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Merge pull request #659 from ryanbreen/master 

Minor grammar and content tweaks to improve readability of docs/agent/check
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Armon Dadgar 2015-01-29 15:15:58 -08:00
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@ -3,39 +3,42 @@ layout: "docs"
page_title: "Check Definition"
sidebar_current: "docs-agent-checks"
description: |-
One of the primary roles of the agent is the management of system and application level health checks. A health check is considered to be application level if it associated with a service. A check is defined in a configuration file, or added at runtime over the HTTP interface.
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 the management of system and
application level health checks. A health check is considered to be application
level if it associated with a service. A check is defined in a configuration file,
or added at runtime over the HTTP interface.
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 does the health check and exits with an appropriate exit code, potentially
generating some output. A script is paired with an invocation interval (e.g.
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 passing, `429 Too Many Requests` is warning and anything else is failing.
This type of check should be preferred over a script that for example uses `curl`.
* 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 web app can periodically curl the
endpoint, and if the app fails, then the TTL will expire and the health check
enters a critical state. This is conceptually similar to a dead man's switch.
set to the failed state. This mechanism, conceptually similar to a dead man's switch,
relies on the application to directly report its health. For example, a healthy 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.
## Check Definition
A check definition that is a script looks like:
A script check:
```javascript
{
@ -48,7 +51,7 @@ A check definition that is a script looks like:
}
```
An HTTP based check looks like:
A HTTP check:
```javascript
{
@ -61,7 +64,7 @@ An HTTP based check looks like:
}
```
A TTL based check is very similar:
A TTL check:
```javascript
{
@ -74,18 +77,18 @@ A TTL based check is very similar:
}
```
Each type of definitions must include a `name`, and may optionally
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, so if names
might conflict then unique ID's should be provided.
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 may be used for human
readable descriptions. The field is set to any output that a script
generates, and similarly the TTL update hooks can update the `notes`
as well.
The `notes` field is opaque to Consul but can be used to provide a human-readable
description of the current state of the check. 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
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).
@ -93,8 +96,8 @@ 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 convey
a specific meaning. Specifically:
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
@ -106,7 +109,7 @@ by human operators.
## Service-bound checks
Health checks may also be optionally bound to a specific service. This ensures
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:
@ -123,12 +126,12 @@ provided by adding a `service_id` field to a check configuration:
```
In the above configuration, if the web-app health check begins failing, it will
only affect the availability of the web-app service and no other services
provided by the node.
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 provided at once using the `checks` (plural)
Multiple check definitions can be defined using the `checks` (plural)
key in your configuration file.
```javascript