alknet/tasks/http/client/shared-http-client.md

id, name, status, depends_on, scope, risk, impact, level

id	name	status	depends_on	scope	risk	impact	level
http/client/shared-http-client	Implement shared HTTP client (ClientWithMiddleware + retry + Retry-After, OQ-40)	completed	http/crate-init	narrow	low	component	implementation

Description

Implement the shared HTTP client in src/client/http_client.rs. This is the reqwest_middleware::ClientWithMiddleware used by all from_openapi/from_mcp forwarding handlers. The client owns connection pooling, keep-alive, TLS, and a retry stack. It is constructed once and reused across all forwarding handlers; credential injection happens per-request (from OperationContext.capabilities), not at client construction — the client is shared across all operations, the credentials are per-call.

The middleware stack (OQ-40 resolved)

The shared type is reqwest_middleware::ClientWithMiddleware, not a bare reqwest::Client — both retry and Retry-After are middleware on the stack, and middleware requires the ClientWithMiddleware wrapper. The stack has two layers:

1. RetryTransientMiddleware (from reqwest-retry) — exponential backoff on transient failures (connection errors, 5xx). The "retry N times with increasing intervals" part. Configured via an ExponentialBackoff policy at client construction.

2. Inlined RetryAfterMiddleware — parses the Retry-After header on 429/503 and sleeps before the next request to that URL. The "respect what the server told you" part. Inlined (MIT, ~50 lines of real logic) from melotic/reqwest-retry-after, not pulled as a dependency: the crate is complementary to reqwest-retry (whose default strategy does not honor Retry-After), and inlining lets the upstream's unbounded HashMap<Url, SystemTime> storage be bounded for a long-running process.

Pooling, keep-alive, TLS

Pooling, keep-alive, and TLS come from reqwest::ClientBuilder defaults; outbound TLS uses the system trust store (standard HTTPS to external APIs like OpenAI, Anthropic). Custom CA bundle + client certs are an optional config for self-hosted API gateways (two-way-door implementation detail; the credential comes from Capabilities, the TLS trust comes from the system).

Hot-reload (rebuild-and-swap)

Hot-reload of the pooling/retry config is rebuild-and-swap: a config change rebuilds the ClientWithMiddleware and swaps it via ArcSwap (the same pattern ConfigIdentityProvider uses, ADR-035). A rebuild drops the connection pool / keep-alive state, which is acceptable — a config change wanting a fresh pool is the case that triggers it. The retry policy is baked into the middleware at ClientBuilder::build() time; live policy mutation is not supported by reqwest-retry, so cheap per-policy updates are not part of the model.

API

/// Configuration for the shared HTTP client (two-way-door, OQ-40 resolved).
pub struct HttpClientConfig {
    /// Pool max idle connections per host (reqwest default if None).
    pub pool_max_idle_per_host: Option<usize>,
    /// Request timeout (reqwest default if None).
    pub request_timeout: Option<Duration>,
    /// Retry policy for transient failures (ExponentialBackoff).
    pub retry_policy: ExponentialBackoff,
    /// Custom CA bundle path for self-hosted API gateways (optional).
    pub ca_bundle: Option<PathBuf>,
    /// Client cert for mutual TLS (optional, from Capabilities at call time,
    /// not here — this is the trust config, not the credential).
    pub client_cert: Option<ClientCertConfig>,
}

/// The shared HTTP client. Constructed once, reused across all forwarding
/// handlers. Wrapped in `ArcSwap` for rebuild-and-swap hot-reload.
pub struct SharedHttpClient {
    inner: ArcSwap<ClientWithMiddleware>,
    config: ArcSwap<HttpClientConfig>,
}

impl SharedHttpClient {
    pub fn new(config: HttpClientConfig) -> Self { ... }

    /// Get the current client (for forwarding handlers to use).
    pub fn client(&self) -> Arc<ClientWithMiddleware> {
        self.inner.load_full()
    }

    /// Rebuild the client with new config (rebuild-and-swap hot-reload).
    pub fn reload(&self, config: HttpClientConfig) { ... }
}

The inlined RetryAfterMiddleware

The inlined middleware is ~50 lines: a bounded HashMap<Url, SystemTime> holding per-URL retry-after deadlines, a before hook that sleeps if the deadline is in the future, and an after hook that parses the Retry-After header on 429/503 and records the deadline. The bound (e.g., LRU eviction at N entries) prevents unbounded growth in a long-running process — the upstream's unbounded HashMap<Url, SystemTime> is the reason it's inlined rather than depended on.

Downstream layering boundary

The agent crate's provider SSE normalization sits on top of this ClientWithMiddleware: it consumes the reqwest::Response stream the forwarding handler produces and emits call.responded events. It does not replace the client or own transport/pooling/retry. alknet-http owns transport; the agent crate owns provider-specific SSE → Vercel-UI-message mapping. The aisdk core/client.rs reference for HTTP client construction is not carried forward — its env-var config and hand-rolled retry are the anti-patterns discarded in favor of the middleware stack above.

Acceptance Criteria

• SharedHttpClient struct in src/client/http_client.rs
• Wraps ArcSwap<ClientWithMiddleware> for rebuild-and-swap
• HttpClientConfig with pool, timeout, retry policy, optional CA bundle
• RetryTransientMiddleware (from reqwest-retry) in the middleware stack
• Inlined RetryAfterMiddleware (~50 lines, bounded HashMap<Url, SystemTime>)
• RetryAfterMiddleware parses Retry-After header on 429/503
• RetryAfterMiddleware sleeps before next request to a URL with an active deadline
• RetryAfterMiddleware storage is bounded (LRU eviction or equivalent)
• Pooling/keep-alive/TLS from reqwest::ClientBuilder defaults
• System trust store for outbound TLS by default
• Custom CA bundle optional (self-hosted API gateways)
• reload(config) rebuilds and swaps via ArcSwap
• Credential injection is per-request (NOT at client construction)
• No std::env::var reads (no-env-vars invariant — ADR-014)
• No shared global client (alknet-http owns its client)
• Unit test: client() returns a usable ClientWithMiddleware
• Unit test: reload() swaps the client (new client returned by client())
• Unit test: RetryAfterMiddleware records deadline from Retry-After: <seconds>
• Unit test: RetryAfterMiddleware records deadline from Retry-After: <HTTP-date>
• Unit test: bounded storage evicts old entries (no unbounded growth)
• cargo test -p alknet-http succeeds
• cargo clippy -p alknet-http --all-targets succeeds with no warnings

References

• docs/architecture/crates/http/http-adapters.md — HTTP client (§"HTTP client (reqwest)")
• docs/architecture/crates/http/overview.md — OQ-40 resolved (ClientWithMiddleware + middleware stack)
• docs/architecture/decisions/014-secret-material-flow-and-capability-injection.md — ADR-014 (no env vars)
• https://docs.rs/reqwest-retry/ — RetryTransientMiddleware / ExponentialBackoff
• https://github.com/melotic/reqwest-retry-after — RetryAfterMiddleware source (MIT, inlined, not a dependency)

Notes

The shared HTTP client is the no-env-vars-compliant outbound transport. The middleware stack (RetryTransientMiddleware + inlined RetryAfterMiddleware) is the resolved shape (OQ-40). The RetryAfterMiddleware is inlined (not a dependency) so its storage can be bounded for a long-running process — the upstream's unbounded HashMap is the reason. Hot-reload is rebuild-and-swap via ArcSwap (same pattern as ConfigIdentityProvider, ADR-035). Credential injection is per-request (from OperationContext.capabilities), not at client construction — the client is shared, the credentials are per-call. The agent crate's SSE normalization sits on top of this client; it does not replace it.

Summary

Implemented SharedHttpClient (ArcSwap) with HttpClientConfig (pool/timeout/retry/optional CA bundle+client cert), RetryTransientMiddleware from reqwest-retry, and inlined RetryAfterMiddleware (~90 lines, bounded HashMap with LRU eviction by earliest deadline, parses Retry-After seconds + HTTP-date, sleeps before next request on 429/503). reload() rebuilds and swaps via ArcSwap. No env-var reads; per-request credential injection only. Added arc-swap, httpdate, http, url deps + rustls TLS feature to reqwest. 24 unit tests. Build/clippy/test all clean.