Service Discovery Client Configuration

Intro#

Semaphore can resolve service address not only through DNS, but using a specific service discovery server such as Consul by HashiCorp.

By default, Semaphore resolves service hosts through the default DNS performing a regular query with no actions from the Semaphore side. We can set up several service discovery configurations and use them as address resolvers in every service independently.

Defining clients#

To configure a service discovery client, we use discovery block:

discovery "consul" {
  address = "http://localhost:8500"
}

By default, the block label ("consul" in the example above) defines not only the name of the configuration, but the provider type as well. We still can define a configuration with a custom name, using provider field to tell Semaphore what adapter it should use:

discovery "production-1" {
  address = "http://localhost:8500"
  provider = "consul"
}

Semaphore supports defining several discovery clients what might be useful in some rare cases:

discovery "production-old" {
  address = "http://production-old:850"
  provider = "consul"
}

discovery "production-new" {
  address = "http://production-new:850"
  provider = "consul"
}

// and so on

Using discovery clients#

To use a discovery service, we should set resolver field in service configuration, referencing to the discovery client name:

service "com.semaphore" "awesome-dogs" {
  transport = "http"
  codec     = "json"
  host      = "http://awesome-dogs"
  resolver  = "consul" // or "production-old"

  method "List" {
    response = "com.semaphore.Dogs"
    request = "com.semaphore.Void"

    options {
      endpoint = "/"
      method = "GET"
    }
  }
}

When we set resolver property, Semaphore uses host property as a service name and scheme. But the port should be received from the Service Discovery response. In this case, host will be parsed, "awesome-dogs" will be used as the service name, and "http://" will be used as the scheme for the resolved address.

Let's assume, Consul returns "192.168.1.15" as the resolved address, and "8080" as the service port. The service URL will be http://192.168.1.15:8080.

The another way to set a resolver for a service is using service selectors:

services {
    select "com.semaphore.*" {
        resolver = "consul"
    }
}

Example#

You can find an example in Semaphore repositories, under examples/consul directory.