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

---
layout: "docs"
page_title: "Service Discovery in Nomad"
sidebar_current: "docs-jobspec-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://www.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`.

* `consul.token`: Token is used to provide a per-request ACL token.This options
  overrides the agent's default token

* `consul.auth`: The auth information to use for http access to the Consul
  Agent.

* `consul.ssl`: This boolean option sets the transport scheme to talk to the Consul
  Agent as `https`. This option is unset by default and so the default transport
  scheme for the consul api client is `http`.

* `consul.verifyssl`: This option enables SSL verification when the transport
 scheme for the Consul API client is `https`. This is set to true by default.


## Service Definition Syntax

The service block 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"
                interval = "10s"
                timeout = "2s"
            }
            check {
                type = "script"
                name = "check_table"
                cmd = "/usr/local/bin/check_mysql_table_status"
                agrs = ["--verbose"]
                interval = "60s"
                timeout = "5s"
            }
        }
        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)-$(task-name)`. Users can
  explicitly name the service by specifying this option. If multiple services
  are defined for a Task then only one task can have the default name, all
  the services have to be explicitly named.  Users can add the following to
  the service names: `${JOB}`, `${TASKGROUP}`, `${TASK}`, `${BASE}`.  Nomad
  will replace them with the appropriate value of the Job, Task Group, and
  Task names while registering the Job. `${BASE}` expands to
  `${JOB}-${TASKGROUP}-${TASK}`.  Names must be adhere to
  [RFC-1123 §2.1](https://tools.ietf.org/html/rfc1123#section-2) and are
  limited to alphanumeric and hyphen characters (i.e. `[a-z0-9\-]`), and be
  less than 64 characters in length.

* `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 supports the `script`,
  `http` and `tcp` Consul Checks. Script checks are not supported for the qemu
  driver since the Nomad client doesn't have access to the file system of a
  tasks using the Qemu driver.

### 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.

* `interval`: This indicates the frequency of the health checks that Consul will
  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`. We default it to `http`

* `cmd`: This is the command that the Nomad client runs for doing script based
  health check.

* `args`: Additional arguments to the `cmd` for script based health checks.

## Assumptions

* Consul 0.6.4 or later 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 need 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 in the network namespace of the container.