open-nomad/website/source/docs/jobspec/servicediscovery.html.md

layout	page_title	sidebar_current	description
docs	Service Discovery in Nomad	docs-service-discovery	Learn how to add service discovery to jobs

Service Discovery

Nomad enables dynamic scheduling on compute resources to run services at scale on compute nodes. Size of an application cluster varies depending on volume of traffic, health of services, etc thereby the network topology of services are constanly changing. This introduces the challenge of discovery of services, Nomad solves this problem by integrating with Consul to provide service discovery and health checks of services.

Operators have to run Consul agents on a Nomad compute node. Nomad also makes running an application like Consul agent on every single node in a data centre simple by providing system jobs. Nomad connects to Consul on it's default http port but operators can override it.

Configuration

• consul.address: This is a Nomad client config which can be used to override the default Consul Agent HTTP port that Nomad uses to connect to Consul. The default for this is "127.0.0.1:8500"

Service Deginition Syntax

The service blocks in a Task definition defines a service which Nomad will register with Consul. Multiple Service blocks are allowed in a Task definition, which makes it usable for cases when an application listens on multiple ports each exposing a specific service.

Example

A brief example of a service definition in a Task

group "database" {
    task "mysql" {
        driver = "docker"
        service {
            tags = ["master", "mysql"]
            port = "db"
            check {
                type = "tcp"
                delay = "10s"
                timeout = "2s"
            }
        }
        reources {
            cpu = 500
            memory = 1024
            network {
                mbits = 10
                port "db" {
                }
            }
        }
    }
}

• name: Nomad automatically determines the name of a Task. By default the name of a service is $(job-name)-$(task-group)-$(task-name). Users can explictly name the service by specifying this option. If multiple services are defined for a Task then all the services have to be explictly named. Nomad will add the prefix $(job-name)-${task-group}-${task-name} prefix to each user defined names.

• tags: A list of tags associated with this Service.

• port: The port indicates the port associated with the Service. Users are reruired to specify a valid port label here which they have defined in the resources block. This could be a label to either a dynamic or a static port. If an incorrect port label is specified, Nomad doesn't register the service with Consul.

• check: A check block defines a health check associated with the service. Mutliple check blocks are allowed for a service. Nomad currently supports only the http and tcp Consul Checks.

Check Syntax

• type: This indicates the check types supported by Nomad. Valid options are currently http and tcp. In the future Nomad will add support for more Consul checks.

• delay: This indicates the frequency of the health checks that Consul with perform.

• timeout: This indicates how long Consul will wait for a health check query to succeed.

• path: The path of the http endpoint which Consul will query to query the health of a service if the type of the check is http. Nomad will add the ip of the service and the port, users are only required to add the relative url of the health check endpoint.

• protocol: This indicates the protocol for the http checks. Valid options are http and https.

Assumptions

• Consul 0.6 is needed for using the TCP checks.

• The Service Discovery feature in Nomad depends on Operators making sure that the Nomad client can reach the consul agent.

• Nomad assumes that it controls the life cycle of all the externally discoverable services running on a host.

• Tasks running inside Nomad also needs to reach out to the Consul agent if they want to use any of the Consul APIs. Ex: A task running inside a docker container in the bridge mode won't be able to talk to a Consul Agent running on the loopback interface of the host since the container in the bridge mode has it's own network interface and doesn't see interfaces on the global network namespace of the host. There are a couple of ways to solve this, one way is to run the container in the host networking mode, or make the Consul agent listen on an interface on the network namespace of the container.