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 Data Firehose Amazon S3 Batch |
Amazon S3 Amazon SNS Amazon Simple Email Service AWS CloudFormation Amazon CloudWatch Logs Amazon EventBridge 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.