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 invocation	Asynchronous invocation	Polling invocation
AWS CLI Elastic Load Balancing (Application Load Balancer) Amazon Cognito Amazon Lex Amazon Alexa Amazon API Gateway Amazon CloudFront via Lambda@Edge Amazon Kinesis Data Firehose Amazon S3 Batch	Amazon S3 Amazon SNS Amazon Simple Email Service AWS CloudFormation Amazon CloudWatch Logs Amazon CloudWatch Events AWS CodeCommit AWS Config AWS IoT AWS IoT Events AWS CodePipeline	Amazon DynamoDB Amazon Kinesis Amazon Managed Streaming for Apache Kafka (Amazon MSK) Amazon SQS

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.

AWS service	Invocation type	Retry behavior
Amazon API Gateway	Synchronous	None – returns error to the client
Amazon S3	Asynchronous	Retries with exponential backoff
Amazon SNS	Asynchronous	Retries with exponential backoff
Amazon DynamoDB Streams	Synchronous from poller	Retries until data expiration (24 hours)
Amazon Kinesis	Synchronous from poller	Retries until data expiration (24 hours to 7 days)
AWS CLI	Synchronous/Asynchronous	Configured by CLI call
AWS SDK	Synchronous/Asynchronous	Application-specific
Amazon SQS	Synchronous from poller	Retries until Message Retention Period expires or is sent to a dead-letter queue

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