Cosmos: Microbenchmark and some optimizations

[analogrelay]

I wrote up a quick microbenchmark of the driver's execute_operation code path, which excludes the I/O entirely by using a fixed Mock Transport (gated behind an internal feature flag). Using that, I was able to get a 40+% improvement in performance (again, these times exclude I/O, but should reflect CPU usage reductions).

Before:

point_read              time:   [16.691 µs 16.734 µs 16.783 µs]

After

point_read              time:   [9.7474 µs 9.7864 µs 9.8319 µs]

Obviously, these are tiny numbers, but I suspect they add up under high concurrency. We won't know for sure until this is run in the benchmarks. They should be safe, so I'm game to merge and see, but we could also try running the tip of this branch in the perf infra (cc @tvaron3 ) to get real results.

The optimizations I worked on were predominantly to reduce duplicate parsing and HashMap::insert/HashMap::get calls. Rather than calling HashMap::get_optional_str(...) for each header when materializing a CosmosResponseHeaders, we now iterate the map. Rather than re-parsing url::Url, the AccountRegion type now directly deserializes the gateway response into an AccountEndpoint, allowing us to reuse the same url::Url for each request. Rather than constantly format!-ing the CosmosResourceReference, we now produce a single ResourcePaths from it and allow getting slices from it for the various purposes.

The benchmarks include instructions for running both profiled and unprofiled runs, and criterion does a great job of tracking results and showing you improvements over time. Having benchmark infra like this in place will be useful as we keep going.

[Copilot]

Pull request overview

This PR adds a Cosmos driver microbenchmark harness (no I/O via a mock transport) and applies several CPU-side optimizations in the driver hot path to reduce repeated parsing/allocations, aligning with the repo’s performance framework direction (Issues #3094 / #3095).

Changes:

• Add a new azure_data_cosmos_benchmarks crate (Criterion + optional pprof/dhat workflows) using an in-memory mock transport via a new internal driver feature flag.
• Optimize the driver request path + signing-link computation by precomputing/borrowing paths (ResourcePaths, Arc<str> container paths) and reusing parsed account endpoints.
• Reduce header processing overhead by iterating headers once and carrying parsed CosmosResponseHeaders through the pipeline to avoid redundant lookups/parsing.

Reviewed changes

Copilot reviewed 29 out of 30 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
sdk/cosmos/azure_data_cosmos_driver/src/testing.rs	Adds a feature-gated internal testing/mocking surface for benchmarks/harnesses.
sdk/cosmos/azure_data_cosmos_driver/src/models/resource_reference.rs	Caches container paths using Arc<str> and switches path accessors to borrowed strings.
sdk/cosmos/azure_data_cosmos_driver/src/models/mod.rs	Renames header-name module export and re-exports ResourcePaths internally.
sdk/cosmos/azure_data_cosmos_driver/src/models/cosmos_resource_reference.rs	Introduces ResourcePaths + compute_paths() to reduce duplicate allocations and adds tests.
sdk/cosmos/azure_data_cosmos_driver/src/models/cosmos_headers.rs	Consolidates header-name constants and optimizes CosmosResponseHeaders::from_headers.
sdk/cosmos/azure_data_cosmos_driver/src/models/account_reference.rs	Adds Deserialize for AccountEndpoint to parse once during account metadata load.
sdk/cosmos/azure_data_cosmos_driver/src/lib.rs	Exposes testing module behind __internal_mocking.
sdk/cosmos/azure_data_cosmos_driver/src/fault_injection/http_client.rs	Updates header handling to the new header-name constants approach.
sdk/cosmos/azure_data_cosmos_driver/src/driver/transport/transport_pipeline.rs	Parses CosmosResponseHeaders once in the transport layer and passes them forward.
sdk/cosmos/azure_data_cosmos_driver/src/driver/transport/http_client_factory.rs	Makes factory/config types public for feature-gated mocking API.
sdk/cosmos/azure_data_cosmos_driver/src/driver/transport/cosmos_transport_client.rs	Makes request/response/transport traits public for feature-gated mocking API.
sdk/cosmos/azure_data_cosmos_driver/src/driver/transport/authorization_policy.rs	Avoids signing-link allocations by carrying ResourcePaths into auth context.
sdk/cosmos/azure_data_cosmos_driver/src/driver/runtime.rs	Allows injecting a custom HTTP client factory under __internal_mocking.
sdk/cosmos/azure_data_cosmos_driver/src/driver/routing/routing_systems.rs	Reuses pre-parsed regional endpoint URLs.
sdk/cosmos/azure_data_cosmos_driver/src/driver/pipeline/retry_evaluation.rs	Adjusts transport result destructuring for new outcome shape.
sdk/cosmos/azure_data_cosmos_driver/src/driver/pipeline/operation_pipeline.rs	Uses compute_paths() and consumes parsed cosmos headers from transport results.
sdk/cosmos/azure_data_cosmos_driver/src/driver/pipeline/components.rs	Carries parsed CosmosResponseHeaders in transport outcomes; raw headers only on errors.
sdk/cosmos/azure_data_cosmos_driver/src/driver/cosmos_driver.rs	Uses pre-parsed AccountEndpoint for write-region selection and updates tests.
sdk/cosmos/azure_data_cosmos_driver/src/driver/cache/account_metadata_cache.rs	Changes AccountRegion.database_account_endpoint from String to AccountEndpoint.
sdk/cosmos/azure_data_cosmos_driver/Cargo.toml	Adds the __internal_mocking feature flag.
sdk/cosmos/azure_data_cosmos_benchmarks/benches/point_read.rs	Adds Criterion point-read microbenchmark with optional pprof profiling.
sdk/cosmos/azure_data_cosmos_benchmarks/benches/heap_profile.rs	Adds dhat-based single-iteration heap profiling bench.
sdk/cosmos/azure_data_cosmos_benchmarks/benches/common.rs	Shared mock transport + driver setup/priming for benchmarks.
sdk/cosmos/azure_data_cosmos_benchmarks/README.md	Documents how to run benchmarks and generate profiles.
sdk/cosmos/azure_data_cosmos_benchmarks/Cargo.toml	New (non-published) benchmarks crate definition and dev-dependencies.
sdk/cosmos/azure_data_cosmos_benchmarks/.gitignore	Ignores dhat output JSON.
sdk/cosmos/azure_data_cosmos/src/models/cosmos_response.rs	Adds an internal constructor that accepts pre-parsed headers from the driver bridge.
sdk/cosmos/azure_data_cosmos/src/driver_bridge.rs	Avoids redundant header parsing by passing pre-parsed headers through.
Cargo.toml	Adds benchmarks crate to the workspace and adds workspace deps for dhat/pprof.
Cargo.lock	Locks new benchmark-related dependencies.

Comments suppressed due to low confidence (1)

sdk/cosmos/azure_data_cosmos_driver/src/driver/pipeline/operation_pipeline.rs:449

• normalized clones request_path via to_string() in the common case where it already starts with '/'. Since compute_paths() always builds non-empty paths with a leading '/', this adds an extra allocation on the hot path. Consider passing request_path directly to Url::set_path() (or using Cow<'_, str> to allocate only when a leading '/' needs to be added).

        let request_path = paths.request_path();
        let normalized = if request_path.starts_with('/') {
            request_path.to_string()
        } else if request_path.is_empty() {
            String::new()
        } else {
            format!("/{}", request_path)
        };
        base.set_path(&normalized);

[heaths]

Approved for files outside sdk/cosmos. I had some questions/(slight) concerns under sdk/cosmos.

[FabianMeiswinkel]

LGTM

[FabianMeiswinkel]

LGTM

[tvaron3]

LGTM

[FabianMeiswinkel]

cspell - either we disable it everywhere (except maybe md files) or we enforce cspell and whitelist exceptions - but disabling cspell for entire files defeats the purpose of having it (silently accidental regressions in those files will happen)

[FabianMeiswinkel]

LGTM