feat: Failure cooldown after exhausted transient failures - #887
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If a model is unavailable, we would retry the configured number of times and then fall back to the next model in the list. But we would do this on every request. Now we briefly remember the model is not available. We don't retry the model until after a cooldown period. Adds `HttpBackendConfig::failure_cooldown`: after a completion call exhausts retries on a transient failure (transport, timeout, 408/429/5xx), the client skips that model for the cooldown window. Returns `LlmClientError::TemporarilyUnavailable` which triggers ordered fallback and maps to HTTP 503. Lock free using AtomicU64. Fixes: #594 Assisted-by: Pi:GPT 6 Astra medium Reviewed-by: Pi:GLM 5.3 high Signed-off-by: Graham King <grahamk@nvidia.com>
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Navigate logical layers of code changes, visualize relationships, and explore their blast radius. WalkthroughThe client now supports per-model cooldowns after exhausted transient completion failures. Calls to a cooling-down backend return a typed temporary-unavailability error. Ordered routing can fall back to another candidate, and the server maps the error to HTTP 503. ChangesCompletion cooldown
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~25 minutes Merge Risk: 🟡 Moderate · up to The new default-on cooldown can block a healthy backend that serves the same model through a different API format. When the cooldown ends, it can also let a burst of requests through to a backend that is still recovering. Both should be addressed or explicitly accepted before merging. 🚥 Pre-merge checks | ✅ 3 | ❌ 2❌ Failed checks (2 warnings)
✅ Passed checks (3 passed)
Full details: Linked Issues checkExplanation Issue Resolution Implement the opt-in Full details: Docstring CoverageExplanation Docstring coverage is 42.31% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 26 functions across 10 files. (3 skipped: 3 unsupported.)
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Review comments at @crates/libsy-llm-client/src/client.rs:
- Line 133: Change `unavailable_until` to key cooldown deadlines by both
`ModelId` and `WireFormat`, and use the resolved backend format consistently
when initializing and looking up deadlines. Add a test showing that a transient
failure in one format does not prevent a successful call using another format
during cooldown.
- Line 265: Update the cooldown recovery flow around `until`, `cooldown_epoch`,
and `send_with_retries` to atomically allow only one caller to claim the
half-open probe after expiry; return `TemporarilyUnavailable` to concurrent
callers while it runs. Update the circuit state from the probe result and
release the claim if the probe is cancelled, and add a test verifying concurrent
calls at expiry produce exactly one probe.
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crates/libsy-llm-client/Cargo.tomlcrates/libsy-llm-client/README.mdcrates/libsy-llm-client/src/backend.rscrates/libsy-llm-client/src/client.rscrates/libsy-llm-client/src/observability.rscrates/libsy-llm-client/src/run.rscrates/protocol/src/client.rscrates/switchyard-runner/src/config.rscrates/switchyard-runner/src/failure.rscrates/switchyard-server/src/lib.rscrates/switchyard-server/tests/client_deadline.rscrates/switchyard-server/tests/server.rsdocs/reference/toml_schema.md
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@grahamking GPT-6 Astra suggests we may want to disable cooldown whenever forward_auth = true. The issue being different keys can be used and the cooldown will impact the model regardless of the key.
Initial claim by Astra was this should block the PR but I disagree. I'm not sure how often we'd encounter this scenario? Flagging it here in case you think it's worth a follow up PR/issue.
Thanks Ryan! Signed-off-by: Graham King <grahamk@nvidia.com>
…o#887) If a model is unavailable, we would retry the configured number of times and then fall back to the next model in the list. But we would do this on every request. Now we briefly remember the model is not available. We don't retry the model until after a cooldown period. Signed-off-by: Graham King <grahamk@nvidia.com> Signed-off-by: David Gardner <dagardner@nvidia.com>
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If a model is unavailable, we would retry the configured number of times
and then fall back to the next model in the list. But we would do this
on every request.
Now we briefly remember the model is not available. We don't retry the
model until after a cooldown period.
Adds
HttpBackendConfig::failure_cooldown: after a completion call exhausts retries on a transient failure (transport, timeout, 408/429/5xx), the client skips that model for the cooldown window.Returns
LlmClientError::TemporarilyUnavailablewhich triggers ordered fallback and maps to HTTP 503.Lock free using AtomicU64.
Fixes: #594
Assisted-by: Pi:GPT 6 Astra medium
Reviewed-by: Pi:GLM 5.3 high
Signed-off-by: Graham King grahamk@nvidia.com
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New Features
Documentation