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

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

# Service Discovery

Nomad schedules workloads of various types across a cluster of generic hosts.
Because of this, placement is not known in advance and you will need to use
service discovery to connect tasks to other services deployed across your
cluster. Nomad integrates with [Consul](https://consul.io) to provide service
discovery and monitoring.

Note that in order to use Consul with Nomad, you will need to configure and
install Consul on your nodes alongside Nomad, or schedule it as a system job.
Nomad does not currently run Consul for you.

## Configuration

* `consul.address`: This is a Nomad client configuration 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 Definition 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 allow registering multiple services for a task that exposes multiple ports.

### 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"
            }
        }
        resources {
            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)-$(task-name). Users can explicitly
  name the service by specifying this option. If multiple services are defined
  for a Task then only one task can have the default name, all the services have 
  to be explicitly named. Nomad will add the prefix ```$(job-name)-${task-group}-${task-name}``` 
  prefix to each user defined name.

* `tags`: A list of tags associated with this Service.

* `port`: The port indicates the port associated with the Service. Users are
  required 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.
  Multiple 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.