Access Congito authorized AppSync API from a Lambda function

With user pools as your authentication mechanism, there is no way to get tokens to authorize with AWS AppSync without signing in, at some point. With that said, you could mitigate the overhead of that a bit. Ultimately, it boils down to what you suggested in your question, using a 'fake user'. It's not an uncommon solution, and involves having some admin level user, the credentials for whom would only be accessed from this Lambda.

First of all, I would recommend you not use the traditional SRP login, for latency reasons. One alternative would be to use AdminInitiateAuth/ADMIN_NO_SRP_AUTH, to offload the overhead of SRP calculations from your Lambda to the Cognito back end, with ADMIN_NO_SRP_AUTH turned on in your pool. I saw from your other question (Authenticate AppSync request with adminInitiateAuth) that you were thinking of doing this, however it is important to point out this is simply a different API used to sign a user in. You can read more about it here: https://docs.aws.amazon.com/cognito/latest/developerguide/amazon-cognito-user-pools-authentication-flow.html#amazon-cognito-user-pools-server-side-authentication-flow

An additional alternative could be a custom authentication flow. The same link above has more details on how to do this, but in short, you could set up a quicker flow that fits whatever specific needs your add has.

To improve overhead a bit more, you could then keep those tokens cached, to some degree. That could just mean keeping it in memory in the Lambda and checking their validity/expiry before using them, or even attaching a remote caching mechanism.

@Gerharddc, what if do you try to add policies that allow unauthenticated access to your Cognito Identity Pool?

I have this piece of code that defines authenticated and unauthenticated access to my Identity Pool (you can check the full code here in my personal project).

IdentityPoolUnauthorizedIAMRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Principal:
              Federated: cognito-identity.amazonaws.com
            Action:
              - sts:AssumeRoleWithWebIdentity
            Condition:
              StringEquals:
                cognito-identity.amazonaws.com:aud: !Ref CognitoIdentityPool
              ForAnyValue:StringLike:
                cognito-identity.amazonaws.com:amr: unauthenticated
      Policies:
        - PolicyName: CognitoUserSignInUnauthorizedPolicy
          PolicyDocument:
            Version: 2012-10-17
            Statement:
              - Effect: Allow
                Action:
                  - mobileanalytics:PutEvents
                  - mobiletargeting:PutEvents
                  - cognito-sync:*
                  - cognito-identity:*
                Resource: '*'

  IdentityPoolAuthorizedIAMRole:
    Type: AWS::IAM::Role
    Properties:
      AssumeRolePolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Principal:
              Federated: cognito-identity.amazonaws.com
            Action:
              - sts:AssumeRoleWithWebIdentity
            Condition:
              StringEquals:
                cognito-identity.amazonaws.com:aud: !Ref CognitoIdentityPool
              ForAnyValue:StringLike:
                cognito-identity.amazonaws.com:amr: authenticated
      Policies:
        - PolicyName: CognitoUserSignInAuthorizedPolicy
          PolicyDocument:
            Version: 2012-10-17
            Statement:
              - Effect: Allow
                Action:
                  - mobileanalytics:PutEvents
                  - mobiletargeting:PutEvents
                  - cognito-sync:*
                  - cognito-identity:*
                Resource: '*'

  # Assigns the roles to the Identity Pool
  CognitoIdentityPoolRoleAttachment:
    Type: AWS::Cognito::IdentityPoolRoleAttachment
    Properties:
      IdentityPoolId: !Ref CognitoIdentityPool
      Roles:
        unauthenticated: !GetAtt IdentityPoolUnauthorizedIAMRole.Arn
        authenticated: !GetAtt IdentityPoolAuthorizedIAMRole.Arn

When I declare my AWS AppSync template, I add this template that permits users to access my AppSync endpoint when signed in:

AppSyncIAMPolicy:
    Type: AWS::IAM::Policy
    Description: Allow user consume AppSync when signed in
    DependsOn: AppSyncGraphQLApi
    Properties:
      PolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Action:
              - appsync:GraphQL
            Resource:
              - !Join ['/', [!GetAtt AppSyncGraphQLApi.Arn, '*']]
      PolicyName: !Sub ${StackName}-appsync-iam-policy
      Roles:
        - !Sub ${IdentityPoolAuthorizedIAMRoleRef}

My point is: you can attach a policy that allows users not signed (unauthenticated) access AppSync. Instead - !Sub ${IdentityPoolAuthorizedIAMRoleRef} you can in the template above, you can try - !Sub ${IdentityPoolUnauthorizedIAMRoleRef}.

Of course, you can specify which endpoints you allow unauthenticated access, for example:

AppSyncIAMPolicy:
    Type: AWS::IAM::Policy
    Description: Allow user consume AppSync when NOT signed in
    DependsOn: AppSyncGraphQLApi
    Properties:
      PolicyDocument:
        Version: 2012-10-17
        Statement:
          - Effect: Allow
            Action:
              - appsync:GraphQL
            Resource:
              - arn:aws:appsync:us-west-2:123456789012:apis/YourGraphQLApiId/types/Query/fields/<Field-1>
              - arn:aws:appsync:us-west-2:123456789012:apis/YourGraphQLApiId/types/Query/fields/<Field-2>
      PolicyName: !Sub ${StackName}-appsync-iam-policy-unauthenticated
      Roles:
        - !Sub ${IdentityPoolUnauthorizedIAMRoleRef}