» 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", // Deprecated
    "proxy": {
      "destination_service_name": "redis",
      "destination_service_id": "redis1",
      "local_service_name": "127.0.0.1",
      "local_service_port": 9090,
      "config": {},
      "upstreams": []
    }
    "connect": {
      "native": false,
      "sidecar_service": {}
      "proxy": {  // Deprecated
        "command": [],
        "config": {}
      }
    },
    "weights": {
      "passing": 5,
      "warning": 1
    }
  }
}

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

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.

» Connect

The kind field is used to optionally identify the service as a Connect proxy instance with the value connect-proxy. For typical non-proxy instances the kind field must be omitted. The proxy field is also required for Connect proxy registrations and is only valid if kind is connect-proxy. The only required proxy field is destination_service_name. For more detail please see complete proxy configuration example

The connect field can be specified to configure Connect for a service. This field is available in Consul 1.2.0 and later. The native value can be set to true to advertise the service as Connect-native. The sidecar_service field is an optional nested service definition its behavior and defaults are described in Sidecar Service Registration. If native is true, it is an error to also specify a sidecar service registration.

» Checks

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.

There are several check types that have differing required options as documented here. 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.

» DNS SRV Weights

The weights field is an optional field to specify the weight of a service in DNS SRV responses. If this field is not specified, its default value is: "weights": {"passing": 1, "warning": 1}. When a service is critical, it is excluded from DNS responses. Services with warning checks are included in responses by default, but excluded if the optional param only_passing = true is present in agent DNS configuration or ?passing is used via the API.

When DNS SRV requests are made, the response will include the weights specified given the state of the service. This allows some instances to be given higher weight if they have more capacity, and optionally allows reducing load on services with checks in warning status by giving passing instances a higher weight.

» Enable Tag Override and Anti-Entropy

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.

You can optionally disable the anti-entropy feature for this service using the enable_tag_override flag. External agents can modify tags on services in the catalog, so subsequent sync operations can either maintain tag modifications or revert them. If enable_tag_override is set to TRUE, the next sync cycle may revert some service properties, but the tags would maintain the updated value. If enable_tag_override is set to FALSE, the next sync cycle will revert any updated service properties, including tags, to their original value.

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.

» 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.

» Service Definition Parameter Case

For historical reasons Consul's API uses CamelCased parameter names in responses, however it's configuration file uses snake_case for both HCL and JSON representations. For this reason the registration HTTP APIs accept both name styles for service definition parameters although APIs will return the listings using CamelCase.

Note though that all config file formats require snake_case fields. We always document service definition examples using snake_case and JSON since this format works in both config files and API calls.