[Endpoints BDD 5/5] Add a result cache to the BDD endpoint provider

[alextwoods]

Summary

Adds a single-entry result cache to the BDD-generated endpoint provider. A client resolving
endpoints for the same configuration and request shape repeatedly re-walks the decision diagram
and rebuilds an identical Endpoint on every request. The provider now holds the last
(params, endpoint) pair and returns the cached endpoint when the incoming params still match,
turning a 27–346 ns resolution into a 1–5 ns comparison.

The cache is scoped to the BDD provider only. The rules2-generated provider is unchanged.

Caching an endpoint is only safe if the key covers everything that can change the answer, so most
of the change is in deriving that key at codegen time rather than in the cache itself. However, optimization to make the cache actually worthwhile involves limiting the cache key to as small a set as possible and optimizing the order to reduce the cost of cache misses.

The cache key

cacheParamsMatch is a flat Objects.equals chain, one term per parameter. Objects.equals tries identity before equals, so a parameter whose reference is stable across requests (a Region, a client context param) settles on the identity check, while one that arrives fresh falls through and still matches by value.

Only parameters the BDD actually references are used. A new codegen class, BddParameterReferences, walks the BDD's conditions and results with the existing expression parser and classifies each declared parameter as unreferenced, first-element-only, or fully read. Unreferenced parameters cannot affect the endpoint and are omitted. This matters most on S3, where Key, Prefix and CopySource are declared but read by zero rules — and Key changes on nearly every request. 14 of 17 S3 parameters are in the key.

List parameters are compared in bounded time. When the BDD only ever reads element 0 of a stringArray (DynamoDB's ResourceArnList), the key compares presence plus that one element and ignores the tail. Otherwise elements are compared pairwise up to a cap of 8, above which the provider reports a miss and resolves — List.equals is unbounded while resolution is largely indifferent to list length, so an uncapped key check could cost more than the work it avoids.

Terms are ordered booleans, then reference-stable strings, then the rest, each in declaration order. This only affects how quickly a mismatch is found; the chain compares every keyed parameter before returning true, so ordering cannot affect correctness.

Benchmarks

Service	Params declared	In cache key	Cache hit	Cache miss	Resolution avoided on a hit	Break-even hit rate
Standard regional (Connect)	4	4	1.1 ns	1.7 ns	27–56 ns (96%)	6%
DynamoDB	9	9	4.6 ns	3.4 ns	28–70 ns (84%)	13%
DynamoDB (batch, ARN list set)	9	9	5.4 ns	4.3 ns	28–70 ns (81%)	16%
S3	17	14	4.9 ns	4.5 ns	82–346 ns (94%)	6%

Changes affecting the non-BDD rules path

Three changes are not BDD-specific. They remove per-request allocations from the endpoint path and apply for all existing rules based services. Two exist because the cache benefits from stable references to compare cheaply, but each stands on its own as an allocation removal.

ClientEndpointProvider.sanitizedEndpointString() — new @SdkProtectedApi default method returning the client endpoint with query and user-info stripped, which is what the rules engine receives as SDK::Endpoint. AwsEndpointProviderUtils.endpointBuiltIn() now delegates to it instead of constructing a URI and converting it to a string on every request. StaticClientEndpointProvider overrides it with a value computed once at construction, so any client with an endpoint override drops a URI construction plus a toString per request and returns a stable reference.

AccountIdEndpointMode.endpointModeValue() — new method on this @SdkPublicApi enum returning a per-constant interned literal. Generated code called mode.name().toLowerCase(), allocating a fresh string per request for a value that has three possible results.

EndpointResolverUtilsSpec** static list hoisting** — staticContextParams array values are emitted as private static final List<String> constants (STATIC_LIST_{OP}_{PARAM}) instead of being rebuilt inline, so setStaticContextParams hands the builder a shared immutable list rather than an equal new one per request.

License

• I confirm that this pull request can be released under the Apache 2 license