battery-pack-rs/battery-pack/service-architecture

Service Architecture

How a request flows through this service, why the middleware stack is ordered the way it is, and how to scale it. The stack lives in src/routes.rs::router.

Why axum, and when to drop to hyper

The service template uses axum: its extractors and Router make handlers and middleware cheap to write, at a small per-request cost from cloning state and running extractors. Drop to raw hyper (the hyper-util examples) only when you need what axum hides: custom connection lifecycle, protocol upgrades, or the last allocation off a hot path.

The layer stack (read this before reordering it)

Layers wrap the router, so the last one added is the outermost and a request flows through them top down. The order is deliberate. Several layers are optional features, so a given service may not have all of them; the list shows the full stack with every feature enabled:

1. SetRequestId / PropagateRequestId (outermost): honor an incoming x-request-id or mint one, and echo it back. It runs first so everything inside has a correlation id.
2. TraceLayer: opens the request span carrying that id, so all handler logs correlate.
3. telemetry_middleware: opens the wide-event metric. It is the outermost status recorder, so it captures the final status even when an inner layer (not the handler) produced it.
4. rate limit (GovernorLayer) (when enabled): inside the recorder, so a rejected 429 is still counted as a request.
5. CatchPanic (when enabled): converts a handler panic into a 500 inside the recorder, so the 500 is recorded and the panic does not unwind through the metric guard.
6. OnEarlyDrop (when enabled): diagnostic only, logs a warning if the response future or body is dropped before completion. It does not alter the status.
7. timeout / body-timeout (when enabled): bound total request time (408) and slow-body reads.
8. DefaultBodyLimit (innermost): rejects oversized bodies with 413 before they are buffered into memory.

Anything that can produce a status (rate limit, panic, timeout, body limit) sits inside telemetry_middleware so the status is recorded, while request-id and tracing sit outside it so the id and span exist when it runs. /health is mounted on a separate router with none of this, so probes are not metered or rate-limited.

Scaling the rate limit

The service template ships a single global token bucket (GlobalKeyExtractor). To limit per client instead:

• Swap GlobalKeyExtractor for PeerIpKeyExtractor.
• Serve with into_make_service_with_connect_info::<SocketAddr>() so the extractor can read the peer address from request extensions (tests then need a SocketAddr injected).
• Run a periodic governor_conf.limiter().retain_recent() task. The keyed store grows one entry per key and is never auto-evicted, so without this it leaks memory under a wide key space.

No new dependency or feature is needed: PeerIpKeyExtractor ships with the tower_governor the rate-limit feature already pulls, so this is a code change.

Behind a load balancer the peer IP is the balancer, so key off a trusted forwarded-for header instead of the socket address.

Optional additions

The service template deliberately leaves these out (workload-specific, easy to misuse). Add each with cargo bp add backend-service -F <feature>, then wire it as below.

• Read caching (cargo bp add backend-service -F cache, which adds moka): front the store's reads with its future::Cache. A cache changes the service's consistency contract; its failure mode is silent stale reads. Size the TTL against an explicit staleness budget.
• Load shedding (cargo bp add backend-service -F load-shed, which adds the tower layers): use ConcurrencyLimitLayer paired with LoadShedLayer, do not hand-roll a counter check. The pair caps concurrent requests and sheds the overflow with a 503; a bare concurrency limit without LoadShedLayer queues the overflow without bound. Use the always-on IN_FLIGHT metric only to size the cap from observed concurrency, not as the shedding mechanism. Place it inside the recorder so the 503 is counted, but outside the rate limit and timeout.

Invariants

• Do not move a status-producing layer (rate limit, catch-panic, timeout, body limit) outside telemetry_middleware, or that status stops being recorded.
• Keep request-id and tracing outside telemetry_middleware; it reads the id they set.
• Keep DefaultBodyLimit innermost so oversized bodies are rejected before buffering.
• A new route needs a matching arm in classify_operation (src/middleware.rs) or it records as Other.
• /health stays on the bypass router: no metrics, no rate limit.
• Pull the pack's optional features with cargo bp add backend-service -F <feature>, not a plain cargo add, so dependency versions stay the ones the pack pins.

References

• axum: https://docs.rs/axum, hyper-util examples: https://github.com/hyperium/hyper-util
• tower_governor: https://docs.rs/tower_governor, moka: https://docs.rs/moka, tower (load-shed, concurrency-limit): https://docs.rs/tower
• Observability of the stack: see the telemetry skill.