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Watches

Watches are a way of specifying a view of data (list of nodes, KV pairs, health checks, etc) which is monitored for any updates. When an update is detected, an external handler is invoked. A handler can be any executable. As an example, you could watch the status of health checks and notify an external system when a check is critical.

Watches are implemented using blocking queries in the HTTP API. Agent's automatically make the proper API calls to watch for changes, and inform a handler when the data view has updated.

Watches can can be configured as part of the agent's configuration, causing them to run once the agent is initialized. Reloading the agent configuration allows for adding or removing watches dynamically.

Alternatively, the watch command enables a watch to be started outside of the agent. This can be used by an operator to inspect data in Consul, or to easily pipe data into processes without being tied to the agent lifecycle.

In either case, the type of the watch must be specified. Each type of watch supports different parameters, both required and optional. These options are specified in a JSON body when using agent configuration, or as CLI flags for the watch command.

Handlers

The watch specifiation specifies the view of data to be monitored. Once that view is updated the specified handler is invoked. The handler can be any executable.

A handler should read it's input from stdin, and expect to read JSON formatted data. The format of the data depends on the type of the watch. Each watch type documents the format type, and because they map directly to an HTTP API, handlers should expect the input to match the format of the API.

Additionally, the CONSUL_INDEX environmental variable will be set. This maps to the X-Consul-Index value from the HTTP API.

Global Parameters

In addition to the parameters supported by each option type, there are a few global parameters that all watches support:

• datacenter - Can be provided to override the agent's default datacenter.
• token - Can be provided to override the agent's default ACL token.
• handler - The handler to invoke when the data view updates.

Watch Types

The following types are supported, with more documentation below:

• key - Watch a specific KV pair
• keyprefix - Watch a prefix in the KV store
• services - Watch the list of available services
• nodes - Watch the list of nodes
• service- Watch the instances of a service
• checks - Watch the value of health checks

Type: key

The "key" watch type is used to watch a specific key in the KV store. It requires that the "key" parameter be specified.

This maps to the /v1/kv/ API internally.

Here is an example configuration:

{
    "type": "key",
    "key": "foo/bar/baz",
    "handler": "/usr/bin/my-key-handler.sh"
}

Or, using the watch command:

$ consul watch -type key -key foo/bar/baz /usr/bin/my-key-handler.sh

An example of the output of this command:

{
    "Key": "foo/bar/baz",
    "CreateIndex": 1793,
    "ModifyIndex": 1793,
    "LockIndex": 0,
    "Flags": 0,
    "Value": "aGV5",
    "Session": ""
}

Type: keyprefix

The "keyprefix" watch type is used to watch a prefix of keys in the KV store. It requires that the "prefix" parameter be specified.

This maps to the /v1/kv/ API internally.

Here is an example configuration:

{
    "type": "keyprefix",
    "prefix": "foo/",
    "handler": "/usr/bin/my-prefix-handler.sh"
}

Or, using the watch command:

$ consul watch -type keyprefix -prefix foo/ /usr/bin/my-prefix-handler.sh

An example of the output of this command:

[
    {
        "Key": "foo/bar",
        "CreateIndex": 1796,
        "ModifyIndex": 1796,
        "LockIndex": 0,
        "Flags": 0,
        "Value": "TU9BUg==",
        "Session": ""
    },
    {
        "Key": "foo/baz",
        "CreateIndex": 1795,
        "ModifyIndex": 1795,
        "LockIndex": 0,
        "Flags": 0,
        "Value": "YXNkZg==",
        "Session": ""
    },
    {
        "Key": "foo/test",
        "CreateIndex": 1793,
        "ModifyIndex": 1793,
        "LockIndex": 0,
        "Flags": 0,
        "Value": "aGV5",
        "Session": ""
    }
]

Type: services

The "services" watch type is used to watch the list of available services. It has no parameters.

This maps to the /v1/catalog/services API internally.

An example of the output of this command:

{
    "consul": [],
    "redis": [],
    "web": []
}

Type: nodes

The "nodes" watch type is used to watch the list of available nodes. It has no parameters.

This maps to the /v1/catalog/nodes API internally.

An example of the output of this command:

[
    {
        "Node": "nyc1-consul-1",
        "Address": "192.241.159.115"
    },
    {
        "Node": "nyc1-consul-2",
        "Address": "192.241.158.205"
    },
    {
        "Node": "nyc1-consul-3",
        "Address": "198.199.77.133"
    },
    {
        "Node": "nyc1-worker-1",
        "Address": "162.243.162.228"
    },
    {
        "Node": "nyc1-worker-2",
        "Address": "162.243.162.226"
    },
    {
        "Node": "nyc1-worker-3",
        "Address": "162.243.162.229"
    }
]

Type: service

The "service" watch type is used to monitor the providers of a single service. It requires the "service" parameter, but optionally takes "tag" and "passingonly". The "tag" parameter will filter by tag, and "passingonly" is a boolean that will filter to only the instances passing all health checks.

This maps to the /v1/health/service API internally.

Here is an example configuration:

{
    "type": "service",
    "key": "redis",
    "handler": "/usr/bin/my-service-handler.sh"
}

Or, using the watch command:

$ consul watch -type service -service redis /usr/bin/my-service-handler.sh

An example of the output of this command:

[
    {
        "Node": {
            "Node": "foobar",
            "Address": "10.1.10.12"
        },
        "Service": {
            "ID": "redis",
            "Service": "redis",
            "Tags": null,
            "Port": 8000
        },
        "Checks": [
            {
                "Node": "foobar",
                "CheckID": "service:redis",
                "Name": "Service 'redis' check",
                "Status": "passing",
                "Notes": "",
                "Output": "",
                "ServiceID": "redis",
                "ServiceName": "redis"
            },
            {
                "Node": "foobar",
                "CheckID": "serfHealth",
                "Name": "Serf Health Status",
                "Status": "passing",
                "Notes": "",
                "Output": "",
                "ServiceID": "",
                "ServiceName": ""
            }
        ]
    }
]

Type: checks

The "checks" watch type is used to monitor the checks of a given service or in a specific state. It optionally takes the "service" parameter to filter to a specific service, or "state" to filter to a specific state. By default, it will watch all checks.

This maps to the /v1/health/state/ API if monitoring by state, or /v1/health/checks/ if monitoring by service.

An example of the output of this command:

[
    {
        "Node": "foobar",
        "CheckID": "service:redis",
        "Name": "Service 'redis' check",
        "Status": "passing",
        "Notes": "",
        "Output": "",
        "ServiceID": "redis",
        "ServiceName": "redis"
    }
]