Cognito User Pool

Valid Triggers

Serverless supports all Cognito User Pool Triggers as specified here. Use this guide to understand the event objects that will be passed to your function.

Simple event definition

This will create a Cognito User Pool with the specified name. You can reference the same pool multiple times.

functions:
  preSignUp:
    handler: preSignUp.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool
          trigger: PreSignUp
  customMessage:
    handler: customMessage.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool
          trigger: CustomMessage

Multiple pools event definitions

This will create multiple Cognito User Pools with their specified names:

functions:
  preSignUpForPool1:
    handler: preSignUp.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool1
          trigger: PreSignUp
  preSignUpForPool2:
    handler: preSignUp.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool2
          trigger: PreSignUp

You can also deploy the same function for different user pools:

functions:
  preSignUp:
    handler: preSignUp.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool1
          trigger: PreSignUp
      - cognitoUserPool:
          pool: MyUserPool2
          trigger: PreSignUp

Special Trigger Considerations

Custom Sender Triggers

There are two types of Custom Sender Triggers, CustomSMSSender and CustomEmailSender, both documented by AWS.

In order to use these triggers, you must supply a kmsKeyId and (optionally) the lambdaVersion of the function. Only 1 kmsKeyId can be supplied per Cognito User Pool.

functions:
  customSMSSenderFunction:
    handler: customSMSSender.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool1
          trigger: CustomSMSSender
          # Required with CustomSMSSender and CustomEmailSender Triggers
          # Can either be KMS Key ARN string or reference to KMS Key Resource ARN (see customEmailSenderFunction below)
          kmsKeyId: 'arn:aws:kms:eu-west-1:111111111111:key/12345678-9abc-def0-1234-56789abcdef1'
  customEmailSenderFunction:
    handler: customEmailSender.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool2
          trigger: CustomEmailSender
          kmsKeyId:
            Fn::GetAtt: ['kmsKey', 'Arn']

resources:
  Resources:
    # Used to show use in supplying kmsKeyId in customEmailSenderFunction, not needed when kmsKeyId is a string as shown in customSMSSenderFunction
    kmsKey:
      Type: AWS::KMS::Key
      Properties:
        Description: MyKMSKey
        Enabled: true
        KeyPolicy:
          Version: '2012-10-17'
          Id: my-kms-key
          Statement:
            Sid: Enable IAM User Permissions
            Principal:
              AWS:
                - Fn::Sub: arn:aws:iam::${AWS::AccountId}:root
            Effect: Allow
            Action: kms:*
            Resource: '*'

    # Used to show how CustomSenderSources can be used with overriding the generated User Pool, as described below
    # This makes it such that when a user signs up, they are automatically sent a verification email, triggering our
    # CustomEmailSender
    CognitoUserPoolMyUserPool2:
      Type: AWS::Cognito::UserPool
      Properties:
        UsernameAttributes:
          - 'email'
        AutoVerifiedAttributes:
          - 'email'
        EmailVerificationMessage: 'email message: {####}'
        EmailVerificationSubject: 'email subject: {####}'

NOTE: The only supported value for lambdaVersion is V1_0, as documented by AWS.

Custom Sender Triggers Handlers

For custom senders, the event.triggerSource type does not get populated by the type of custom sender, rather may be populated by another trigger source. Instead, event.request.type is populated with either customEmailSenderRequestV1 or customSMSSenderRequestV1, respectively documented by AWS here and here

// customSender.js
function handler(event, context, callback) {
  if (event.request.type === 'customEmailSenderRequestV1') {
    // ...
  }
  if (event.request.type === 'customSMSSenderRequestV1') {
    // ...
  }
}

Custom Message Trigger Handlers

For custom messages, you will need to check event.triggerSource type inside your handler function:

// customMessage.js
function handler(event, context, callback) {
  if (event.triggerSource === 'CustomMessage_AdminCreateUser') {
    // ...
  }
  if (event.triggerSource === 'CustomMessage_ResendCode') {
    // ...
  }
}

Using existing pools

Sometimes you might want to attach Lambda functions to existing Cognito User Pools. In that case you just need to set the existing event configuration property to true. All the other config parameters can also be used on existing user pools:

IMPORTANT: You can only attach 1 existing Cognito User Pool per function.

NOTE: Using the existing config will add an additional Lambda function and IAM Role to your stack. The Lambda function backs-up the Custom Cognito User Pool Resource which is used to support existing user pools.

functions:
  users:
    handler: users.handler
    events:
      - cognitoUserPool:
          pool: legacy-user-pool
          trigger: CustomMessage
          existing: true

Overriding a generated User Pool

A Cognito User Pool created by an event can be overridden by using the logical resource name in Resources:

functions:
  preSignUp:
    handler: preSignUpForPool1.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool
          trigger: PreSignUp
  postConfirmation:
    handler: postConfirmation.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool
          trigger: PostConfirmation

resources:
  Resources:
    CognitoUserPoolMyUserPool:
      Type: AWS::Cognito::UserPool

Forcing deploying of triggers

A Cognito User Pool with triggers attached may not be correctly updated by AWS Cloudformation on subsequent deployments. To circumvent this issue you can use the forceDeploy flag which will try to force Cloudformation to update the triggers no matter what. This flag has to be used in conjunction with the existing: true flag.

functions:
  preSignUp:
    handler: preSignUp.handler
    events:
      - cognitoUserPool:
          pool: MyUserPool1
          trigger: PreSignUp
          existing: true
          forceDeploy: true