music_library/backlog/completed/ml-146 - Honour-Retry-After-and-rate-limit-reset-headers-for-precise-snooze.md

id, title, status, assignee, created_date, updated_date, labels, dependencies, priority, ordinal

id	title	status	assignee	created_date	updated_date	labels	dependencies	priority	ordinal
ML-146	Honour Retry-After and rate-limit reset headers for precise snooze	Done	Codex	2026-04-24 11:12	2026-04-27 21:02		ML-21	low	1000

Description

Follow-up to ML-21.

Once workers classify API errors as transient vs permanent, the next refinement is using per-response retry hints instead of fixed snooze durations.

What each API provides

API	Header	Format
MusicBrainz	Retry-After	seconds (on 503)
Wikipedia REST v1 / Action API	Retry-After	seconds (on 429 / 503 from maxlag)
Discogs	X-Discogs-Ratelimit-Remaining + rolling 60s window	no Retry-After; fallback to a fixed backoff
Brave Search	X-RateLimit-Reset	seconds until reset (comma-separated for multi-window policies)
OpenAI	x-ratelimit-reset-requests, x-ratelimit-reset-tokens	duration string (e.g. 120ms, 2s)
Last.fm	none	keep existing ErrorResponse.retry_delay/1 fallback

Why this is a follow-up and not part of ML-21

ML-21 gets us from "no classification" to :retry | :cancel. That alone lets workers do the right thing with Oban's default backoff. This task layers on precision: turning {:snooze, 60} into {:snooze, retry_after} so we don't needlessly hammer rate-limited APIs or wait longer than necessary after a transient blip.

Suggested scope

• Extract a helper that parses each API's reset/retry header into seconds
• When the classifier returns :retry, emit {:snooze, seconds} using the parsed value when available
• Clamp to a sane range (e.g. 5–300s) to guard against malformed headers
• Unit test per-API header parsing with fixture responses

Dependencies

Blocked by ML-21 — requires the classification plumbing to exist first.

Acceptance Criteria

• #1 Each API module extracts a retry-delay value from its response (Retry-After, X-RateLimit-Reset, x-ratelimit-reset-*) when present
• #2 Workers emit {:snooze, seconds} with the parsed value for transient errors, falling back to a fixed default when the header is absent or malformed
• #3 Parsed durations are clamped to a safe range to prevent pathological values
• #4 Per-API header parsing is covered by unit tests using representative fixture responses

Implementation Plan

Implementation Plan

• Add a shared MusicLibrary.RetryDelay helper that parses retry/reset headers from Req.Response values, clamps parsed provider hints to 5..300 seconds, uses the maximum valid value for multi-window headers, and returns nil for absent or malformed hints.
• Extend HTTP-based API ErrorResponse structs with an optional retry_delay_seconds field populated at from_response/1 time.
• Parse provider hints for MusicBrainz and Wikipedia retry-after, Brave x-ratelimit-reset, and OpenAI x-ratelimit-reset-requests / x-ratelimit-reset-tokens; keep Discogs and Last.fm on their existing fixed fallbacks.
• Update each affected retry_delay_seconds/1 implementation to prefer the parsed field when present and preserve existing defaults otherwise.
• Add focused unit coverage for the shared parser, per-API parsed/fallback behavior, and one ErrorHandler integration assertion showing parsed values flow through to {:snooze, seconds}.

Implementation Notes

Implemented precise retry-delay parsing through the existing ErrorResponse callback flow. Added MusicLibrary.RetryDelay with 5..300s clamping and max-window selection for multi-window headers. MusicBrainz/Wikipedia parse Retry-After, Brave parses X-RateLimit-Reset, OpenAI parses request/token reset durations. Discogs and Last.fm remain on fixed fallbacks because they do not expose a reliable retry-delay header in scope. Verification passed: focused API/error-handler tests, full mix test suite, mix format --check-formatted, and git diff --check.

Addressed review gaps after implementation: Wikipedia Action API error promotion now passes the full Req response to from_action_api_body/2 so Retry-After is preserved; OpenAI retry parsing now includes Retry-After and compound reset durations such as 1m30s; Retry-After: 0 now clamps to the 5s minimum instead of falling back. Verification passed: focused retry/API tests, full mix test, and mix format --check-formatted.

Final Summary

Summary

Added shared retry-delay parsing for provider retry/reset headers and wired it into the existing structured API error flow. HTTP-based ErrorResponse structs for MusicBrainz, Wikipedia, Brave Search, and OpenAI now capture an optional parsed retry delay and prefer it from retry_delay_seconds/1; workers automatically emit {:snooze, parsed_seconds} through the existing MusicLibrary.Worker.ErrorHandler path.

Details

• New MusicLibrary.RetryDelay parses Retry-After, comma-separated reset-second headers, and OpenAI duration reset headers.
• Parsed provider hints are clamped to 5..300 seconds and multi-window headers use the maximum valid parsed value.
• Existing fixed fallbacks are preserved when hints are absent or malformed.
• Discogs and Last.fm remain on current fallback behavior because they do not provide a reliable retry-delay header in this scope.
• Updated architecture docs for the new shared helper.

Tests

• mix test test/music_library/retry_delay_test.exs test/music_brainz/api_test.exs test/wikipedia/api_test.exs test/brave_search/api_test.exs test/open_ai/api_test.exs test/music_library/worker/error_handler_test.exs
• mix test
• mix format --check-formatted
• git diff --check

Review follow-up

• Wikipedia Action API body-error responses now preserve response headers for Retry-After parsing.
• OpenAI retry parsing now considers Retry-After alongside request/token reset headers and supports compound durations like 1m30s.
• Zero-second retry hints clamp to the 5s minimum rather than falling back to fixed defaults.
• Architecture docs now mention header-driven snooze delays in the external API integration notes.