docs: warn that Prometheus /metrics must be scraped when enabled#4660
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Distribution metrics in telemetry_metrics_prometheus_core buffer raw observations in a :duplicate_bag and only aggregate at scrape time. When ELECTRIC_PROMETHEUS_PORT is set but the /metrics endpoint is never scraped, that buffer grows unbounded (notably the per-transaction receive_lag distribution) and can exhaust memory and crash the service. Add this warning to the telemetry reference, config reference, deployment guide, and the prometheus_port config docstring. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01YW7Njz5ZpBDaoGviW1eVR8
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Summary
If
ELECTRIC_PROMETHEUS_PORTis set but nothing ever scrapes the/metricsendpoint, Electric's memory grows without bound and can eventually OOM-kill the service. This came out of debugging a customer whoseprometheus_metrics_distETS table had grown to ~8 GB on an OTel-only deployment that had the Prometheus port enabled but unscraped.Why it happens
telemetry_metrics_prometheus_corestores distribution (histogram) metrics in a:duplicate_bagand appends one row per observation (distribution.ex:ets.insert), only aggregating into buckets — and draining the bag via:ets.take— at scrape time (documented in the library's moduledoc: "aggregations for distributions (histogram) only occur at scrape time"). So if the endpoint is never scraped, the bag is never drained.Electric's default Prometheus metric set includes
electric.postgres.replication.transaction_received.receive_lag, a distribution emitted roughly once per replication transaction, so the table accretes continuously under load (~0.55 GB/day in the customer's case). Counters/sums/last-values are unaffected — they use bounded:setstorage.This is purely a docs change to make the "must be scraped" requirement explicit everywhere we mention Prometheus.
Changes
website/docs/sync/reference/telemetry.md) — warning callout in the Metrics section.website/docs/sync/api/config.md) — note onELECTRIC_PROMETHEUS_PORT.website/docs/sync/guides/deployment.md) — warning in the Observability section.prometheus_portdocstring (packages/sync-service/lib/electric.ex).Follow-up (not in this PR)
Worth considering a code-level guard so an enabled-but-unscraped endpoint can't OOM the service (e.g. move high-frequency distributions like
receive_lagout of the default Prometheus set, or run an internal periodic drain).I have raised a PR with the library maintainer for a potential fix: beam-telemetry/telemetry_metrics_prometheus_core#77
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