Request validation for WebSocket APIs in API Gateway - Amazon API Gateway

Request validation for WebSocket APIs in API Gateway

You can configure API Gateway to perform validation on a route request before proceeding with the integration request. If the validation fails, API Gateway fails the request without calling your backend, sends a "Bad request body" gateway response to the client, and publishes the validation results in CloudWatch Logs. Using validation this way reduces unnecessary calls to your API backend.

Model selection expressions

You can use a model selection expression to dynamically validate requests within the same route. Model validation occurs if you provide a model selection expression for either proxy or non-proxy integrations. You might need to define the $default model as a fallback when no matching model is found. If there is no matching model and $default isn't defined, the validation fails. The selection expression looks like Route.ModelSelectionExpression and evaluates to the key for Route.RequestModels.

When you define a route for a WebSocket API, you can optionally specify a model selection expression. This expression is evaluated to select the model to be used for body validation when a request is received. The expression evaluates to one of the entries in a route's requestmodels.

A model is expressed as a JSON schema and describes the data structure of the request body. The nature of this selection expression enables you to dynamically choose the model to validate against at runtime for a particular route. For information about how to create a model, see Data models for REST APIs.

Set up request validation using the API Gateway console

The following example shows you how to set up request validation on a route.

First, you create a model, and then you create a route. Next, you configure request validation on the route you just created. Lastly, you deploy and test your API. To complete this tutorial, you need a WebSocket API with $request.body.action as the route selection expression and an integration endpoint for your new route.

You also need wscat to connect to your API. For more information, see Use wscat to connect to a WebSocket API and send messages to it.

To create a model
  1. Sign in to the API Gateway console at

  2. Choose a WebSocket API.

  3. In the main navigation pane, choose Models.

  4. Choose Create model.

  5. For Name, enter emailModel.

  6. For Content type, enter application/json.

  7. For Model schema, enter the following model:

    { "$schema": "", "type" : "object", "required" : [ "address"], "properties" : { "address": { "type": "string" } } }

    This model requires that the request contains an email address.

  8. Choose Save.

In this step, you create a route for your WebSocket API.

To create a route
  1. In the main navigation pane, choose Routes.

  2. Choose Create route.

  3. For Route key, enter sendMessage.

  4. Choose an integration type and specify an integration endpoint. For more information see Integrations for WebSocket API in API Gateway.

  5. Choose Create route.

In this step, you set up request validation for the sendMessage route.

To set up request validation
  1. On the Route request tab, under Route request settings, choose Edit.

  2. For Model selection expression, enter ${request.body.messageType}.

    API Gateway uses the messageType property to validate the incoming request.

  3. Choose Add request model.

  4. For Model key, enter email.

  5. For Model, choose emailModel.

    API Gateway validates incoming messages with the messageType property set to email against this model.


    If API Gateway can't match the model selection expression to a model key, then it selects the $default model. If there is no $default model, then the validation fails. For production APIs, we recommend that you create a $default model.

  6. Choose Save changes.

In this step, you deploy and test your API.

To deploy and test your API
  1. Choose Deploy API.

  2. Choose the desired stage from the dropdown list or enter the name of a new stage.

  3. Choose Deploy.

  4. In the main navigation pane, choose Stages.

  5. Copy your API's WebSocket URL. The URL should look like wss://

  6. Open a new terminal and run the wscat command with the following parameters.

    wscat -c wss://
    Connected (press CTRL+C to quit)
  7. Use the following command to test your API.

    {"action": "sendMessage", "messageType": "email"}
    {"message": "Invalid request body", "connectionId":"ABCD1=234", "requestId":"EFGH="}

    API Gateway will fail the request.

    Use the next command to send a valid request to your API.

    {"action": "sendMessage", "messageType": "email", "address": ""}