Comparing Lambda invocation modes

Lambda functions can be invoked either synchronously or asynchronously, depending upon the trigger. In synchronous invocations, the caller waits for the function to complete execution and the function can return a value. In asynchronous operation, the caller places the event on an internal queue, which is then processed by the Lambda function.

Synchronous invocations are well suited for short-lived Lambda functions. Although Lambda functions can run for up to 15 minutes, synchronous callers may have shorter timeouts. For example, API Gateway has a 29-second integration timeout, so a Lambda function running for more than 29 seconds will not return a value successfully. In synchronous invocations, if the Lambda function fails, retries are the responsibility of the trigger.

In asynchronous invocations, the caller continues with other work and cannot receive a return value from the Lambda function. The function can send the result to a destination, configurable based on success or failure. The internal queue between the caller and the function ensures that messages are stored durably. The Lambda service scales up the concurrency of the processing function as this internal queue grows. If an error occurs in the Lambda function, the retry behavior is determined by the Lambda service.

To learn more, read Invoking AWS Lambda functions and Introducing AWS Lambda Destinations.