Developing With Service Discovery

What is Service Discovery?

Service Discovery is a way of letting services discover and connect with each other. Typically, services can only talk to each other if they expose a public endpoint - and even then, requests will have to go over the internet. With ECS Service Discovery each service you create is given a private address and DNS name - meaning each service can talk to each other without ever leaving the local network (VPC) and without exposing a public endpoint.

How Do I use Service Discovery?

Service Discovery is enabled for all services set up using the Copilot CLI. We'll show you how to use it by using an example. Imagine we have an app called kudos and two services, api and front-end.

In this example we'll imagine our front-end service has a public endpoint and wants to call our api service using its service discovery endpoint.

// Calling our api service from the front-end service using Service Discovery
func ServiceDiscoveryGet(w http.ResponseWriter, req *http.Request, ps httprouter.Params) {
    endpoint := fmt.Sprintf("http://api.%s/some-request", os.Getenv("COPILOT_SERVICE_DISCOVERY_ENDPOINT"))
    resp, err := http.Get(endpoint)
    if err != nil {
        http.Error(w, err.Error(), http.StatusInternalServerError)
        return
    }
    defer resp.Body.Close()
    body, _ := ioutil.ReadAll(resp.Body)
    w.WriteHeader(http.StatusOK)
    w.Write(body)
}

The important part is that our front-end service is making a request to our api service through a special endpoint:

endpoint := fmt.Sprintf("http://api.%s/some-request", os.Getenv("COPILOT_SERVICE_DISCOVERY_ENDPOINT"))

COPILOT_SERVICE_DISCOVERY_ENDPOINT is a special environment variable that the Copilot CLI sets for you when it creates your service. It's of the format {app name}.local - so in this case in our kudos app, the request would be to http://api.kudos.local/some-request. In this case our api service is running on port 80, but if it was running on another port, say 8080, we'd need to include the port in the request, as well http://api.kudos.local:8080/some-request.

When our front-end makes this request, the endpoint api.kudos.local resolves to a private IP address and is routed privately within your VPC.