feat(tonic-xds): compute gRFC A42 request hash from header hash policy by madhurishgupta · Pull Request #2686 · grpc/grpc-rust

madhurishgupta · 2026-06-15T18:50:40Z

Summary

Implement gRFC A42: xDS Ring Hash LB Policy in tonic-xds. It lands the per-request hash computation.

What this PR does

Adds a xds::resource::hash_policy module with HashPolicyConfig and the request-hash logic as impl methods:
- HashPolicyConfig::request_hash(headers, policies) — folds each matching policy's hash into one value using XXH64 (seed 0, per A42) and Envoy's reference rotate-XOR combination (combined = rotl(combined, 1) ^ new), honoring the terminal flag; returns None if no policy produced a hash (picker then uses a random hash).
Adds RouteDecision.request_hash: Option, carried from the routing layer to the LB picker via request extensions.
resolve_route computes request_hash (currently with an empty policy list — TODO to populate from RDS RouteAction.hash_policy in a later PR).

Tests

Added UTs.
Currently this path won't be called in production.

Plan (A42 series)

(this PR) Request-hash computation + plumbing.
Ring-hash picker + member tracking — builds an A42-conformant ring (min/max ring size, XXH64-keyed vnodes) over the cluster's healthy endpoints and routes each request by its hash; falls through to the next ready endpoint on a miss.
CDS wiring — parse lb_policy: RING_HASH and ring_hash_lb_config (validating hash_function == XX_HASH), and select the ring-hash picker from the cluster's LB policy.
RDS wiring — parse RouteAction.hash_policy to populate the policy list (replacing the empty scaffold here).

linux-foundation-easycla · 2026-06-15T18:50:47Z

The committers listed above are authorized under a signed CLA.

✅ login: madhurishgupta / name: Madhurish Nitish Gupta (45848fb)

YutaoMa · 2026-06-16T22:49:42Z

Specify this is scoped for tonic-xds in PR title for better discoverability.

Adds the request-hash computation that the ring-hash LB picker (A42) will consume, ahead of the picker itself: - HashPolicyConfig::Header type (RDS parsing deferred to a later PR) - RouteDecision.request_hash field - compute_request_hash(): XXH64 seed-0 with Envoy's rotate-XOR combination and terminal short-circuit - resolve_route populates request_hash from a default header policy ("x-ring-hash-key") as scaffolding until RDS hash_policy parsing lands Adds the xxhash-rust dependency. Core-first sequencing: the data plane is built and tested before xDS config wiring.

content-type is hashed correctly by the generic header path (tonic always sets content-type=application/grpc before routing), matching grpc-go, so no special-casing is owed.

Per gRFC A42, a terminal hash policy stops further combination once any hash has been generated, even if the terminal policy itself produced no hash (e.g. its header is absent). The previous code only short-circuited when the terminal policy matched, diverging from Envoy/grpc-go. Move the terminal check out of the matched branch and gate it on the accumulated hash. Add a test covering a terminal policy with an absent header following a matching policy.

…ffer Hash the comma-joined header values straight into a streaming Xxh64 hasher instead of building a concatenated Vec. XXH64 is boundary-independent, so the digest is unchanged (existing pinned-digest tests still pass); this removes the multi-value allocation and unifies the single- and multi-value paths.

madhurishgupta force-pushed the madhurishgupta/a42-pr1-request-hash branch from 45848fb to 5745c41 Compare June 15, 2026 20:19

ankurmittal reviewed Jun 15, 2026

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