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This page maps every real module under src/ to what it does. It is a companion to overview.md, which covers the request flow, the redirect-based delivery trade-off, and the two required diagrams. Every module and file path below was checked against the code on main, not assumed from names.

Entry point

src/main.rs

Builds the Tokio runtime by hand (not #[tokio::main]) so worker_threads can be a runtime value read from TOKIO_WORKER_THREADS, cgroup-aware via modules::utils::cgroup::effective_cpu_count rather than num_cpus::get() directly (a cgroup-limited container otherwise oversizes the runtime for the host's full core count). Sets mimalloc::MiMalloc as the global allocator. Installs a SIGTERM/SIGINT-triggered graceful shutdown with a configurable drain deadline (SHUTDOWN_TIMEOUT_SECS, default 20s — comfortably under Kubernetes' default 30s terminationGracePeriodSeconds).

src/bin/healthcheck.rs, src/bin/benchmark.rs

Two extra binaries: a standalone healthcheck probe, and a load-generating benchmark client (serves fixture images locally and drives the resize endpoint against them — see .bench-baseline/BASELINE.md for how its output feeds the performance numbers cited in overview.md).

Router and middleware — src/modules/router/

  • router.rs — hand-written route table (build_app): GET /health (unauthenticated, Kubernetes probe target), GET /api/images/files/{key} (unsigned download route), GET /{signature}/{*rest} (the imgproxy-compatible signed resize route, mounted at the root — axum/matchit prefers a literal path segment like api/health over this route's dynamic first segment, so it never shadows the others), and, only when built with --features otel, GET /metrics behind metrics_auth::require_metrics_auth. There is no gen_server-generated router anywhere in this file or this codebase — see "Removed" below.
  • middlewares.rs — MiddlewareConfig (env-driven: REQUEST_TIMEOUT_SECS, MAX_CONCURRENT_REQUESTS, RATE_LIMIT_BURST, RATE_LIMIT_PERIOD_MS) plus apply_common_middlewares, which layers, outer to inner: CORS, tower_governor's per-IP token-bucket GovernorLayer (429 on excess — its per-key state is pruned every 60s via limiter.retain_recent() so it doesn't grow unbounded over the process lifetime), a conditional- GET middleware for the download route (ETag/If-None-Match/304, computed directly from the requested key without touching storage), a Semaphore-backed saturation/timeout guard (503 on either), and tower-http's CompressionLayer (br/deflate/gzip/zstd) closest to the app.

API handlers — src/modules/api/

  • handler.rs — ApiService, the shared application state: resize_service, signing, presets/allowed_options (issue #52), and (under otel) metrics_auth. ApiService::create builds every sub-config from EnvConfig and fails closed at startup for signing, /metrics auth, and preset parsing — a misconfiguration is a boot-time error, not a per-request surprise.
  • resize.rs — GET /{signature}/{processing_options}/{plain|base64 source}.{extension}. Splits the path (modules::url::split), verifies the signature, parses options+source (SignedRequest::parse_with_config), resolves .auto via modules::negotiation::resolve, calls ResizeService::resize, and builds an explicit 301 (not axum's Redirect::permanent, which issues 308) with Cache-Control: public, max-age=31536000, immutable and, only for a negotiated .auto request, Vary: Accept.
  • download.rs — GET /api/images/files/{key}, deliberately unsigned (this route only ever serves bytes the resize route already produced and cached under a content-derived key; the strict key grammar in services::storage::key_validation is the guard here, not a signature). Derives Content-Type from the key's own extension.

URL grammar and signing

src/modules/url/

Implements the imgproxy-compatible signed-URL grammar /{signature}/{processing_options}/{plain|base64 source}.{extension}:

  • mod.rs — split (cheap extraction of the signature segment and the exact signed byte string, run before any full parse so an unauthenticated caller can't use parse-error content as an oracle) and SignedRequest::parse/parse_with_config (full options+source grammar, presets and the processing-option allowlist applied ahead of the plain grammar parse).
  • options.rs — ProcessingOptions, parsing /-delimited code:arg1:arg2 segments (rs, q, bl, g, el, bg, ar, sm (strip-metadata, default true — issue #5), fq, webpo, jpgo/mb (JPEG progressive/chroma-subsampling tuning and max-bytes search — issue #76), crop/gravity, rotate/flip/trim/extend/padding/zoom/dpr/min-width/min-height, watermark, preset references).
  • presets.rs — PresetRegistry (parses the PRESETS env var: named, reusable option-segment lists, imgproxy-compatible; a preset named default is auto-prepended to every request) and AllowedOptions (parses ALLOWED_PROCESSING_OPTIONS, an allowlist of which option codes a deployment permits at the top level of a URL).
  • source.rs — SourceSpec/parse_source: either plain/{percent-encoded URL}.{extension} or a single base64url-encoded segment, with the trailing .{extension} mandatory and mapped to ImageFormat (including the .auto negotiation trigger).

src/modules/signing/

  • verify.rs — compute_signature/verify_signature/sign: HMAC-SHA256(key, salt || signed_path), base64url (no padding), constant-time comparison via the subtle crate.
  • config.rs — SigningConfig::from_env, which fails closed at startup (issue #27): a deployment must either configure a real key/salt or explicitly set ALLOW_UNSIGNED_REQUESTS=true to use the /unsigned/... escape hatch.

src/modules/negotiation.rs

resolve(format, accept) — resolves ImageFormat::Auto against the request's Accept header, preferring AVIF, then WebP, falling back to JPEG, weighted by each entry's q parameter. Any non-Auto format passes through unchanged with negotiated = false.

src/modules/metrics_auth/

Bearer-token authentication for GET /metrics (issue #77), gated behind the otel feature since that's the only build where /metrics is ever mounted at all. MetricsAuthConfig::from_env fails closed at startup, mirroring SigningConfig.

Services — src/services/

resize/handler.rs — ResizeService

The orchestrator tying cache, image processing, and storage together:

  • resize(params) — generates the cache key, checks the cache, and on a miss runs single-flight coalescing before calling do_resize_work. See overview.md's "Concurrency model" and its cache-miss sequence diagram for the full mechanics of InFlightMap/InFlightGuard (issue #37).
  • do_resize_work — the actual download → process → upload pipeline for a confirmed miss.
  • download(params) — serves the unsigned /api/images/files/{key} route by reading directly from storage; a missing/expired key surfaces as a "not found" message that AppError::classify_download_error maps to 404.
  • resize_batch — bounded-concurrency batch resizing over futures::stream::buffer_unordered.

image/handler.rs — ImageService

Owns the two semaphores (download_semaphore, processing_semaphore), the SSRF-guarded fetch path, and the decode/resize/encode pipeline:

  • fetch_validated/download_image — see overview.md's "Security boundary" section for the SSRF guard itself (implemented in source_guard.rs, described next). download_image also enforces the streaming size cap.
  • process_image — acquires processing_semaphore (non-blocking try_acquire_owned, 503 on exhaustion) and runs decode/resize/encode on spawn_blocking, with a tx.is_closed() check to skip queued-but-not-yet-started work when the caller has already disconnected.
  • process_image_blocking_with_limits* — header-only dimension peek and resolution check (decompression-bomb guard) before any full decode; EXIF autorotate, trim, and explicit crop are applied in that order (matching imgproxy's own pipeline ordering) before the actual resize; animated GIF/WebP sources are detected and routed to encode_animation separately.
  • decode_with_limits — dispatches by source format, each with a fallback to the plain image-crate decoder (decode_with_image_crate) on failure:
  • JPEG — decode_jpeg_scaled (mozjpeg/libjpeg-turbo, DCT-scaled when a resize makes a smaller decode safe, full-size otherwise; issue #67).
  • WebP — decode_webp_libwebp (real libwebp via FFI, issue #66) — replaces the image crate's pure-Rust image-webp decoder.
  • AVIF — avif_codec::decode (libavif/dav1d, issue #67/#68) — the first release where AVIF is accepted as a source format at all; previously any .avif source was rejected outright.
  • PNG/GIF — always decode_with_image_crate (the image crate's own decoders); untouched by the above.
  • encode_with_max_bytes — binary-searches JPEG quality down until encoded output fits a requested max_bytes budget (issue #76), bounded to a fixed number of extra encode attempts.

image/avif_codec.rs

AVIF encode and decode via libavif, both directions (issue #67/#68): encode (AOM backend, DEFAULT_AVIF_SPEED = 6) and decode/peek_dimensions (dav1d backend, the latter re-running the same megapixel-overflow-checked resolution guard ImageService::check_source_resolution does, as defense in depth around libavif's own header parse). Replaces the pure-Rust ravif/rav1e encoder and adds AVIF source decode, which this service previously had no path for at all.

image/source_guard.rs

Pure, deterministic SSRF validation logic (the one exception is resolve_validated_addr, which performs the actual DNS lookup): validate_scheme, is_allowed_source/matches_allowed_prefix, is_blocked_ip_with_policy (and its IPv4/IPv6 halves), and IPv4-literal decoding covering decimal/octal/hex smuggling forms. See overview.md's "Security boundary" for the full threat model this covers and why redirect revalidation and DNS pinning are both necessary.

cache/handler.rs — CacheService

generate_key(params) — the SHA-256, length-prefixed, versioned cache key described in overview.md's "Cache key design". CACHE_KEY_VERSION is currently 11; its doc comment on this file is the authoritative history of what each version bump added — including v10 (metadata-strip default, issue #5), v11 (the AVIF encoder cutover to libavif/AOM) — and, in one case (issue #67's WebP decoder swap to libwebp), why a bump was deliberately not taken.

storage/

  • core.rs — the StorageBackend trait: upload_image/upload_image_with_ttl, check_cache, get_image, delete. ttl: None means "never expires."
  • handler.rs — StorageService/StorageConfig/StorageType, selecting a backend by Cargo feature and (when more than one is compiled in) STORAGE_TYPE. Also enforces that a configured key_prefix (from STORAGE_SUB_PATH) matches what CacheService::generate_key actually produces.
  • s3_handler.rs (feature s3) — S3/MinIO-compatible backend via aws-sdk-s3.
  • local_fs_handler.rs (feature local_fs) — local-directory backend.
  • in_memory_handler.rs (feature in_memory, test-only) — unbounded HashMap-backed backend, #[cfg(all(test, feature = "in_memory"))] — does not exist in a release build at all, regardless of the Cargo feature flag; selecting it via STORAGE_TYPE outside tests fails fast at startup instead of running an uncapped cache in production.
  • key_validation.rs — validate_cache_key, the strict grammar (<prefix><64 lowercase hex>.<jpg|png|webp|avif|gif>) every backend is checked against before a key ever reaches it, closing path-traversal and S3 IDOR in one place rather than per backend.

health/handler.rs

health() — returns the literal string "OK". Deliberately unauthenticated; see overview.md's "Security boundary" for why.

metrics/handler.rs

metrics_handler() — encodes the global Prometheus registry (prometheus::gather()) as text. Only compiled/mounted under the otel feature; protected by modules::metrics_auth at the router layer, not in this handler itself.

Models — src/models/params.rs

ResizeQuery (the fully-parsed representation of a resize request — url, width/height, ResizeType, ImageFormat, quality/format-quality overrides, crop/gravity, geometry operations, watermark, etc.) and DownloadPathParams ({ key: String }, hand-written — see "Removed" below).

Configuration

  • src/modules/env/env.rs — EnvConfig, the envconfig-derived top-level environment binding (HTTP host/port, storage credentials, MAX_IMAGE_SIZE_MB, MAX_SRC_RESOLUTION_MP, ALLOWED_SOURCES, ALLOW_LOOPBACK_SOURCE_ADDRESSES/ALLOW_LINK_LOCAL_SOURCE_ADDRESSES, max_redirects, presets/allowlist strings, etc.).
  • src/config/performance.rs — PerformanceConfig, derived from EnvConfig plus a small set of named default profiles (default/high-throughput/low-latency/memory-constrained), each setting max_image_size, max_src_resolution_mp, max_redirects, download/processing concurrency limits, and the SSRF-guard override flags.

Utility modules — src/modules/utils/

  • date.rs — a minimal hand-rolled RFC 7231 IMF-fixdate formatter (no date/time crate is a dependency of this crate), used for the download route's Last-Modified header.
  • etag.rs — if_none_match_satisfied, RFC 7232 §3.2 strong-comparison matching against the single server-computed ETag the conditional-download middleware produces.
  • err.rs — AppError and its classify_download_error/classify_resize_error constructors, mapping internal failures to HTTP status codes (see overview.md's rejection-state diagram for the concrete mapping). Every variant renders with Cache-Control: no-store.
  • cgroup.rs — effective_cpu_count, reading cgroup v2 cpu.max or cgroup v1 cpu.cfs_quota_us/cpu.cfs_period_us directly (falling back to num_cpus::get()), used by main.rs to size the Tokio runtime.

Tracing — src/modules/tracer/

OpenTelemetry tracing/metrics initialization (init.rs), compiled only under the otel feature.

Removed since the last documentation pass

The following existed in an earlier version of this service and no longer do. They are recorded here only as history, not as current architecture:

  • The generated OpenAPI server (gen-server, a packages/ directory, openapi.yaml). Issue #53 replaced it with the hand-written router in src/modules/router/router.rs and the hand-written handlers in src/modules/api/. Verified: grep -rc 'gen-server\|gen_server\|openapi' Cargo.toml and find . -iname 'openapi.yaml' both return nothing in this tree.
  • rayon — the old CPU-bound processing pool. Replaced by tokio::task::spawn_blocking bounded by processing_semaphore; see image/handler.rs's doc comment on process_image for why rayon's intra-job work-stealing was never actually used. Not a dependency in Cargo.toml.
  • o2o, lru, axum-extra — none appear in Cargo.toml's dependency list; no code under src/ references them.