When a client needs to consume multiple services, setting up a separate endpoint for each service and having the client manage each endpoint can be challenging. For example, an e-commerce application might provide services such as search, reviews, cart, checkout, and order history. Each service has a different API that the client must interact with, and the client must know about each endpoint in order to connect to the services. If an API changes, the client must be updated as well. If you refactor a service into two or more separate services, the code must change in both the service and the client.

Place a gateway in front of a set of applications, services, or deployments. Use application Layer 7 routing to route the request to the appropriate instances.

With this pattern, the client application only needs to know about and communicate with a single endpoint. If a service is consolidated or decomposed, the client does not necessarily require updating. It can continue making requests to the gateway, and only the routing changes.

A gateway also lets you abstract backend services from the clients, allowing you to keep client calls simple while enabling changes in the backend services behind the gateway. Client calls can be routed to whatever service or services need to handle the expected client behavior, allowing you to add, split, and reorganize services behind the gateway without changing the client.

This pattern can also help with deployment, by allowing you to manage how updates are rolled out to users. When a new version of your service is deployed, it can be deployed in parallel with the existing version. Routing lets you control what version of the service is presented to the clients, giving you the flexibility to use various release strategies, whether incremental, parallel, or complete rollouts of updates. Any issues discovered after the new service is deployed can be quickly reverted by making a configuration change at the gateway, without affecting clients.

• The gateway service may introduce a single point of failure. Ensure it is properly designed to meet your availability requirements. Consider resiliency and fault tolerance capabilities when implementing.
• The gateway service may introduce a bottleneck. Ensure the gateway has adequate performance to handle load and can easily scale in line with your growth expectations.
• Perform load testing against the gateway to ensure you don't introduce cascading failures for services.
• Gateway routing is level 7. It can be based on IP, port, header, or URL.

Use this pattern when:

• A client needs to consume multiple services that can be accessed behind a gateway.
• You wish to simplify client applications by using a single endpoint.
• You need to route requests from externally addressable endpoints to internal virtual endpoints, such as exposing ports on a VM to cluster virtual IP addresses.

This pattern may not be suitable when you have a simple application that uses only one or two services.