headroom/docs/observability.md

Observability — proxy metrics

The Headroom Rust proxy exposes Prometheus-format metrics on the /metrics endpoint of every running proxy instance. The metric catalogue below covers Phase D (Bedrock route instrumentation) and Phase G PR-G3 (proxy-wide observability).

All metric names + label keys are constants in crates/headroom-proxy/src/observability/metric_names.rs, so any rename catches one file in code review.

Metric catalogue

Bedrock route (Phase D PR-D3)

Name	Type	Labels	Purpose
bedrock_invoke_count_total	Counter	model, region, auth_mode	One increment per Bedrock /invoke or /converse request.
bedrock_invoke_latency_seconds	Histogram	model, region	Latency from proxy entry to upstream completion. Buckets target 50ms–60s.
bedrock_eventstream_message_count_total	Counter	model, region, event_type	One increment per parsed binary EventStream message.

Proxy-wide (Phase G PR-G3)

Cache + compression

Name	Type	Labels	Purpose
proxy_cache_hit_rate_per_session	Histogram	provider	Per-session cache hit rate. Phase H canary gate.
proxy_compression_ratio_by_strategy	Histogram	strategy, content_type	compressed_tokens / original_tokens per shrunk block.
proxy_compression_rejected_by_token_check_total	Counter	strategy	Compressor ran but failed the shrink check.

Cache-safety alarm

Name	Type	Labels	Purpose
proxy_passthrough_bytes_modified_total	Counter	path	Bytes mutated on a passthrough path. Must stay 0 outside the compression hot path — any non-zero rate fires the cache-safety alarm.

The alarm metric is wired in crates/headroom-proxy/src/proxy.rs: when the dispatcher returns Outcome::NoCompression or Outcome::Passthrough, the post-dispatcher byte length is compared to the original buffered length and any delta increments the counter (by the byte delta) under the request's path label. The PR-E4 prompt_cache_key injector runs AFTER the alarm check, so its intentional byte mutations do not trip the alarm.

Upstream rate limits

Name	Type	Labels	Purpose
proxy_rate_limit_remaining_requests	Gauge	provider	Last-seen remaining requests in the current window.
proxy_rate_limit_remaining_tokens	Gauge	provider	Last-seen remaining tokens in the current window.
proxy_rate_limit_remaining_input_tokens	Gauge	provider	Anthropic-only input-token bucket.
proxy_rate_limit_remaining_output_tokens	Gauge	provider	Anthropic-only output-token bucket.

OpenAI Responses telemetry

Name	Type	Labels	Purpose
proxy_service_tier_count_total	Counter	tier	Service-tier distribution observed at the proxy.
proxy_response_status_count_total	Counter	status	Terminal status distribution (completed, incomplete, failed, cancelled, in_progress).

Image log redaction (Python-side)

Name	Type	Labels	Purpose
proxy_image_generation_call_log_redacted_total	Counter	none	Base64-encoded image payloads redacted from request logs. Driven from headroom.proxy.request_logger.redactions_total().

C3 remediation: Image redaction is purely a Python-proxy operation (the request logger walks JSON and replaces over- threshold image payloads with placeholders). The counter lives Python-side so we have one source of truth instead of two. The Rust proxy previously held a dead counter for this metric; that has been removed.

How to query

The proxy renders Prometheus text-format on GET /metrics:

curl -s http://127.0.0.1:8787/metrics

Phase H canary gate

The canary script that decides "ship Rust, retire Python" uses all four of these queries against proxy_cache_hit_rate_per_session to confirm parity vs the Python baseline. A single percentile is not enough — a regression that only shows up at the tail (a small class of long sessions losing cache hits) would slip through a median-only check.

# p50, p95, p99 of cache hit rate over the last 5 minutes, per provider.
histogram_quantile(0.50, sum by (provider, le) (rate(proxy_cache_hit_rate_per_session_bucket{provider!="__init__"}[5m])))
histogram_quantile(0.95, sum by (provider, le) (rate(proxy_cache_hit_rate_per_session_bucket{provider!="__init__"}[5m])))
histogram_quantile(0.99, sum by (provider, le) (rate(proxy_cache_hit_rate_per_session_bucket{provider!="__init__"}[5m])))

# Mean cache hit rate over the last 5 minutes, per provider. The
# `sum / count` form is the cleanest "average without a quantile"
# query and is what the Python baseline reports.
sum by (provider) (rate(proxy_cache_hit_rate_per_session_sum{provider!="__init__"}[5m]))
  /
sum by (provider) (rate(proxy_cache_hit_rate_per_session_count{provider!="__init__"}[5m]))

The canary fails if ANY of p50, p95, p99, or mean regresses below the Python baseline for any provider over the canary window.

Other common queries

# Cache-safety alarm. Should always be 0 (post-`__init__` row).
sum(rate(proxy_passthrough_bytes_modified_total{path!="__init__"}[5m]))

# Per-strategy compression value at p50 (post-H1 fix: each strategy
# reports its own before/after; pre-fix this was the same aggregate
# ratio repeated per strategy).
histogram_quantile(0.50, sum by (strategy, le) (rate(proxy_compression_ratio_by_strategy_bucket{strategy!="__init__"}[1h])))

# Per-strategy compression value at p95 and p99 (catch outlier
# strategies that fail to shrink at the tail).
histogram_quantile(0.95, sum by (strategy, le) (rate(proxy_compression_ratio_by_strategy_bucket{strategy!="__init__"}[1h])))
histogram_quantile(0.99, sum by (strategy, le) (rate(proxy_compression_ratio_by_strategy_bucket{strategy!="__init__"}[1h])))

# Strategies that ran but failed the token-check (compressor ran
# but its output was not strictly smaller, so the original was
# kept). High rate here means the compressor needs tuning.
sum by (strategy) (rate(proxy_compression_rejected_by_token_check_total{strategy!="__init__"}[1h]))

# Upstream rate-limit headroom (smaller = closer to throttle).
proxy_rate_limit_remaining_tokens{provider="anthropic"}

# Image-redaction rate (Python-side).
rate(proxy_image_generation_call_log_redacted_total[5m])

All queries above include a {... != "__init__"} filter so the sentinel zero-rows the boot-touch contract emits do not skew the result. See "Wiring → H3 force-zero" below.

Wiring

Every metric registration is OnceLock-backed and lazy: the first call to a *_counter() / *_gauge() / *_histogram() helper registers the family with the shared registry. handle_metrics force-touches every Phase G PR-G3 family before scraping.

H3 force-zero

The prometheus crate v0.13 skips empty MetricVecs from gather() entirely — neither HELP/TYPE lines nor rows appear until the family has been incremented at least once with a label tuple. Operators expect to see the catalogue from boot, so handle_metrics increments each counter / gauge MetricVec by 0 under a sentinel __init__ label tuple before the first scrape. HELP/TYPE then surface from boot and dashboards/alarms see a predictable scrape shape.

Counters with the __init__ label increment by 0, so the alarm-able "must stay 0" semantic of proxy_passthrough_bytes_modified_total is preserved (the family becomes visible, the rate stays 0). PromQL queries should filter {... != "__init__"} so the sentinel rows are excluded from aggregations (the catalogue above does this).

Histograms are NOT force-zeroed: a synthetic observe(0.0) would contribute a real sample to the per-label distribution and pollute percentile readings. The two histogram families (proxy_cache_hit_rate_per_session and proxy_compression_ratio_by_strategy) only surface in the scrape after the first real session, by design.

H4 prometheus crate version pin

The H3 contract above relies on the prometheus crate's v0.13 gather() semantics — empty MetricVec families are omitted from the scrape. This is implementation-defined behaviour. If crates/headroom-proxy/Cargo.toml ever bumps the prometheus dependency, retest the alarm contract:

1. Start a fresh proxy.
2. curl /metrics and confirm every counter / gauge family has HELP/TYPE + an __init__ row.
3. Confirm histograms (*_cache_hit_rate_per_session, *_compression_ratio_by_strategy) DO NOT appear (no observe() calls yet).
4. Drive one cache-hit session, scrape again, confirm histograms now appear.
5. Confirm passthrough_bytes_modified_total stays at 0 across passthrough requests.

The crate version is pinned exactly (= "0.13.4", no caret) in Cargo.toml precisely so a silent semver bump cannot break the contract without a code-review trigger.

C2 alarm wiring

proxy_passthrough_bytes_modified_total fires from proxy.rs when a dispatcher arm that promised byte-equal passthrough (Outcome::NoCompression or Outcome::Passthrough) produces a final body of a different byte length. The check runs BEFORE the PR-E4 prompt_cache_key injector so the injector's intentional byte mutations do not trip the alarm.

H1 per-strategy ratio wiring

proxy_compression_ratio_by_strategy samples one observation per strategy using the strategy's OWN before/after token counts (plumbed through Outcome::Compressed.per_strategy_tokens from the manifest in live_zone_anthropic / live_zone_openai / live_zone_responses). Pre-H1 the same aggregate ratio was emitted per strategy when multiple strategies ran on one body, making Phase H per-strategy dashboards read garbage.

H2 aborted-stream gate

The proxy_cache_hit_rate_per_session histogram observes ONLY when the SSE stream completed:

• Anthropic: state.status == StreamStatus::MessageStop after the channel closes.
• OpenAI Chat: state.usage.is_some() (the final usage chunk only arrives at stream completion).
• OpenAI Responses: state.terminal_status().is_some().

A client disconnect mid-stream closes the channel without setting the terminal flag — under H2 we log + skip rather than observe a garbage half-stream sample.

Cardinality discipline

Every label vocabulary is bounded by code, not customer input:

• model / region: read from path params + Config::bedrock_region.
• auth_mode: 3-variant enum (payg, oauth, subscription).
• provider: 3 values (anthropic, openai_chat, openai_responses).
• strategy: &'static str from the compressor's BlockAction::Compressed.
• content_type: &'static str from headroom_core::transforms::ContentType.
• tier: validated through crate::observability::metric_names::service_tier::validate(raw: &str). Returns one of {auto, default, flex, on_demand, priority, scale} or the sentinel "other" for anything else. The raw inbound value is never used as a label. A malicious client posting {"service_tier":"<random>"} per request gets bucketed to "other" and a tracing::warn! is emitted so wire-format drift surfaces loudly in logs.
• status: 5-variant enum.

There is no code path where a malicious client can drive label cardinality unbounded.

See also

• crates/headroom-proxy/src/observability/ — implementation.
• REALIGNMENT/09-phase-G-rtk-observability.md — spec.
• REALIGNMENT/10-phase-H-python-retirement.md — H1 acceptance gate.