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207 lines
7.3 KiB
Markdown
207 lines
7.3 KiB
Markdown
---
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layout: "docs"
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page_title: "Check Definition"
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sidebar_current: "docs-agent-checks"
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description: |-
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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.
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---
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# Checks
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One of the primary roles of the agent is management of system-level and application-level health
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checks. A health check is considered to be application-level if it is associated with a
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service. If not associated with a service, the check monitors the health of the entire node.
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A check is defined in a configuration file or added at runtime over the HTTP interface. Checks
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created via the HTTP interface persist with that node.
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There are three different kinds of checks:
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* Script + Interval - These checks depend on invoking an external application
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that performs the health check, exits with an appropriate exit code, and potentially
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generates some output. A script is paired with an invocation interval (e.g.
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every 30 seconds). This is similar to the Nagios plugin system.
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* HTTP + Interval - These checks make an HTTP `GET` request every Interval (e.g.
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every 30 seconds) to the specified URL. The status of the service depends on the HTTP response code:
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any `2xx` code is considered passing, a `429 Too Many Requests` is a warning, and anything else is a failure.
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This type of check should be preferred over a script that uses `curl` or another external process
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to check a simple HTTP operation. By default, HTTP checks will be configured
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with a request timeout equal to the check interval, with a max of 10 seconds.
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It is possible to configure a custom HTTP check timeout value by specifying
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the `timeout` field in the check definition.
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* <a name="TTL"></a>Time to Live (TTL) - These checks retain their last known state for a given TTL.
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The state of the check must be updated periodically over the HTTP interface. If an
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external system fails to update the status within a given TTL, the check is
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set to the failed state. This mechanism, conceptually similar to a dead man's switch,
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relies on the application to directly report its health. For example, a healthy app
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can periodically `PUT` a status update to the HTTP endpoint; if the app fails, the TTL will
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expire and the health check enters a critical state. The endpoints used to
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update health information for a given check are the
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[pass endpoint](https://consul.io/docs/agent/http/agent.html#agent_check_pass)
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and the [fail endpoint](https://consul.io/docs/agent/http/agent.html#agent_check_fail).
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TTL checks also persist
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their last known status to disk. This allows the Consul agent to restore the
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last known status of the check across restarts. Persisted check status is
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valid through the end of the TTL from the time of the last check.
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## Check Definition
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A script check:
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```javascript
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{
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"check": {
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"id": "mem-util",
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"name": "Memory utilization",
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"script": "/usr/local/bin/check_mem.py",
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"interval": "10s"
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}
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}
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```
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A HTTP check:
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```javascript
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{
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"check": {
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"id": "api",
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"name": "HTTP API on port 5000",
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"http": "http://localhost:5000/health",
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"interval": "10s",
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"timeout": "1s"
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}
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}
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```
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A TTL check:
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```javascript
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{
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"check": {
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"id": "web-app",
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"name": "Web App Status",
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"notes": "Web app does a curl internally every 10 seconds",
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"ttl": "30s"
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}
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}
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```
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Each type of definition must include a `name` and may optionally
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provide an `id` and `notes` field. The `id` is set to the `name` if not
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provided. It is required that all checks have a unique ID per node: if names
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might conflict, unique IDs should be provided.
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The `notes` field is opaque to Consul but can be used to provide a human-readable
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description of the current state of the check. With a script check, the field is
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set to any output generated by the script. Similarly, an external process updating
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a TTL check via the HTTP interface can set the `notes` value.
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Checks may also contain a `token` field to provide an ACL token. This token is
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used for any interaction with the catalog for the check, including
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[anti-entropy syncs](/docs/internals/anti-entropy.html) and deregistration.
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Both script and HTTP checks must include an `interval` field. This field is
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parsed by Go's `time` package, and has the following
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[formatting specification](http://golang.org/pkg/time/#ParseDuration):
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> A duration string is a possibly signed sequence of decimal numbers, each with
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> optional fraction and a unit suffix, such as "300ms", "-1.5h" or "2h45m".
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> Valid time units are "ns", "us" (or "µs"), "ms", "s", "m", "h".
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To configure a check, either provide it as a `-config-file` option to the
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agent or place it inside the `-config-dir` of the agent. The file must
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end in the ".json" extension to be loaded by Consul. Check definitions can
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also be updated by sending a `SIGHUP` to the agent. Alternatively, the
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check can be registered dynamically using the [HTTP API](/docs/agent/http.html).
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## Check Scripts
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A check script is generally free to do anything to determine the status
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of the check. The only limitations placed are that the exit codes must obey
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this convention:
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* Exit code 0 - Check is passing
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* Exit code 1 - Check is warning
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* Any other code - Check is failing
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This is the only convention that Consul depends on. Any output of the script
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will be captured and stored in the `notes` field so that it can be viewed
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by human operators.
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## Initial Health Check Status
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By default, when checks are registered against a Consul agent, the state is set
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immediately to "critical". This is useful to prevent services from being
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registered as "passing" and entering the service pool before they are confirmed
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to be healthy. In certain cases, it may be desirable to specify the initial
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state of a health check. This can be done by specifying the `status` field in a
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health check definition, like so:
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```javascript
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{
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"check": {
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"id": "mem",
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"script": "/bin/check_mem",
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"interval": "10s",
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"status": "passing"
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}
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}
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```
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The above service definition would cause the new "mem" check to be
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registered with its initial state set to "passing".
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## Service-bound checks
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Health checks may optionally be bound to a specific service. This ensures
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that the status of the health check will only affect the health status of the
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given service instead of the entire node. Service-bound health checks may be
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provided by adding a `service_id` field to a check configuration:
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```javascript
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{
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"check": {
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"id": "web-app",
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"name": "Web App Status",
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"service_id": "web-app",
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"ttl": "30s"
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}
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}
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```
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In the above configuration, if the web-app health check begins failing, it will
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only affect the availability of the web-app service. All other services
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provided by the node will remain unchanged.
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## Multiple Check Definitions
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Multiple check definitions can be defined using the `checks` (plural)
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key in your configuration file.
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```javascript
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{
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"checks": [
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{
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"id": "chk1",
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"name": "mem",
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"script": "/bin/check_mem",
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"interval": "5s"
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},
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{
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"id": "chk2",
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"name": "/health",
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"http": "http://localhost:5000/health",
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"interval": "15s"
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},
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{
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"id": "chk3",
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"name": "cpu",
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"script": "/bin/check_cpu",
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"interval": "10s"
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},
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...
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]
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}
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```
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