» 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 is associated with a service. A service is defined in a configuration file or added at runtime over the HTTP interface.

» Service Definition

To configure a service, either provide the service definition 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 or .hcl extension to be loaded by Consul. Check definitions can be updated by sending a SIGHUP to the agent. Alternatively, the service can be registered dynamically using the HTTP API.

A service definition is a configuration that looks like the following. This example shows all possible fields, but note that only a few are required.

{
  "service": {
    "name": "redis",
    "tags": ["primary"],
    "address": "",
    "meta": {
      "meta": "for my service"
    },
    "port": 8000,
    "enable_tag_override": false,
    "checks": [
      {
        "args": ["/usr/local/bin/check_redis.py"],
        "interval": "10s"
      }
    ],
    "kind": "connect-proxy",
    "proxy_destination": "redis",
    "connect": {
      "native": false,
      "proxy": {
        "command": [],
        "config": {}
      }
    }
  }
}

A service definition must include a name and may optionally provide an id, tags, address, port, check, meta and enable_tag_override. 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.

For Consul 0.9.3 and earlier you need to use enableTagOverride. Consul 1.0 supports both enable_tag_override and enableTagOverride but the latter is deprecated and has been removed in Consul 1.1.

The tags property is a list of values that are opaque to Consul but can be used to distinguish between primary or secondary 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.

The meta object is a map of max 64 key/values with string semantics. Key can contain only ASCII chars and no special characters (A-Z a-z 0-9 _ and -). For performance and security reasons, values as well as keys are limited to 128 characters for keys, 512 for values. This object has the same limitations as the node meta object in node definition. All those meta data can be retrieved individually per instance of the service and all the instances of a given service have their own copy of it.

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 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 secondary, 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, args 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.

The enable_tag_override can optionally be specified to disable the anti-entropy feature for this service. If enable_tag_override is set to TRUE then external agents can update this service in the catalog and modify the tags. Subsequent local sync operations by this agent will ignore the updated tags. For example, if an external agent modified both the tags and the port for this service and enable_tag_override 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 enable_tag_override 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 enable_tag_override configuration and all other service configuration items are independent of one another. Updating the tags for the service registered on one node is independent of the same service (by name) registered on another node. If enable_tag_override is not specified the default value is false. See anti-entropy syncs for more info.

For Consul 0.9.3 and earlier you need to use enableTagOverride. Consul 1.0 supports both enable_tag_override and enableTagOverride but the latter is deprecated and has been removed as of Consul 1.1.

The kind field is used to optionally identify the service as an unmanaged Connect proxy instance with the value connect-proxy. For typical non-proxy instances the kind field must be omitted. The proxy_destination field is also required for unmanaged proxy registrations and is only valid if kind is connect-proxy. It's value must be the name of the service that the proxy is handling traffic for.

The connect field can be specified to configure Connect for a service. This field is available in Consul 1.2 and later. The native value can be set to true to advertise the service as Connect-native. If the proxy field is set (even to an empty object), then this will enable a managed proxy for the service. The fields within proxy are used to configure the proxy and are specified in the proxy docs. If native is true, it is an error to also specifiy a managed proxy instance.

» Multiple Service Definitions

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

{
  "services": [
    {
      "id": "red0",
      "name": "redis",
      "tags": [
        "primary"
      ],
      "address": "",
      "port": 6000,
      "checks": [
        {
          "args": ["/bin/check_redis", "-p", "6000"],
          "interval": "5s",
          "ttl": "20s"
        }
      ]
    },
    {
      "id": "red1",
      "name": "redis",
      "tags": [
        "delayed",
        "secondary"
      ],
      "address": "",
      "port": 7000,
      "checks": [
        {
          "args": ["/bin/check_redis", "-p", "7000"],
          "interval": "30s",
          "ttl": "60s"
        }
      ]
    },
    ...
  ]
}

» Service and Tag Names with DNS

Consul exposes service definitions and tags over the DNS 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.