Chapter 17: When Things Go Wrong — Dead Letter Queues

A deployment introduced a bug in the BookReportProjector — it crashes on books without an ISBN (the field is optional). The server retries the message three times, then moves it to the dead-letter queue (DLQ). Meanwhile, new books are being added but the marketing dashboard is not updating.

How the DLQ Works

When a handler fails to process a message:

The message is retried (up to max_retries times, with exponential backoff).
After all retries are exhausted, the message is moved to the DLQ.
The handler continues processing subsequent messages — one failure does not block the stream.

Discovering the Problem

List DLQ entries:

$ protean dlq list --domain bookshelf
Dead Letter Queue:
  ID          Stream                Handler              Error                  Time
  msg-001     bookshelf::book-fact  BookReportProjector   KeyError: 'isbn'      2024-03-15 10:23:45
  msg-002     bookshelf::book-fact  BookReportProjector   KeyError: 'isbn'      2024-03-15 10:24:12

Inspecting a Failed Message

Get the full details of a failed message:

$ protean dlq inspect msg-001 --domain bookshelf
Message ID: msg-001
Stream: bookshelf::book-fact
Handler: BookReportProjector
Error: KeyError: 'isbn'
Traceback:
  File "bookshelf/projections.py", line 42, in on_book_report
    report.isbn = event.isbn  # isbn is None for this book!
    ...
Payload:
  {"id": "abc-123", "title": "Brave New World", "author": "Aldous Huxley", "isbn": null, ...}
Retries: 3/3
First failure: 2024-03-15 10:23:45
Last failure: 2024-03-15 10:23:52

Now we can see the issue: the projector assumes isbn is always present.

The Fix-and-Replay Cycle

Fix the bug — handle the None case in the projector:

@on(BookFactEvent)
def on_book_report(self, event):
    report = BookReport(
        book_id=event.id,
        title=event.title,
        author=event.author,
        price=event.price,
        isbn=event.isbn or "",  # Handle None
    )
    current_domain.repository_for(BookReport).add(report)

Deploy the fix and restart the server.
Replay the failed messages:

# Replay a single message
$ protean dlq replay msg-001 --domain bookshelf

# Or replay all failed messages
$ protean dlq replay-all --domain bookshelf
Replayed 2 messages. 0 failures.

Verify the marketing dashboard now shows all books.

Purging Abandoned Messages

If a message is truly unrecoverable (bad data that will never process successfully), purge it:

$ protean dlq purge --domain bookshelf
Purged 0 messages from DLQ.

DLQ Configuration

The DLQ behavior is configured in domain.toml:

[server.stream_subscription]
max_retries = 3
retry_delay_seconds = 1
enable_dlq = true

max_retries — how many times to retry before moving to DLQ.
retry_delay_seconds — base delay between retries (exponential backoff is applied).
enable_dlq — set to false to disable DLQ (failed messages are dropped instead).

What We Built

Understanding of the fix-and-replay cycle: discover, inspect, fix, replay, verify.
Using protean dlq list, inspect, replay, and purge.
Configuring retry behavior and DLQ settings.

In the next chapter, we will set up monitoring so the team knows about problems before customers report them.