Error Handling for Async Function Invocation

[gangliao]

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scatter-gather model shows that we can aggregate data packets received from different previous function instances if aggregate function's concurrency == 1. If the function doesn't have enough capacity to handle all incoming requests, events might wait in the event queue.

Even if your function doesn't return an error, it's possible for it to receive the same event from Lambda multiple times because the queue itself is eventually consistent. In Flock, we use a bitmap in the aggregate function to ensure that data is reassembled and processed exactly once.

However, If the function can't keep up with incoming events, events might also be deleted from the queue without being sent to the function. Or, when the queue is very long, new events might age out before Lambda has a chance to send them to your function. In rare cases, we may lose data packets or events.

• https://docs.aws.amazon.com/lambda/latest/dg/invocation-async.html

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We can configure the aggregate function with a dead-letter queue to save discarded events for further processing. A dead-letter queue is used when an event fails all processing attempts or expires without being processed. A dead-letter queue is part of a function's version-specific configuration, so it is locked in when you publish a version.

We can configure an Amazon SQS queue or Amazon SNS topic as a dead-letter queue for discarded events. For dead-letter queues, Lambda only sends the content of the event, without details about the response. To reprocess events in a dead-letter queue, we can set it as an event source for our Lambda function. Or, we can manually retrieve the events from the aggregate function.

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