AWSLambdaSharpTemplatePartialBatch 5.1.3

AWSLambdaSharpTemplatePartialBatch

Overview

This library adds partial batch support for SQS-triggered Lambdas to Kralizek's excellent AWSLambdaSharpTemplate.

SQS Partial Batch Quick Start

• Call UsePartialBatchQueueMessageHandler instead of UseQueueMessageHandler
• Derive your entry point class from PartialBatchEventFunction instead of EventFunction<SQSEvent>
• Write your IMessageHandler<TInput> (and provide its type to UsePartialBatchQueueMessageHandler) as usual
• Ensure that the SQS event source is configured for partial batch support from the AWS side (the next section describes how this can be done)

UsePartialBatchQueueMessageHandler adds partial batch support by registering an IRequestResponseHandler<SQSEvent, SQSBatchResponse> (alongside the usual IEventHandler<SQSEvent>). Any exceptions thrown by your handler will be caught, logged, and provided to AWS, using a SQSBatchResponse (generated on your behalf) that conveys which messages failed to process.

Handling SQS messages with partial batch responses

The SQS Event Source can be configured to send batches of more than one message to the Lambda. When a class that derives from EventFunction<SQSEvent> is used as the Lambda entry point, any exceptions thrown from the message handler will propagate to the Lambda runtime, causing the entire batch that's being processed by that Lambda invocation to fail. All of the messages in the failed batch will be retried (subject to SQS configuration).

As an alternative to this default behavior, partial batch support can be enabled by deriving the entry point class from PartialBatchEventFunction (or RequestResponseFunction<SQSEvent, SQSBatchResponse>) instead, and configuring the SQS Event Source to look in the Lambda response body for batch item failure information. When partial batch support is enabled, exceptions thrown from the handler are caught, and failed messages are reported to Lambda in the response payload. Only failed messages will be retried (subject to SQS configuration).

The Event Source configuration can be done using CloudFormation, or by turning on "Report batch item failures" in the AWS console (trigger configuration), or by some other means such as the AWS CLI.

Configuring the SQS Event Source is critical to partial batch support working properly. If this step is omitted, yet an entry point class deriving from RequestResponseFunction<SQSEvent, SQSBatchResponse> is used, exceptions thrown from the message handler will be logged but then ignored (not retried).

Parallel execution of SQS messages

Parallel processing is supported, and configured the same way as with the baseline AWSLambdaSharpTemplate library. Please see Parallel execution of SQS messages.

SQS Partial Batch Event Log

Optionally, implementations of ISqsEventLogger can be registered, to inspect the progress of batches and individual messages. An instance of each ISqsEventLogger implementation registered as transient will be created for each batch. Raw SQSEvent and SQSEvent.SQSMessage instances are provided for inspection.

• BatchReceivedAsync is called at the start of a batch.
• MessageReceivedAsync is called for each message in the batch, before processing begins.
• PartialBatchItemFailureAsync is called for each message that experienced a failure. The exception thrown from the message handler is supplied as an argument.
• MessageCompletedAsync is called for each message, after the message is done processing, or the message fails to process succesfully.
• BatchCompletedAsync is called after all messages in a batch have been processed.

MessageReceivedAsync, PartialBatchItemFailureAsync, and MessageCompletedAsync may be called from various threads simultaneously, when parallel execution is enabled.