Invoking Lambda functions with Amazon SNS notifications

You can use a Lambda function to process Amazon Simple Notification Service (Amazon SNS) notifications. Amazon SNS supports Lambda functions as a target for messages sent to a topic. You can subscribe your function to topics in the same account or in other AWS accounts. For a detailed walkthrough, see Tutorial: Using AWS Lambda with Amazon Simple Notification Service.

Lambda supports SNS triggers for standard SNS topics only. FIFO topics aren't supported.

Lambda processes SNS messages asynchronously by queuing the messages and handling retries. If Amazon SNS can't reach Lambda or the message is rejected, Amazon SNS retries at increasing intervals over several hours. For details, see Reliability in the Amazon SNS FAQs.

Warning

Lambda asynchronous invocations process each event at least once, and duplicate processing of records can occur. To avoid potential issues related to duplicate events, we strongly recommend that you make your function code idempotent. To learn more, see How do I make my Lambda function idempotent in the AWS Knowledge Center.

Topics

• Adding an Amazon SNS topic trigger for a Lambda function using the console

• Manually adding an Amazon SNS topic trigger for a Lambda function

• Sample SNS event shape

• Tutorial: Using AWS Lambda with Amazon Simple Notification Service

Adding an Amazon SNS topic trigger for a Lambda function using the console

To add an SNS topic as a trigger for a Lambda function, the easiest way is to use the Lambda console. When you add the trigger via the console, Lambda automatically sets up the necessary permissions and subscriptions to start receiving events from the SNS topic.

To add an SNS topic as a trigger for a Lambda function (console)

1. Open the Functions page of the Lambda console.

2. Choose the name of a function you want to add the trigger for.

3. Choose Configuration, and then choose Triggers.

4. Choose Add trigger.

5. Under Trigger configuration, in the dropdown menu, choose SNS.

6. For SNS topic, choose the SNS topic to subscribe to.

Manually adding an Amazon SNS topic trigger for a Lambda function

To set up an SNS trigger for a Lambda function manually, you need to complete the following steps:

• Define a resource-based policy for your function to allow SNS to invoke it.

• Subscribe your Lambda function to the Amazon SNS topic.

  Note

  If your SNS topic and your Lambda function are in different AWS accounts, you also need to grant extra permissions to allow cross-account subscriptions to the SNS topic. For more information, see Grant cross-account permission for Amazon SNS subscription.

You can use the AWS Command Line Interface (AWS CLI) to complete both of these steps. First, to define a resource-based policy for a Lambda function that allows SNS invocations, use the following AWS CLI command. Be sure to replace the value of --function-name with your Lambda function name, and the value of --source-arn with your SNS topic ARN.

aws lambda add-permission --function-name example-function \
    --source-arn arn:aws:sns:us-east-1:123456789012:sns-topic-for-lambda \
    --statement-id function-with-sns --action "lambda:InvokeFunction" \
    --principal sns.amazonaws.com

To subscribe your function to the SNS topic, use the following AWS CLI command. Replace the value of --topic-arn with your SNS topic ARN, and the value of --notification-endpoint with your Lambda function ARN.

aws sns subscribe --protocol lambda \
    --region us-east-1 \
    --topic-arn arn:aws:sns:us-east-1:123456789012:sns-topic-for-lambda \
    --notification-endpoint arn:aws:lambda:us-east-1:123456789012:function:example-function

Sample SNS event shape

Amazon SNS invokes your function asynchronously with an event that contains a message and metadata.

Example Amazon SNS message event

{
  "Records": [
    {
      "EventVersion": "1.0",
      "EventSubscriptionArn": "arn:aws:sns:us-east-1:123456789012:sns-lambda:21be56ed-a058-49f5-8c98-aedd2564c486",
      "EventSource": "aws:sns",
      "Sns": {
        "SignatureVersion": "1",
        "Timestamp": "2019-01-02T12:45:07.000Z",
        "Signature": "tcc6faL2yUC6dgZdmrwh1Y4cGa/ebXEkAi6RibDsvpi+tE/1+82j...65r==",
        "SigningCertURL": "https://sns.us-east-1.amazonaws.com/SimpleNotificationService-ac565b8b1a6c5d002d285f9598aa1d9b.pem",
        "MessageId": "95df01b4-ee98-5cb9-9903-4c221d41eb5e",
        "Message": "Hello from SNS!",
        "MessageAttributes": {
          "Test": {
            "Type": "String",
            "Value": "TestString"
          },
          "TestBinary": {
            "Type": "Binary",
            "Value": "TestBinary"
          }
        },
        "Type": "Notification",
        "UnsubscribeUrl": "https://sns.us-east-1.amazonaws.com/?Action=Unsubscribe&SubscriptionArn=arn:aws:sns:us-east-1:123456789012:test-lambda:21be56ed-a058-49f5-8c98-aedd2564c486",
        "TopicArn":"arn:aws:sns:us-east-1:123456789012:sns-lambda",
        "Subject": "TestInvoke"
      }
    }
  ]
}