that does not would have hung either way.
| let engine_may_have_changed = matches!( | ||
| launched_status, | ||
| WorkerStatus::Failed | WorkerStatus::NotReady | ||
| ) || (launched_status == WorkerStatus::Pending | ||
| && worker.kv_engine_id().is_none()); | ||
| if probe_result.is_ok() && engine_may_have_changed { |
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🟡 Nit: the gate that decides when to re-read is the part of this change with the most behavioural surface, and it is the only part with no test. It lives inline in the probe closure, so nothing pins that Ready and Draining skip the read, that Pending with an id skips it (the case the reply to the earlier Pending thread explicitly carved out), or that Pending without one takes it. The two tests added cover the layers on either side — refresh_kv_engine_id_after_recovery in isolation (manager.rs:1238) and minting off the refreshed value (kv_transfer.rs:261) — but a regression that flipped this predicate to, say, fire on every Ready probe would put a full gRPC metadata round-trip on every health probe of every PD worker and no test would notice.
Lifting it to fn engine_id_may_be_stale(launched_status: WorkerStatus, worker: &dyn Worker) -> bool next to compute_next_status (which is unit-tested that way already) makes all four cases a cheap table test.
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Lifted to engine_id_may_be_stale(launched_status, worker) next to compute_next_status, with a table test over Failed, NotReady, Pending with and without an id, Ready and Draining.
A metadata read that connects and answers GetModelInfo but not GetServerInfo comes back without an engine id, and storing that None would have cleared a known-good id and dropped the worker to the legacy no-engine-id path for every request until its next failure. Only an id the engine reports is written; a read without one keeps the previous id and warns. Signed-off-by: Alex McC <319643551+hello-alexmcc@users.noreply.github.com>
| } | ||
| // A partial read (server info tolerated as missing) or an engine | ||
| // that reports no id: a known id must never be cleared by it. | ||
| Ok(Ok(None)) => warn!( |
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🟡 Nit: keeping the previous id is the right call, but the fallback is terminal — the worker is promoted on this same probe and never re-read again, so a single partial read leaves #2491 fully reproducible for the life of the process.
The refresh has no bearing on compute_next_status, so the probe that took this arm still drives Failed → Ready (or NotReady → Ready) on success_threshold. Once the worker is Ready, the gate at manager.rs:497-501 is false for every subsequent probe (Ready is neither Failed/NotReady, and the Pending clause needs launched_status == Pending), and nothing on the request path re-reads — kv_transfer.rs:103 only reads worker.kv_engine_id(). So the id stays stale until the worker happens to cycle through Failed again, which needs another outage.
Failure scenario: restarted vLLM prefill answers health_check and GetModelInfo, GetServerInfo transiently fails (engine still assembling its server-args snapshot — exactly the window a recovery probe lands in). fetch_grpc_metadata tolerates that (discover_metadata.rs:286, Err(e) => warn!(...)), so this arm runs, the pre-restart id is kept, the worker goes Ready, and every handoff through it is minted for the dead process's transfer address → 480 s stall per request, one warn! line total. Same for the Ok(Err)/Err(_) arms below.
Cheapest close: treat "the id is unconfirmed" like the Pending case — e.g. widen the gate to also fire while the worker's id has not been confirmed since its last recovery (a flag set on entry to these three arms, cleared on Ok(Ok(Some(_)))), so the next probe interval retries instead of one shot at promotion time.
Minor, same arm: when previous is None (the Pending-without-id path added in the previous push), "keeping the previous one" reads as if there were one, and this warns once per probe until promotion — worth distinguishing "engine reports no id" from "read failed, prior id retained".
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Closed as suggested: the worker carries an unconfirmed flag set on entry to the three fallback arms and cleared on a confirmed id, and the gate fires on a Ready probe while it is set, so the next interval retries rather than the id staying stale until the next outage. The table test covers Ready-unconfirmed, and the hanging-server test asserts the flag is left set. The no-id warning now distinguishes an engine that reports none (previous None) from a read that kept a prior id.
The gate that decides whether a successful probe re-reads the engine id is lifted next to the status state machine and covered by a table test: always after Failed or NotReady, while Pending only without an id, never on a Ready or Draining probe. Signed-off-by: Alex McC <319643551+hello-alexmcc@users.noreply.github.com>
… probes A re-read that failed, expired or came back without an id on the probe that promoted the worker left it serving with a possibly stale id until its next outage. The worker now remembers that its id is unconfirmed, and a Ready probe re-reads while that holds; a confirmed id clears it. The warnings distinguish an engine that reports no id from a read that failed and kept the prior one. Signed-off-by: Alex McC <319643551+hello-alexmcc@users.noreply.github.com>
| } | ||
| // A partial read (server info tolerated as missing) or an engine | ||
| // that reports no id: a known id must never be cleared by it. | ||
| Ok(Ok(None)) => match worker.kv_engine_id() { |
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🔴 Important: this arm is not just the transient partial-read case — it is also the steady state of every PD worker whose engine never reports an id, and those now re-read on every probe for the life of the process.
worker.set_kv_engine_id_confirmed(false) at line 748 runs before the match and is only undone by Ok(Ok(Some(_))). Ok(Ok(None)) is returned both by a partial read (get_server_info tolerated as a warn! inside fetch_grpc_metadata, discover_metadata.rs:286) and by an engine that simply has no kv_engine_id in its server info — the case this very push added a branch for ("PD worker reports no KV engine id yet"), and the case kv_transfer.rs:26 documents as supported ("the servicer predates kv_engine_id reporting"). discover_grpc_kv_engine_id collapses the two into the same Ok(None), so the flag can never be cleared for the second kind of worker.
Failure scenario: a gRPC PD worker with kv_connector set whose engine reports no kv_engine_id (older servicer, or a connector that does not carry one). It registers Pending with kv_engine_id: None, so engine_id_may_be_stale already fires on the pending probe → Ok(None) → unconfirmed. It is promoted to Ready, and the new WorkerStatus::Ready => !worker.kv_engine_id_confirmed() arm (line 717) is then true for every subsequent probe. So each health probe — every check_interval_secs, forever — additionally does GrpcClient::connect + get_model_info() + get_server_info() (a fresh channel each time, fetch_grpc_metadata does not reuse one), emits one warn!, and holds its MAX_CONCURRENT_HEALTH_PROBES slot for up to timeout_secs longer. Before this push the read happened once per recovery; now it is unbounded and permanent, on workers where it can never succeed.
Two ways out, either is small:
- Distinguish the two in
discover_grpc_kv_engine_id: propagate theget_server_infofailure (a variant offetch_grpc_metadatathat does not tolerate it for this call site) so a partial read isErr→ unconfirmed → retried, while a complete read that reports no id is a confirmed "this engine has none" → flag cleared, no retry. - Or bound it: keep the flag for a fixed number of probes, so the worst case is a few extra reads rather than one per probe forever.
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Took the first way out: discover_grpc_kv_engine_id now calls server info directly and propagates its failure, so a partial read is Err (unconfirmed, retried next probe) and a completed read without an id is Ok(None), which confirms the worker (no retry, known id kept, debug-level when there was none). A worker whose engine never reports an id therefore reads once while pending and never again.
| }, | ||
| }; | ||
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| /// The re-read gate: after a failure always, while pending only without |
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🟡 Nit: this doc comment (and the test name below it) now contradict the case added right underneath — "never on a Ready or Draining probe" is no longer true, (WorkerStatus::Ready, &unconfirmed, true) is exactly a Ready probe that re-reads. Worth ".. and on a Ready probe whose last re-read was not confirmed" so the summary still matches the table.
Also uncovered: nothing pins the clearing side of the flag. a_hanging_engine_id_read_expires_and_keeps_the_previous_id asserts the flag is left set on expiry, and the table test seeds unconfirmed by hand, but no test drives refresh_kv_engine_id_after_recovery to a successful read and asserts kv_engine_id_confirmed() is back to true. If set_kv_engine_id_confirmed(true) at line 760 were ever dropped, every PD worker would fall into a re-read on every probe forever and the suite would stay green.
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Doc and test name updated to cover the Ready-until-confirmed case.
… that reports no id The recovery re-read reused the registration discovery, which tolerates a failed server-info call and returns no id for it, so a partial read and an engine that never reports an id looked the same. Left unconfirmed, a worker of the second kind would have re-read on every probe for the life of the process. The re-read now calls server info directly and propagates its failure: a read that did not complete is retried on the next probe, a completed read without an id confirms that there is nothing to retry, and a known id is never cleared by either. Signed-off-by: Alex McC <319643551+hello-alexmcc@users.noreply.github.com>
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Hardware confirmation: #2464's vLLM-over-Mooncake row ( |
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CI status: the two failing lanes here are not this change. |
2 participants
Description
Problem
Fixes #2491. After a vLLM prefill using
MooncakeConnectorrestarts on the same address, every PD request through it hangs until the engine's 480 s transfer timeout. The gateway learnskv_engine_idonce, at registration, and stores it in the immutableWorkerSpec; a worker that goesFailed → Readyat the same URL is the same object with the old id. vLLM's decode keeps the prefill's transfer address cached per engine id, and that address is a random port per process, so a handoff minted for the old id pulls from a dead address with no error on either side. Found by the PD sweep on #2464 (the 1p2d sole-prefill outage cases, and the "15-minute restart" on 2p1d).Solution
The engine id in force lives on the worker (
BasicWorker.kv_engine_id, anArcSwapOptionseeded from the spec) behindWorker::kv_engine_id()/refresh_kv_engine_id(). When a probe succeeds on a worker whose status wasFailedorNotReady, the manager re-reads the id over gRPC (discover_grpc_kv_engine_id, the sameGetServerInfopath registration uses) before the worker is promoted, and logs the change.connector_mode_for_workermints from that id, so both the gRPC sequential path and the HTTP PD router are covered. A read that fails keeps the previous id and warns; HTTP-discovered vLLM workers report no engine id and are unchanged.Changes
model_gateway/src/worker/worker.rs,builder.rs: the refreshable engine id and the two trait methods.model_gateway/src/worker/manager.rs:refresh_kv_engine_id_after_recovery, run from the probe task on a successful probe afterFailed/NotReady.model_gateway/src/workflow/steps/local/discover_metadata.rs,mod.rs:discover_grpc_kv_engine_id.model_gateway/src/routers/common/kv_transfer.rs: mint fromworker.kv_engine_id(); test that a refreshed id is what gets minted.Test Plan
cargo clippy -p smg -p smg-golang --all-targets -- -D warnings: clean.cargo test -p smg --lib -- kv_transfer worker::manager worker::worker::tests discover_metadata: passes.-k "1p2d and not http") is rebased onto this branch;test_sole_leg_outage_is_reported_promptlyand the stream check after it are the cases that hung.Checklist
make fmt/cargo +nightly fmt --all