Public Overpass instances go bad without warning — we just lived
through a day where overpass.private.coffee (our single upstream)
returned 504s after 60s while lz4.overpass-api.de served the same
query in 0.6s. Survive that by trying a list of upstreams in order
until one responds usefully.
Server changes:
- OVERPASS_URLS env — comma-separated list, tried in order. Falls
back to OVERPASS_URL for backward compat, then to a built-in
default pair (lz4.overpass-api.de, overpass-api.de).
- Per-upstream timeout of 10s via AbortSignal.timeout, so a saturated
instance fails over in seconds, not minutes.
- Client abort propagation via AbortSignal.any — if the browser
cancels (e.g. user panned the map), the in-flight upstream fetch is
aborted too. Stops wasting upstream capacity on requests nobody
wants.
- Rate-limited-body detection (Overpass returns 200 with a
`rate_limited` marker when throttling) triggers failover.
- Per-upstream labels on overpass_upstream_requests_total and
overpass_upstream_duration_seconds so the dashboard can break out
health + latency by instance. Histogram buckets extended to 30s.
Compose: OVERPASS_URLS passthrough with the default pair hard-wired,
overridable via SOPS.
Tests: 8 cases covering the new fetchWithFailover helper — happy
path, 5xx failover, rate_limited failover, network error failover,
timeout tagging, all-upstreams-fail, client abort stops the loop,
per-attempt latency observation.
Follow-ups left out of scope:
- Client-side debounce (UI concern).
- Moving `.observe()` after body read for accuracy in measurement.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
963902514b