Helped with a SQS event processor

[Iamrodos]

We have an SQS-triggered Lambda that processes events, where each event can have multiple handlers (send email, update records, trigger downstream events, etc.). Before durable functions, if handler 3 of 5 failed, the SQS retry would reprocess handlers 1-2 unnecessarily. These handlers were not always idempotent, causing duplicate side effects. Its been a TODO to resolve and durable functions turned into an easy way to do this.

We wrapped our event processor in @durable_execution and use ctx.step() to checkpoint:

1. Event creation - The initial write to DynamoDB
2. Each handler - Individual steps so completed handlers are skipped on replay

for i, handler in enumerate(handlers):
    if ctx:  # Durable mode - wrap each handler in a step
        ctx.step(lambda _: handler(event), name=f"handler-{i}-{handler.__name__}")
    else:  # Standard mode
        handler(event)

We use a DURABLE_EXECUTION_ENABLED env var to switch between durable and standard BatchProcessor paths. This lets us roll back instantly without code changes if issues arise.

We include the handler index in step names (handler-0-send_email) to avoid collisions when multiple handlers share the same __name__.

The only issue we hit was a circular dependency on CloudFromation when trying to write the role for the Lambda. The IAM role needs the Lambda ARN for checkpoint permissions, but the Lambda needs the Role ARN. Would like to find a way to solve that.