83 lines
3.0 KiB
Markdown
83 lines
3.0 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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---
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# Checks
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One of the primary roles of the agent is the management of system and
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application level health checks. A health check is considered to be application
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level if it associated with a service. A check is defined in a configuration file,
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or added at runtime over the HTTP interface.
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There are two different kinds of checks:
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* Script + Interval - These checks depend on invoking an external application
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which does the health check and exits with an appropriate exit code, potentially
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generating 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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* TTL - These checks retain their last known state for a given TTL. The state
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of the check must be updated periodicadically 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 is used to allow an application to
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directly report it's health. For example, a web app can periodically curl the
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endpoint, and if the app fails, then the TTL will expire and the health check
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enters a critical state.
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## Check Definition
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A check definition that is a script looks like:
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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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A TTL based check is very similar:
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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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Both types of definitions 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, so if names
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might conflict, then unique ID's should be provided.
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The `notes` field is opaque to Consul, but may be used for human
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readable descriptions. The field is set to any output that a script
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generates, and similarly the TTL update hooks can update the `notes`
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as well.
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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 the exit codes must convey
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a specific meaning. Specifically:
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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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