mirror of https://github.com/hashicorp/consul
Armon Dadgar
11 years ago
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---
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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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@ -0,0 +1,54 @@
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---
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layout: "docs"
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page_title: "Service Definition"
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sidebar_current: "docs-agent-services"
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---
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# Services
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One of the main goals of service discovery is to provide a catalog of available
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services. To that end, the agent provides a simple service definition format
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to declare the availability of a service, and to potentially associate it with
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a health check. A health check is considered to be application level if it
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associated with a service. A service is defined in a configuration file,
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or added at runtime over the HTTP interface.
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## Service Definition
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A service definition that is a script looks like:
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{
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"service": {
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"name": "redis",
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"tag": "master",
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"port": 8000,
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"check": {
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"script": "/usr/local/bin/check_redis.py",
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"interval": "10s"
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}
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}
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}
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A service definition must include a `name`, and may optionally provide
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an `id`, `tag`, `port`, and `check`. The `id` is set to the `name` if not
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provided. It is required that all services 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 `tag` is an opaque value to Consul, but can be used to distinguish
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between "master" or "slave" nodes, or any other service level labels.
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The `port` can be used as well to make a service oriented architecture
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simpler to configure. This way the address and port of a service can
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be discovered.
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Lastly, a service can have an associated health check. This is a powerful
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feature as it allows a web balancer to gracefully remove failing nodes, or
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a database to replace a failed slave, etc. The health check is strongly integrated
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in the DNS interface as well. If a service is failing it's health check or
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a node has any failing system-level check, the DNS interface will omit that
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node from any service query.
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There is more information about [checks here](/docs/agent/checks.html). The
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check must be of the script or TTL type. If it is a script type, `script` and
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`interval` must be provided. If it is a TTL type, then only `ttl` must be
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provided. The check name is automatically generated as "service:<service-id>".
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