Basic WebSocket mock in AWS ApiGateway using CloudFormation in AWS

aws-websockets-api-gw-mock.yaml

AWSTemplateFormatVersion: '2010-09-09'
Description: |
  AWS CloudFormation template for Mock WebSocket API Gateway. When deploying this stack please remember to check the option:
   - I acknowledge that AWS CloudFormation might create IAM resources.
  This template can help you to solve issues like:
  - CloudWatch Logs role ARN must be set in account settings to enable logging
  - Execution failed due to configuration error: statusCode should be an integer which defined in request template
  - This custom domain name cannot map to WEBSOCKET protocol APIs
  - Error during WebSocket handshake: Unexpected response code: 500

Resources:
  ApiGwAccountConfig:
    Type: "AWS::ApiGateway::Account"
    Properties:
      CloudWatchRoleArn: !GetAtt "ApiGatewayLoggingRole.Arn"
  ApiGatewayLoggingRole:
    Type: "AWS::IAM::Role"
    Properties:
      AssumeRolePolicyDocument:
        Version: "2012-10-17"
        Statement:
          - Effect: Allow
            Principal:
              Service:
                - "apigateway.amazonaws.com"
            Action: "sts:AssumeRole"
      Path: "/"
      ManagedPolicyArns:
        - !Sub "arn:${AWS::Partition}:iam::aws:policy/service-role/AmazonAPIGatewayPushToCloudWatchLogs"

  WebSocketMock:
    Type: AWS::ApiGatewayV2::Api
    Properties:
      Name: WebSocketMock
      ProtocolType: WEBSOCKET
      RouteSelectionExpression: "$request.body.message"
      Description: "Mock WebSocket API Gateway."
  
  ConnectRoute:
    Type: AWS::ApiGatewayV2::Route
    Properties:
      ApiId: !Ref WebSocketMock
      RouteKey: $connect
      RouteResponseSelectionExpression: '$default'
      AuthorizationType: NONE
      ApiKeyRequired: false
      OperationName: ConnectRoute
      Target: !Join
        - '/'
        - - 'integrations'
          - !Ref ConnectInteg
  
  ConnectInteg:
    Type: AWS::ApiGatewayV2::Integration
    Properties:
      ApiId: !Ref WebSocketMock
      Description: Connect Integration
      IntegrationType: MOCK
      RequestTemplates: 
        "200" : '{"statusCode" : 200}'
      TemplateSelectionExpression: '200'
      PassthroughBehavior: 'WHEN_NO_MATCH'
  
  ConnectIntegResp:
    Type: AWS::ApiGatewayV2::IntegrationResponse
    Properties:
      ApiId: !Ref WebSocketMock
      IntegrationId: !Ref ConnectInteg
      IntegrationResponseKey: '$default'
      ResponseTemplates:
        "200" : '{"statusCode" : 200}'
  
  ConnectRouteResponse:
    Type: AWS::ApiGatewayV2::RouteResponse
    Properties:
      RouteId: !Ref ConnectRoute
      ApiId: !Ref WebSocketMock
      RouteResponseKey: $default

  DisconnectRoute:
    Type: AWS::ApiGatewayV2::Route
    Properties:
      ApiId: !Ref WebSocketMock
      RouteKey: $disconnect
      RouteResponseSelectionExpression: '$default'
      AuthorizationType: NONE
      OperationName: DisconnectRoute
      Target: !Join
        - '/'
        - - 'integrations'
          - !Ref DisconnectInteg
  
  DisconnectInteg:
    Type: AWS::ApiGatewayV2::Integration
    Properties:
      ApiId: !Ref WebSocketMock
      Description: Disconnect Integration
      IntegrationType: MOCK
      RequestTemplates: 
        "200" : '{"statusCode" : 200}'
      TemplateSelectionExpression: '200'
      PassthroughBehavior: 'WHEN_NO_MATCH'

  DisconnectIntegResp:
    Type: AWS::ApiGatewayV2::IntegrationResponse
    Properties:
      ApiId: !Ref WebSocketMock
      IntegrationId: !Ref DisconnectInteg
      IntegrationResponseKey: '$default'
      ResponseTemplates:
        "200" : '{"statusCode" : 200}'
  
  DisconnectRouteResponse:
    Type: AWS::ApiGatewayV2::RouteResponse
    Properties:
      RouteId: !Ref DisconnectRoute
      ApiId: !Ref WebSocketMock
      RouteResponseKey: $default
  
  DefaultRoute:
    Type: AWS::ApiGatewayV2::Route
    Properties:
      ApiId: !Ref WebSocketMock
      RouteKey: $default
      RouteResponseSelectionExpression: '$default'
      AuthorizationType: NONE
      OperationName: DefaultRoute
      Target: !Join
        - '/'
        - - 'integrations'
          - !Ref DefaultInteg
  
  DefaultInteg:
    Type: AWS::ApiGatewayV2::Integration
    Properties:
      ApiId: !Ref WebSocketMock
      Description: Default Integration
      IntegrationType: MOCK
      RequestTemplates: 
        "200" : '{"statusCode" : 200}'
      TemplateSelectionExpression: '200'

  DefaultIntegResp:
    Type: AWS::ApiGatewayV2::IntegrationResponse
    Properties:
      ApiId: !Ref WebSocketMock
      IntegrationId: !Ref DefaultInteg
      IntegrationResponseKey: $default
      ResponseTemplates: 
        "200" : '{"statusCode" : 200, "connectionId" : "$context.connectionId"}'
      TemplateSelectionExpression: '200'
  
  DefaultRouteResponse:
    Type: AWS::ApiGatewayV2::RouteResponse
    Properties:
      RouteId: !Ref DefaultRoute
      ApiId: !Ref WebSocketMock
      RouteResponseKey: $default

  Deployment:
    Type: AWS::ApiGatewayV2::Deployment
    DependsOn:
    - ConnectRoute
    - DisconnectRoute
    - DefaultRoute
    Properties:
      ApiId: !Ref WebSocketMock

  Stage:
    Type: AWS::ApiGatewayV2::Stage
    Properties:
      StageName: LATEST
      Description: One and only Stage
      DeploymentId: !Ref Deployment
      ApiId: !Ref WebSocketMock
      DefaultRouteSettings:
        DetailedMetricsEnabled: true
        LoggingLevel: INFO
  

I'm not sure if I'm allowed to comment on this post, but man, yours is the only article or bit of information that I could find in days of searching to figure out how to implement these mock responses to routes. I can't tell you how much I appreciate it.

I'm happy that helped!
There is also a thread here about it with more context.

Dude, that is perfect - I did have just syntax issues in those templates that caused my mock requests to fail, so I ran your script above and just verified how those templates should work. Thanks again!

That is great! Thank you!

I 2nd @adamwatkins sentiment. This saved me a lot of time, thanks @maatthc!

For anybody who stumbles across this and needs the CDK version for the mock.

///
//    Usage:  `new MockPing(this, "mockPing", { routeKey: "ping", sockapi: YOUR_WEB_SOCKET});`
//
export class MockPing extends Construct {
  constructor(scope: Construct, id: string, { sockapi, routeKey }: { routeKey: string; sockapi: WebSocketApi }) {
    super(scope, id);

    const intgration = new cdk.aws_apigatewayv2.CfnIntegration(this, "integration", {
      apiId: sockapi.apiId,
      integrationType: "MOCK",
      requestTemplates: {
        "200": '{"statusCode":200}',
      },
      templateSelectionExpression: "200",
      passthroughBehavior: "WHEN_NO_MATCH",
    });
    const route = new cdk.aws_apigatewayv2.CfnRoute(this, "route", {
      apiId: sockapi.apiId,
      routeKey,
      routeResponseSelectionExpression: "$default",
      operationName: "pingRoute",
      target: new cdk.StringConcat().join("integrations/", intgration.ref),
    });
    new cdk.aws_apigatewayv2.CfnIntegrationResponse(this, "response", {
      apiId: sockapi.apiId,
      integrationId: intgration.ref,
      integrationResponseKey: "/200/",
      responseTemplates: {
        "200": '{"statusCode": 200 }',
      },
    });
    new cdk.aws_apigatewayv2.CfnRouteResponse(this, "routeResponse", {
      apiId: sockapi.apiId,
      routeId: route.ref,
      routeResponseKey: "$default",
    });
   }
}

Thanks @maatthc! This Gist helped answering a lot of questions I had when trying to add Response Integration and Route