---
layout: "docs"
page_title: "Check Definition"
sidebar_current: "docs-agent-checks"
description: |-
  One of the primary roles of the agent is management of system- and application-level health checks. A health check is considered to be application-level if it is associated with a service. A check is defined in a configuration file or added at runtime over the HTTP interface.
---

# Checks

One of the primary roles of the agent is management of system-level and application-level health
checks. A health check is considered to be application-level if it is associated with a
service. If not associated with a service, the check monitors the health of the entire node.

A check is defined in a configuration file or added at runtime over the HTTP interface.  Checks
created via the HTTP interface persist with that node.

There are three different kinds of checks:

 * Script + Interval - These checks depend on invoking an external application
 that performs the health check, exits with an appropriate exit code, and potentially
 generates some output. A script is paired with an invocation interval (e.g.
 every 30 seconds). This is similar to the Nagios plugin system.

 * HTTP + Interval - These checks make an HTTP `GET` request every Interval (e.g.
 every 30 seconds) to the specified URL. The status of the service depends on the HTTP response code:
 any `2xx` code is considered passing, a `429 Too Many Requests` is a warning, and anything else is a failure.
 This type of check should be preferred over a script that uses `curl` or another external process
 to check a simple HTTP operation.

 * Time to Live (TTL) - These checks retain their last known state for a given TTL.
 The state of the check must be updated periodically over the HTTP interface. If an
 external system fails to update the status within a given TTL, the check is
 set to the failed state. This mechanism is used to allow an application to
 directly report its health. For example, a healthy web app can periodically `PUT` a status
 update to the HTTP endpoint; if the app fails, the TTL will expire and the health check
 enters a critical state. This is conceptually similar to a dead man's switch.

## Check Definition

A script check:

```javascript
{
  "check": {
    "id": "mem-util",
    "name": "Memory utilization",
    "script": "/usr/local/bin/check_mem.py",
    "interval": "10s"
  }
}
```

A HTTP check:

```javascript
{
  "check": {
    "id": "api",
    "name": "HTTP API on port 5000",
    "http": "http://localhost:5000/health",
    "interval": "10s"
  }
}
```

A TTL check:

```javascript
{
  "check": {
    "id": "web-app",
    "name": "Web App Status",
    "notes": "Web app does a curl internally every 10 seconds",
    "ttl": "30s"
  }
}
```

Each type of definition must include a `name` and may optionally
provide an `id` and `notes` field. The `id` is set to the `name` if not
provided. It is required that all checks have a unique ID per node: if names
might conflict, unique IDs should be provided.


The `notes` field is opaque to Consul but can be used to provide a human-readable
descriptions. With a script check, the field is set to any output generated by the
script. Similarly, an external process updating a TTL check via the HTTP interface
can set the `notes` value.

To configure a check, 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
check can be registered dynamically using the [HTTP API](/docs/agent/http.html).

## Check Scripts

A check script is generally free to do anything to determine the status
of the check. The only limitations placed are that the exit codes must obey
this convention:

 * Exit code 0 - Check is passing
 * Exit code 1 - Check is warning
 * Any other code - Check is failing

This is the only convention that Consul depends on. Any output of the script
will be captured and stored in the `notes` field so that it can be viewed
by human operators.

## Service-bound checks

Health checks may optionally be bound to a specific service. This ensures
that the status of the health check will only affect the health status of the
given service instead of the entire node. Service-bound health checks may be
provided by adding a `service_id` field to a check configuration:

```javascript
{
  "check": {
    "id": "web-app",
    "name": "Web App Status",
    "service_id": "web-app",
    "ttl": "30s"
  }
}
```

In the above configuration, if the web-app health check begins failing, it will
only affect the availability of the web-app service. All other services
provided by the node will remain unchanged.

## Multiple Check Definitions

Multiple check definitions can be defined using the `checks` (plural)
key in your configuration file.

```javascript
{
  "checks": [
    {
      "id": "chk1",
      "name": "mem",
      "script": "/bin/check_mem",
      "interval": "5s"
    },
    {
      "id": "chk2",
      "name": "/health",
      "http": "http://localhost:5000/health",
      "interval": "15s"
    },
    {
      "id": "chk3",
      "name": "cpu",
      "script": "/bin/check_cpu",
      "interval": "10s"
    },
    ...
  ]
}
```