What is the Lambda invocation priority when dealing with different triggers?

Question

Say I have a lambda handler that is able to process sqs queue invocations and also lambda-to-lambda invocations.

The lambda has a max concurrency limit of 10.

Let's say there is a period of time where the concurrency of the lambda is maxed out due to the high volume of sqs queue messages that are processing.

What happens where there is a lambda-to-lambda invocation in the middle of sqs queue messages being processed and maxing out the concurrency limit? Is the AWS CLI invocation handled after all the messages in the queue are processed? Or does the lambda try to process that invocation at the next available instance?

Uri · Answer

There is no specific priority. The behavior depends on invocation type.

For Asynchronous invocation, the invocation event is added to an internal queue. We then process all the messages in the internal queue. If we try to invoke a function and it already at its limit, we will retry the invocation.

For synchronous invocations (e.g., CLI), if we are at the limit, we return an error. It is up to the client to retry.

For poll based invocations (e.g., SQS), there is a fleet of pollers reading from the queue and trying to invoke the function synchronously. If at the limit, the pollers will retry.

In your case, we can't really tell. It is really timing.

What is the Lambda invocation priority when dealing with different triggers?

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