docker-exporter

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Architecture ​

docker-exporter has no background loop. Everything happens on the request.

Per-scrape collection ​

On each GET /metrics, the exporter:

  1. Lists all containers, running and stopped, so container_state covers every state.
  2. For every container, concurrently fetches stats (bollard::stats(stream=false)) and an inspect (health + restart policy), each with a 5 s timeout; all containers run in parallel.
  3. Encodes the result as Prometheus text.

Failed or timed-out calls are logged; only inspect failures increment docker_exporter_inspect_failures_total. The container is still emitted either way — a stats failure zeroes its CPU/memory/network/block I/O, an inspect failure sets health="none" and restart_policy="unknown" — so one bad container never fails the whole scrape.

With no polling, the process sits idle between scrapes, so scrape duration tracks Docker daemon latency — the concurrent /containers/{id}/stats and /containers/{id}/json calls per container — rather than the exporter's own encoding work.

Working set computation ​

The working set is computed at exposition time from the Docker stats payload:

  • cgroup v2: max(0, usage − inactive_file)
  • cgroup v1: max(0, usage − cache)

This is the calculation cAdvisor gets wrong on ARM64 + cgroup v2, where it reports zero — cAdvisor #3469, closed "not planned" on 2025-12-09. Why cAdvisor reports zero memory on ARM64 →

State across scrapes ​

The Prometheus output is rebuilt from scratch each scrape as MetricFamily protos (no Registry), so nothing stale carries over and counters emit as absolute values. The one piece of cross-scrape state is docker_exporter_inspect_failures_total — an AtomicU64 in the shared AppState that accumulates since start, by design. Everything else is per-request, keeping memory flat and behavior predictable under concurrent scrapes.

Footprint ​

  • Memory: ~7–10 MiB idle, ~10–20 MiB scraping ~30 containers.
  • CPU: near-zero at rest; a scrape's work scales with container count and daemon latency, but steady-state stays under 1% on a Raspberry Pi 5.
  • Image: ~9 MB — a static musl binary on distroless/static, non-root, identical on linux/amd64 and linux/arm64.

See the footprint benchmark → for the methodology and numbers versus cAdvisor, or the full docker-exporter vs cAdvisor comparison → for a feature-by-feature breakdown.