music_library/.agents/skills/oban-worker/SKILL.md

name, description

name	description
oban-worker	Use this skill when creating, modifying, debugging, or reviewing ANY Oban worker. Use PROACTIVELY when the user mentions "worker", "Oban", "background job", "job", "cron", "queue", "perform", "enqueue", "schedule", "async", "snooze", "cancel", "discard", or asks about running work in the background. Also use when adding Oban plugins, changing queue configuration, or writing worker tests.

Oban Worker Development

Project-specific patterns for Oban background job workers. Every worker follows consistent conventions for structure, error handling, queue assignment, and testing.

Checklist Before Creating or Modifying a Worker

1. Does the worker delegate to a context module? perform/1 must be a thin wrapper. No business logic inline.

2. Is the queue correct? Rate-limited APIs get dedicated queues with concurrency 1. DB-intensive operations go to heavy_writes. General tasks go to default.

3. Does error handling use ErrorHandler? Workers calling APIs route HTTP errors through MusicLibrary.Worker.ErrorHandler.to_oban_result/1.

4. Is the worker registered under the correct Oban instance? Production uses MusicLibrary.BackgroundRepo as the Oban instance.

Worker Structure

defmodule MusicLibrary.Worker.FetchArtistInfo do
  use Oban.Worker,
    queue: :default,           # Pick the right queue (see below)
    max_attempts: 3,            # Oban default, override if needed
    unique: [period: 60]        # Optional: prevent duplicate jobs

  @impl true
  def perform(%Oban.Job{args: %{"musicbrainz_id" => mbid}}) do
    # Delegate to context — no business logic here
    case Artists.fetch_info(mbid) do
      {:ok, _artist} ->
        :ok

      # App-layer permanent failures matched FIRST
      {:error, :no_english_wikipedia} ->
        {:cancel, :no_english_wikipedia}

      {:error, :cover_not_available} ->
        {:cancel, :cover_not_available}

      # Everything else routed through ErrorHandler
      {:error, reason} ->
        MusicLibrary.Worker.ErrorHandler.to_oban_result(reason)
    end
  end
end

Queues

Queue	Concurrency	Use For
default	10	General async tasks, non-API operations
heavy_writes	1	DB-intensive or serialized operations
music_brainz	1	MusicBrainz API calls
discogs	1	Discogs API calls
wikipedia	1	Wikipedia API calls
last_fm	1	Last.fm API calls

Rule: Any worker that calls an external API goes on that API's dedicated queue. The concurrency of 1 combined with Req.RateLimiter ensures the API rate limit is never exceeded.

Return Values

Return	Meaning	Oban Behaviour
:ok	Success	Job marked complete
{:ok, result}	Success with result	Job marked complete
{:error, reason}	Transient failure	Oban retries with backoff
{:snooze, seconds}	Retryable with delay	Oban retries after seconds
{:cancel, reason}	Permanent failure	Job cancelled, never retried

CRITICAL: Never use {:discard, reason} — it's deprecated since Oban 2.14. Always use {:cancel, reason}.

Error Handling Flow

perform/1
  ├── App-layer permanent failures (match FIRST)
  │   ├── :no_english_wikipedia → {:cancel, :no_english_wikipedia}
  │   ├── :cover_not_available → {:cancel, :cover_not_available}
  │   └── ...other domain-specific permanent failures
  │
  └── Everything else → MusicLibrary.Worker.ErrorHandler.to_oban_result(reason)
      ├── %ErrorResponse{retryable?: true}  → {:snooze, seconds}
      ├── %ErrorResponse{retryable?: false} → {:cancel, reason}
      └── Other errors                      → {:error, reason}

The order matters: app-layer atoms first, then fall through to ErrorHandler. This ensures domain-specific permanent failures (e.g., no Wikipedia article exists) are handled correctly before generic HTTP error classification kicks in.

Cron Workers

Scheduled via Oban.Plugins.Cron in production config:

# config/prod.exs
config :music_library, Oban,
  plugins: [
    {Oban.Plugins.Cron,
     crontab: [
       {"@daily", MusicLibrary.Worker.SendRecordsOnThisDayEmail},
       {"*/5 * * * *", MusicLibrary.Worker.RefreshScrobbles}
     ]}
  ]

Cron workers follow the same thin-wrapper pattern. The cron expression is the only difference from on-demand workers.

All Cron Workers

Schedule	Worker	Queue
Every 12h	ApplyScrobbleRules	heavy_writes
Every 12h	PruneAssetCache	default
Daily 2 AM	PruneAssets	default
Daily 3 AM	RepoVacuum	heavy_writes
Daily 4 AM	RepoOptimize	heavy_writes
Monthly 1st, 6 AM	RecordRefreshAllMusicBrainzData	music_brainz
Monthly 1st, 7 AM	RecordGenerateAllEmbeddings	heavy_writes
Monthly 1st, 8 AM	ArtistRefreshAllMusicBrainzData	music_brainz
Monthly 1st, 9 AM	ArtistRefreshAllDiscogsData	discogs
Monthly 1st, 10 AM	ArtistRefreshAllWikipediaData	wikipedia
Daily 7 AM	SendRecordsOnThisDayEmail	default
Every 5 min	RefreshScrobbles	last_fm

Oban Plugins (Production)

Plugin	Config	Purpose
Pruner	max_age: 43200 (12h)	Clean up completed jobs
Reindexer	schedule: "@weekly"	Reindex for query performance
Cron	timezone: "Europe/London"	Schedule recurring jobs

Batch Workers (Self-Chaining)

Some workers process data in batches, enqueuing their next batch on completion:

def perform(%{args: %{"page" => page}}) do
  case process_page(page) do
    {:ok, []} ->
      :ok  # No more data, done

    {:ok, _results} ->
      # Enqueue next page
      %{page: page + 1}
      |> __MODULE__.new()
      |> Oban.insert()

      :ok
  end
end

The Records.Batch and Artists.Batch modules provide shared batch infrastructure (stream + transaction + error accumulation). Bulk workers should use these rather than implementing their own batching logic.

Testing Workers

Manual mode

Oban runs in manual testing mode — jobs don't auto-execute. Use perform_job/2:

import Oban.Testing, only: [perform_job: 2]

test "fetches artist info" do
  job = MyWorker.new(%{musicbrainz_id: "abc-123"})
  assert :ok = perform_job(MyWorker, job.args)
end

Assert enqueued downstream jobs

Workers that enqueue other jobs MUST verify with assert_enqueued:

assert_enqueued(worker: FetchArtistImage, args: %{musicbrainz_id: "abc-123"})

perform_job returning {:ok, []} is NOT sufficient — verify the expected downstream workers were enqueued with correct args.

Testing all return states

# Success
assert :ok = perform_job(MyWorker, %{id: valid_id})

# Transient (Oban retries)
assert {:error, :rate_limit} = perform_job(MyWorker, %{id: rate_limited_id})

# Permanent (job cancelled)
assert {:cancel, :no_english_wikipedia} = perform_job(MyWorker, %{id: missing_id})

Non-Fatal Enrichment

Context functions that enrich records (colors, embeddings) use a private best_effort_* helper pattern — log a warning and return unchanged data rather than surfacing {:error, reason}. Workers calling these functions don't need error handling for enrichment failures.