Setting up an Api Gateway Proxy Resource using Cloudformation

I just spent the better part of a day trying to figure out how to do something as seemingly simple as configuring an AWS Api Gateway catch-all endpoint to proxy to another HTTP service. Amazon has documentation detailing how to do this. Using the console. Using the console is great for learning how stuff works, but it is not great for creating reproducible production environments. So I wanted to achieve this with Cloudformation. Which wasn't all that straight-forward.

Like most other things, solving said problem really is quite straight-forward once you know how. My Googlings where unsuccessful in this area, so I'll detail how here, in case someone out there has the same problem.

The REST API

In the interest of providing a complete, working configuration, I'll include all the required parts. If you already have an API configuration, you'll probably be most interested in the AWS::ApiGateway::Method resource.

At the top of the mountain is a Rest API:

  Api:
    Type: 'AWS::ApiGateway::RestApi'
    Properties:
      Name: MyProxyAPI

The proxy resource

In order to proxy all paths to the API, you need two resources: the root resource, and a catch-all resource. Well, a catch-almost-all anyway, since the catch-all does not catch the root resource. The root resource is created for you, the proxy resource is not:

  Resource:
    Type: 'AWS::ApiGateway::Resource'
    Properties:
      ParentId: !GetAtt Api.RootResourceId
      RestApiId: !Ref Api
      PathPart: '{proxy+}'

As you can see, this resource references the Api.RootResourceId as its parent. The path part {proxy+} is a greedy match for any path. If you wanted to only match requests under e.g. /blog/*, you'd have to define two resources:

  BlogResource:
    Type: 'AWS::ApiGateway::Resource'
    Properties:
      ParentId: !GetAtt Api.RootResourceId
      RestApiId: !Ref Api
      PathPart: 'blog'

  Resource:
    Type: 'AWS::ApiGateway::Resource'
    Properties:
      ParentId: !Ref BlogResource
      RestApiId: !Ref Api
      PathPart: '{proxy+}'

The methods

Next up we'll configure the methods. As I want to proxy everything, I just define one ANY method for each resource.

The root resource is a 1:1 from the root path to the root path on your proxy target. For this example, we're proxying to an imaginary S3 bucket website:

  RootMethod:
    Type: 'AWS::ApiGateway::Method'
    Properties:
      HttpMethod: ANY
      ResourceId: !GetAtt Api.RootResourceId
      RestApiId: !Ref Api
      AuthorizationType: NONE
      Integration:
        IntegrationHttpMethod: ANY
        Type: HTTP_PROXY
        Uri: http://my-imaginary-bucket.s3-website-eu-west-1.amazonaws.com/
        PassthroughBehavior: WHEN_NO_MATCH
        IntegrationResponses:
          - StatusCode: 200

Next up is the proxy resource method, and this is what took me an embarrassing amount of time to figure out.

  ProxyMethod:
    Type: 'AWS::ApiGateway::Method'
    Properties:
      HttpMethod: ANY
      ResourceId: !Ref Resource
      RestApiId: !Ref Api
      AuthorizationType: NONE
      RequestParameters:
        method.request.path.proxy: true
      Integration:
        CacheKeyParameters:
          - 'method.request.path.proxy'
        RequestParameters:
          integration.request.path.proxy: 'method.request.path.proxy'
        IntegrationHttpMethod: ANY
        Type: HTTP_PROXY
        Uri: http://my-imaginary-bucket.s3-website-eu-west-1.amazonaws.com/{proxy}
        PassthroughBehavior: WHEN_NO_MATCH
        IntegrationResponses:
          - StatusCode: 200

Let's discuss the key components of this. First of all, setting the resource path to {proxy+} is not enough to be able to use this in the target URL. You also need to specify RequestParameters to state that it is OK to use the proxy parameter from the path in the integration configuration.

As if that wasn't enough, you also have to inform Cloudformation of how you will access the proxy parameter in your integration request path, by specifying Integration.RequestParameters. It is a map of parameters from the method request to parameters in the integration request.

Those two bits are crucial, because now we can finally use {proxy} to insert the proxied path in our integration uri.

Deployment

In order to use the API you need a deployment. Because the deployment does not have a direct dependency on either of the methods, and because we cannot deploy an API with no methods, we use DependsOn to help Cloudformation figure out the order of things:

  Deployment:
    DependsOn:
      - RootMethod
      - ProxyMethod
    Type: 'AWS::ApiGateway::Deployment'
    Properties:
      RestApiId: !Ref Api
      StageName: dev

Choose a stage name of your liking.

The whole shebang

That's all there is to it. Doesn't look very hard when you know what to do.

AWSTemplateFormatVersion: 2010-09-09
Description: An API that proxies requests to another HTTP endpoint

Resources:
  Api:
    Type: 'AWS::ApiGateway::RestApi'
    Properties:
      Name: SomeProxyApi

  Resource:
    Type: 'AWS::ApiGateway::Resource'
    Properties:
      ParentId: !GetAtt Api.RootResourceId
      RestApiId: !Ref Api
      PathPart: '{proxy+}'

  RootMethod:
    Type: 'AWS::ApiGateway::Method'
    Properties:
      HttpMethod: ANY
      ResourceId: !GetAtt Api.RootResourceId
      RestApiId: !Ref Api
      AuthorizationType: NONE
      Integration:
        IntegrationHttpMethod: ANY
        Type: HTTP_PROXY
        Uri: http://my-imaginary-bucket.s3-website-eu-west-1.amazonaws.com/
        PassthroughBehavior: WHEN_NO_MATCH
        IntegrationResponses:
          - StatusCode: 200

  ProxyMethod:
    Type: 'AWS::ApiGateway::Method'
    Properties:
      HttpMethod: ANY
      ResourceId: !Ref Resource
      RestApiId: !Ref Api
      AuthorizationType: NONE
      RequestParameters:
        method.request.path.proxy: true
      Integration:
        CacheKeyParameters:
          - 'method.request.path.proxy'
        RequestParameters:
          integration.request.path.proxy: 'method.request.path.proxy'
        IntegrationHttpMethod: ANY
        Type: HTTP_PROXY
        Uri: http://my-imaginary-bucket.s3-website-eu-west-1.amazonaws.com/{proxy}
        PassthroughBehavior: WHEN_NO_MATCH
        IntegrationResponses:
          - StatusCode: 200

  Deployment:
    DependsOn:
      - RootMethod
      - ProxyMethod
    Type: 'AWS::ApiGateway::Deployment'
    Properties:
      RestApiId: !Ref Api
      StageName: !Ref StageName