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Lambda Event Source Mapping

Links: 102 AWS DVA Index

Event Source Mapping

  • Services:
    • Kinesis Data Streams
    • SQS & SQS FIFO queue
    • DynamoDB Streams
  • Records need to be polled from the source. This means lambda needs to ask the services for some records.
  • Lambda is invoked synchronously.
  • Internally an event source map is created which is responsible for polling kinesis and getting the results back.
  • Once the event source map has data it will invoke the lambda function synchronously for processing

Event Source Mappers

  • There are two categories of event source mappers:
  • Streams: Kinesis & DynamoDB streams
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  • An event source mapping creates an iterator for each shard, processes items in order
  • Start with new items, from the beginning or from timestamp
  • Processed items aren't removed from the stream (other consumers can read them)
  • Low traffic: Use batch window to accumulate records before processing
  • High traffic: You can process multiple batches in parallel
    • Up to 10 batches per shard
    • In-order processing is still guaranteed for each partition key,
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Error Handling

  • By default, if your function returns an error, the entire batch is reprocessed until the function succeeds, or the items in the batch expire.
  • To ensure in-order processing, processing for the affected shard is paused until the error is resolved
  • You can configure the event source mapping to:
    • Discard old events
    • Restrict the number of retries
    • Split the batch on error (to work around Lambda timeout issues)
  • Discarded events can go to a destination.



  • Event Source Mapping will poll SQS (Long Polling)
  • Specify batch size
  • Recommended: Set the queue visibility timeout to 6x the timeout of your Lambda function
  • To use a DLQ
    • Set-up on the SQS queue, not Lambda (DLQ for Lambda is only for async invocations and here we are dealing with synchronous invocations)
    • Or use a Lambda destination for failures
For the lambda to read messages from the queue its execution role must have SQS permission.

Queues & Lambda

  • Lambda also supports in-order processing for FIFO (first-in, first-out) queues, scaling up to the number of active message groups.
  • For standard queues, items aren't necessarily processed in order.
  • Lambda scales up to process a standard queue as quickly as possible.
  • When an error occurs, batches are returned to the queue as individual items and might be processed in a different grouping than the original batch.
  • Occasionally, the event source mapping might receive the same item from the queue twice, even if no function error occurred.
  • Lambda deletes items from the queue after they're processed successfully.
  • You can configure the source queue to send items to a dead-letter queue if they can't be processed.

Lambda Event Mapper Scaling

  • Kinesis Data Streams & DynamoDB Streams:
    • One Lambda invocation per stream shard
    • If you use parallelisation, up to 10 batches processed per shard simultaneously
  • SQS Standard:
    • Lambda adds 60 more instances per minute to scale up
    • Up to 1000 batches of messages processed simultaneously
    • Messages with the same GroupID will be processed in order
    • The Lambda function scales to the number of active message groups

Last updated: 2022-05-24