This post was written by Michael Hume – AWS Solutions Architect Public Sector UKIR.

Customers often want to use Amazon API Gateway REST APIs to send requests to private resources. This feature is useful for building secure architectures using Amazon EC2 instances or container-based services on Amazon ECS or Amazon EKS, which reside within a VPC.

Private integration is possible for REST APIs by using Network Load Balancers (NLB). However, there may be a requirement for private integration with an Application Load Balancer (ALB) or AWS Cloud Map. This capability is built into Amazon API Gateway HTTP APIs, providing customers with three target options and greater flexibility.

You can configure HTTP APIs with a private integration as the front door or entry point to an application. This enables HTTPS resources within an Amazon VPC to be accessed by clients outside of the VPC. This architecture also provides an application with additional HTTP API features such as throttling, cross-origin resource sharing (CORS), and authorization. These features are then managed by the service instead of an application.

HTTP APIs and Application Load Balancers

In the following architecture, an HTTP APIs endpoint is deployed between the client and private backend resources.

HTTP APIs to ALB example

HTTP APIs to ALB example

A VPC link encapsulates connections between API Gateway and targeted VPC resources. HTTP APIs private integration methods only allow access via a VPC link to private subnets. When a VPC link is created, API Gateway creates and manages the elastic network interfaces in a user account. VPC links are shared across different routes and APIs.

Application Load Balancers can support containerized applications. This allows ECS to select an unused port when scheduling a task and then registers that task with a target group and port. For private integrations, an internal load balancer routes a request to targets using private IP addresses to resources that reside within private subnets. As the Application Load Balancer receives a request from an HTTP APIs endpoint, it looks up the listener rule to identify a protocol and port. A target group then forwards requests to an Amazon ECS cluster, with resources on underlying EC2 instances. Targets are added and removed automatically as traffic to an application changes over time. This increases the availability of an application and provides efficient use of an ECS cluster.

Configuration with an ALB

To configure a private integration with an Application Load Balancer.

  1. Create an HTTP APIs endpoint, choose a route and method, and attach an integration to a route using a private resource.

    Attach integration to route

    Attach integration to route

  2. Provide a target service to send the request to an ALB/NLB.

    Integration details

    Integration details

  3. Add both the load balancer and listener’s Amazon Resource Names (ARNs), together with a VPC link.

    Load balancer settings

    Load balancer settings

HTTP APIs and AWS Cloud Map

Modern applications connect to a broader range of resources. This can become complex to manage as network locations dynamically change based on automatic scaling, versioning, and service disruptions. Its challenging, as each service must quickly find the infrastructure location of the resources it needs. Efficient service discovery of any dynamically changing resources is important for application availability.

If an application scales to hundreds or even thousands of services, then a load balancer may not be appropriate. In this case, HTTP APIs private integration with AWS Cloud Map maybe a better choice. AWS Cloud Map is a resource discovery service that provides a dynamic map of the cloud. It does this by registering application resources such as databases, queues, microservices, and other resources with custom names.

For server-side service discovery, if an application uses a load balancer, it must know the load balancer’s endpoint. This endpoint is used as a proxy, which adds additional latency. As AWS Cloud Map provides client-side service discovery, you can replace the load balancer with a service registry. Now, connections are routed directly to backend resources, instead of being proxied. This involves fewer components, making deployments safer and with less management, and reducing complexity.

Configuration with AWS Cloud Map

HTTP APIs to AWS CloudMap example

HTTP APIs to AWS CloudMap example

In this architecture, the Amazon ECS service has been configured to use Amazon ECS Service Discovery. Service discovery uses the AWS Cloud Map API and Amazon Route 53 to create a namespace. This is a logical name for a group of services. It also creates a service, which is a logical group of resources or instances. In this example, it’s a group of ECS clusters. This allows the service to be discoverable via DNS. These resources work together, to provide a service.

Service discovery configuration

Service discovery configuration

To configure a private integration with AWS Cloud Map:

  1. Create an HTTP API, choose a route and method, and attach an integration to a route using a private resource. This is as shown previously for an Application Load Balancer.
  2. Provide a target service to send requests to resources registered with AWS Cloud Map.

    Target service configuration

    Target service configuration

  3. Add both the namespace, service and VPC link.

    Namespace and VPC configuration

    Namespace and VPC configuration

Deployment

To build the solution in this blog, see the AWS CloudFormation templates in the GitHub repository and, the instructions in the README.md file.

Conclusion

This post discusses the benefits of using API Gateway’s HTTP APIs to access private resources that reside within a VPC, and how HTTP APIs provides three different private integration targets for different use cases.

If a load balancer is required, the application operates at layer 7 (HTTP, HTTPS), requires flexible application management and registering of AWS Lambda functions as targets, then use an Application Load Balancer. However, if the application operates at layer 4 (TCP, UDP, TLS), uses non-HTTP protocols, requires extreme performance and a static IP, then use a Network Load Balancer.

As HTTP APIs private integration methods to both an ALB and NLB only allow access via a VPC link. This enhances security, as resources are isolated within private subnets with no direct access from the internet.

If a service does not need a load balancer, then HTTP APIs provide further private integration flexibility with AWS Cloud Map, which automatically registers resources in a service registry. AWS Cloud Map enables filtering by providing attributes when service discovery is enabled. These can then be used as HTTP APIs integration settings to specify query parameters and filter specific resources.

For more information, watch Happy Little APIs (S2E1): Private integrations with HTTP API.