open-consul/website/source/docs/agent/services.html.markdown

---
layout: "docs"
page_title: "Service Definition"
sidebar_current: "docs-agent-services"
description: |-
  One of the main goals of service discovery is to provide a catalog of available services. To that end, the agent provides a simple service definition format to declare the availability of a service and to potentially associate it with a health check. A health check is considered to be application level if it associated with a service. A service is defined in a configuration file or added at runtime over the HTTP interface.
---

# Services

One of the main goals of service discovery is to provide a catalog of available
services. To that end, the agent provides a simple service definition format
to declare the availability of a service and to potentially associate it with
a health check. A health check is considered to be application level if it
associated with a service. A service is defined in a configuration file
or added at runtime over the HTTP interface.

## Service Definition

A service definition that is a script looks like:

```javascript
{
  "service": {
    "name": "redis",
    "tags": ["master"],
    "address": "127.0.0.1",
    "port": 8000,
    "enableTagOverride": false,
    "checks": [
      {
        "script": "/usr/local/bin/check_redis.py",
        "interval": "10s"
      }
    ]
  }
}
```

A service definition must include a `name` and may optionally provide
an `id`, `tags`, `address`, `port`, `check`, and `enableTagOverride`.  The `id` is 
set to the `name` if not provided. It is required that all services have a unique 
ID per node, so if names might conflict then unique IDs should be provided.

The `tags` property is a list of values that are opaque to Consul but can be used to
distinguish between "master" or "slave" nodes, different versions, or any other service
level labels.

The `address` field can be used to specify a service-specific IP address. By
default, the IP address of the agent is used, and this does not need to be provided.
The `port` field can be used as well to make a service-oriented architecture
simpler to configure; this way, the address and port of a service can
be discovered.

Services may also contain a `token` field to provide an ACL token. This token is
used for any interaction with the catalog for the service, including
[anti-entropy syncs](/docs/internals/anti-entropy.html) and deregistration.

A service can have an associated health check. This is a powerful feature as
it allows a web balancer to gracefully remove failing nodes, a database
to replace a failed slave, etc. The health check is strongly integrated in
the DNS interface as well. If a service is failing its health check or a
node has any failing system-level check, the DNS interface will omit that
node from any service query.

The check must be of the script, HTTP, TCP or TTL type. If it is a script type,
`script` and `interval` must be provided. If it is a HTTP type, `http` and
`interval` must be provided. If it is a TCP type, `tcp` and `interval` must be
provided. If it is a TTL type, then only `ttl` must be provided. The check name
is automatically generated as `service:<service-id>`. If there are multiple
service checks registered, the ID will be generated as
`service:<service-id>:<num>` where `<num>` is an incrementing number starting
from `1`.

Note: there is more information about [checks here](/docs/agent/checks.html). 

The `enableTagOverride` can optionally specified to disable the antientropy feature for 
this service. If `enableTagOverride` is set to TRUE then external agents can
reregister this service and modify the tags. Subsequent local sync operations
by this agent will ignore the updated tags. For instance: If an external agent
modified both the tags and the port for this service and `enableTagOverride` 
was set to TRUE then after the next sync cycle the service's port would revert 
to the original value but the tags would maintain the updated value. As a 
counter example: If an external agent modified both the tags and port for this 
service and `enableTagOverride` was set to FALSE then after the next sync 
cycle the service's port AND the tags would revert to the original value and
all modifications would be lost. It's important to note that this applies only
to the locally registered service. If you have multiple nodes all registering
the same service their `enableTagOverride` configuration and all other service
configuration items are independant of one another. Updating the tags for
the service registered on one node is independant of the same service (by name)
registered on another node. If `enableTagOverride` is not specified the default 
value is false.  See [anti-entropy syncs](/docs/internals/anti-entropy.html)
for more info.

To configure a service, either provide it as a `-config-file` option to the
agent or place it inside the `-config-dir` of the agent. The file must
end in the ".json" extension to be loaded by Consul. Check definitions can
also be updated by sending a `SIGHUP` to the agent. Alternatively, the
service can be registered dynamically using the [HTTP API](/docs/agent/http.html).

## Multiple Service Definitions

Multiple services definitions can be provided at once using the `services`
(plural) key in your configuration file.

```javascript
{
  "services": [
    {
      "id": "red0",
      "name": "redis",
      "tags": [
        "master"
      ],
      "address": "127.0.0.1",
      "port": 6000,
      "checks": [
        {
          "script": "/bin/check_redis -p 6000",
          "interval": "5s",
          "ttl": "20s"
        }
      ]
    },
    {
      "id": "red1",
      "name": "redis",
      "tags": [
        "delayed",
        "slave"
      ],
      "address": "127.0.0.1",
      "port": 7000,
      "checks": [
        {
          "script": "/bin/check_redis -p 7000",
          "interval": "30s",
          "ttl": "60s"
        }
      ]
    },
    ...
  ]
}
```

## Service and Tag Names with DNS

Consul exposes service definitions and tags over the [DNS](/docs/agent/dns.html)
interface. DNS queries have a strict set of allowed characters and a
well-defined format that Consul cannot override. While it is possible to
register services or tags with names that don't match the conventions, those
services and tags will not be discoverable via the DNS interface. It is
recommended to always use DNS-compliant service and tag names.

DNS-compliant service and tag names may contain any alpha-numeric characters, as
well as dashes. Dots are not supported because Consul internally uses them to
delimit service tags.