[[how-watcher-works]]
== How {watcher} Works

You <<watch-definition, add watches>> to automatically perform an action when
certain conditions are met. The conditions are generally based on data you've
loaded into the watch, also known as the _Watch Payload_. This payload can be
loaded from different sources - from Elasticsearch, an external HTTP service, or
even a combination of the two.

For example, you could configure a watch to send an email to the sysadmin when a
search in the logs data indicates that there are too many 503 errors in the last
5 minutes.

This topic describes the elements of a watch and how watches operate.

[float]
[[watch-definition]]
=== Watch Definition

A watch consists of a _trigger_, _input_, _condition_, and _actions_. The actions
define what needs to be done once the condition is met. In addition, you can
define _conditions_ and _transforms_ to process and prepare the watch payload before
executing the actions.

<<trigger,Trigger>>::
Determines when the watch is checked. A watch must have a trigger.

<<input,Input>>::
Loads data into the watch payload. If no input is specified, an empty payload is
loaded.

<<condition,Condition>>::
Controls whether the watch actions are executed. If no condition is specified,
the condition defaults to `always`.

<<transform,Transform>>::
Processes the watch payload to prepare it for the watch actions. You can define
transforms at the watch level or define action-specific transforms. Optional.

<<actions,Actions>>::
Specify what happens when the watch condition is met.

[[watch-definition-example]]

For example, the following snippet shows a
{ref}/watcher-api-put-watch.html[Put Watch] request that defines a watch that
looks for log error events:

[source,js]
--------------------------------------------------
PUT _xpack/watcher/watch/log_errors
{
  "metadata" : { <1>
    "color" : "red"
  },
  "trigger" : { <2>
    "schedule" : {
      "interval" : "5m"
    }
  },
  "input" : { <3>
    "search" : {
      "request" : {
        "indices" : "log-events",
        "body" : {
          "size" : 0,
          "query" : { "match" : { "status" : "error" } }
        }
      }
    }
  },
  "condition" : { <4>
    "compare" : { "ctx.payload.hits.total" : { "gt" : 5 }}
  },
  "transform" : { <5>
    "search" : {
        "request" : {
          "indices" : "log-events",
          "body" : {
            "query" : { "match" : { "status" : "error" } }
          }
        }
    }
  },
  "actions" : { <6>
    "my_webhook" : {
      "webhook" : {
        "method" : "POST",
        "host" : "mylisteninghost",
        "port" : 9200,
        "path" : "/{{watch_id}}",
        "body" : "Encountered {{ctx.payload.hits.total}} errors"
      }
    },
    "email_administrator" : {
      "email" : {
        "to" : "sys.admino@host.domain",
        "subject" : "Encountered {{ctx.payload.hits.total}} errors",
        "body" : "Too many error in the system, see attached data",
        "attachments" : {
          "attached_data" : {
            "data" : {
              "format" : "json"
            }
          }
        },
        "priority" : "high"
      }
    }
  }
}
--------------------------------------------------
// CONSOLE
<1> Metadata  - You can attach optional static metadata to a watch.
<2> Trigger   - This schedule trigger executes the watch every 5 minutes.
<3> Input     - This input searches for errors in the `log-events` index and
                loads the response into the watch payload.
<4> Condition - This condition checks to see if there are more than 5 error
                events (hits in the search response). If there are, execution
                continues for all `actions`.
<5> Transform - If the watch condition is met, this transform loads all of the
                errors into the watch payload by searching for the errors using
                the default search type, `query_then_fetch`. All of the watch
                actions have access to this payload.
<6> Actions   - This watch has two actions. The `my_webhook` action notifies a
                3rd party system about the problem. The `email_administrator`
                action sends a high priority email to the system administrator.
                The watch payload that contains the errors is attached to the
                email.

[float]
[[watch-execution]]
=== Watch Execution

[[schedule-scheduler]]
When you add a watch, {watcher} immediately registers its trigger with the
appropriate trigger engine. Watches that have a `schedule` trigger are
registered with the `scheduler` trigger engine.

The scheduler tracks time and triggers watches according to their schedules.
On each node, that contains one of the `.watches` shards, a scheduler, that is
bound to the watcher lifecycle runs. Even though all primaries and replicas are
taken into account, when a watch is triggered, watcher also ensures, that each
watch is only triggered on one of those shards. The more replica shards you
add, the more distributed the watches can be executed. If you add or remove
replicas, all watches need to be reloaded.  If a shard is relocated, the
primary and all replicas of this particular shard will reload.

Because the watches are executed on the node, where the watch shards are, you can create
dedicated watcher nodes by using shard allocation filtering.

You could configure nodes with a dedicated `node.attr.watcher: true` property and
then configure the `.watches` index like this:

[source,js]
------------------------
PUT .watches/_settings
{
  "index.routing.allocation.include": "watcher"
}
------------------------
// CONSOLE
// TEST[skip:indexes don't assign]

When the {watcher} service is stopped, the scheduler stops with it. Trigger
engines use a separate thread pool from the one used to execute watches.

When a watch is triggered, {watcher} queues it up for execution. A `watch_record`
document is created and added to the watch history and the watch's status is set
to `awaits_execution`.

When execution starts, {watcher} creates a watch execution context for the watch.
The execution context provides scripts and templates with access to the watch
metadata, payload, watch ID, execution time, and trigger information. For more
information, see <<watch-execution-context, Watch Execution Context>>.

During the execution process, {watcher}:

. Loads the input data as the payload in the watch execution context. This makes
  the data available to all subsequent steps in the execution process. This step
  is controlled by the input of the watch.
. Evaluates the watch condition to determine whether or not to continue processing
  the watch. If the condition is met (evaluates to `true`), processing advances
  to the next step. If it is not met (evaluates to `false`), execution of the watch
  stops.
. Applies transforms to the watch payload (if needed).
. Executes the watch actions granted the condition is met and the watch is not
  <<watch-acknowledgment-throttling, throttled>>.

When the watch execution finishes, the execution result is recorded as a
_Watch Record_ in the watch history. The watch record includes the execution
time and duration, whether the watch condition was met, and the status of each
action that was executed.

The following diagram shows the watch execution process:

image::images/watch-execution.jpg[align="center"]

[float]
[[watch-acknowledgment-throttling]]
=== Watch Acknowledgment and Throttling

{watcher} supports both time-based and acknowledgment-based throttling. This
enables you to prevent actions from being repeatedly executed for the same event.

By default, {watcher} uses time-based throttling with a throttle period of 5
seconds. This means that if a watch is executed every second, its actions are
performed a maximum of once every 5 seconds, even when the condition is always
met. You can configure the throttle period on a per-action basis or at the
watch level.

Acknowledgment-based throttling enables you to tell {watcher} not to send any more
notifications about a watch as long as its condition is met. Once the condition
evaluates to `false`, the acknowledgment is cleared and {watcher} resumes executing
the watch actions normally.

For more information, see <<actions-ack-throttle>>.

[float]
[[watch-active-state]]
=== Watch Active State

By default, when you add a watch it is immediately set to the _active_ state,
registered with the appropriate trigger engine, and executed according
to its configured trigger.

You can also set a watch to the _inactive_ state. Inactive watches are not
registered with a trigger engine and can never be triggered.

To set a watch to the inactive state when you create it, set the
{ref}/watcher-api-put-watch.html[`active`] parameter to _inactive_. To
deactivate an existing watch, use the
{ref}/watcher-api-deactivate-watch.html[Deactivate Watch API]. To reactivate an
inactive watch, use the
{ref}/watcher-api-activate-watch.html[Activate Watch API].

NOTE:   You can use the {ref}/watcher-api-execute-watch.html[Execute Watch API]
to force the execution of a watch even when it is inactive.

Deactivating watches is useful in a variety of situations. For example, if you
have a watch that monitors an external system and you need to take that system
down for maintenance, you can deactivate the watch to prevent it from falsely
reporting availability issues during the maintenance window.

Deactivating a watch also enables you to keep it around for future use without
deleting it from the system.

[float]
[[scripts-templates]]
=== Scripts and Templates

You can use scripts and templates when defining a watch. Scripts and templates
can reference elements in the watch execution context, including the watch payload.
The execution context defines variables you can use in a script and parameter
placeholders in a template.

{watcher} uses the Elasticsearch script infrastructure, which supports
<<inline-templates-scripts,inline>> and <<stored-templates-scripts, stored>>.
Scripts and templates are compiled
and cached by Elasticsearch to optimize recurring execution. Autoloading is also
supported. For more information, see {ref}/modules-scripting.html[Scripting] and
{ref}/modules-scripting-using.html[How to use scripts] in the Elasticsearch
Reference.

[float]
[[watch-execution-context]]
==== Watch Execution Context

The following snippet shows the basic structure of the _Watch Execution Context_:

[source,js]
----------------------------------------------------------------------
{
  "ctx" : {
    "metadata" : { ... }, <1>
    "payload" : { ... }, <2>
    "watch_id" : "<id>", <3>
    "execution_time" : "20150220T00:00:10Z", <4>
    "trigger" : { <5>
      "triggered_time" : "20150220T00:00:10Z",
      "scheduled_time" : "20150220T00:00:00Z"
    },
    "vars" : { ... } <6>
}
----------------------------------------------------------------------
<1> Any static metadata specified in the watch definition.
<2> The current watch payload.
<3> The id of the executing watch.
<4> A timestamp that shows when the watch execution started.
<5> Information about the trigger event. For a `schedule` trigger, this
    consists of the `triggered_time` (when the watch was triggered)
    and the `scheduled_time` (when the watch was scheduled to be triggered).
<6> Dynamic variables that can be set and accessed by different constructs
    during the execution. These variables are scoped to a single execution
    (i.e they're not persisted and can't be used between different executions
    of the same watch)

[float]
[[scripts]]
==== Using Scripts

You can use scripts to define <<condition-script, conditions>> and
<<transform-script, transforms>>. The default scripting language is
{ref}/modules-scripting-painless.html[Painless].

NOTE: Starting with 5.0, Elasticsearch is shipped with the new
      {ref}/modules-scripting-painless.html[Painless] scripting language.
      Painless was created and designed specifically for use in Elasticsearch.
      Beyond providing an extensive feature set, its biggest trait is that it's
      properly sandboxed and safe to use anywhere in the system (including in
      {watcher}) without the need to enable dynamic scripting.


Scripts can reference any of the values in the watch execution context or values
explicitly passed through script parameters.

For example, if the watch metadata contains a `color` field
(e.g. `"metadata" : {"color": "red"}`), you can access its value with the via the
`ctx.metadata.color` variable. If you pass in a `color`  parameter as part of the
condition or transform definition (e.g. `"params" : {"color": "red"}`), you can
access its value via the `color` variable.

[float]
[[templates]]
==== Using Templates

You use templates to define dynamic content for a watch. At execution time,
templates pull in data from the watch execution context. For example, you can use
a template to populate the `subject` field for an `email` action with data stored
in the watch payload. Templates can also access values explicitly passed through
template parameters.

You specify templates using the https://mustache.github.io[Mustache] scripting
language.

For example, the following snippet shows how templates enable dynamic subjects
in sent emails:

[source,js]
----------------------------------------------------------------------
{
  "actions" : {
    "email_notification" : {
      "email" : {
        "subject" : "{{ctx.metadata.color}} alert"
      }
    }
  }
}
----------------------------------------------------------------------

[float]
[[inline-templates-scripts]]
===== Inline Templates and Scripts

To define an inline template or script, you simply specify it directly in the
value of a field. For example, the following snippet configures the subject of
the `email` action using an inline template that references the `color` value in
the context metadata.

[source,js]
----------------------------------------------------------------------
"actions" : {
  "email_notification" : {
     "email" : {
       "subject" : "{{ctx.metadata.color}} alert"
     }
   }
  }
}
----------------------------------------------------------------------

For a script, you simply specify the inline script as the value of the `script`
field. For example:

[source,js]
----------------------------------------------------------------------
"condition" : {
  "script" : "return true"
}
----------------------------------------------------------------------

You can also explicitly specify the inline type by using a formal object
definition as the field value. For example:

[source,js]
----------------------------------------------------------------------
"actions" : {
  "email_notification" : {
    "email" : {
      "subject" : {
         "source" : "{{ctx.metadata.color}} alert"
      }
    }
  }
}
----------------------------------------------------------------------

The formal object definition for a script would be:

[source,js]
----------------------------------------------------------------------
"condition" : {
  "script" : {
    "source": "return true"
  }
}
----------------------------------------------------------------------

[float]
[[stored-templates-scripts]]
===== Stored Templates and Scripts

If you {ref}/modules-scripting-using.html#modules-scripting-stored-scripts[store]
your templates and scripts, you can reference them by id.

To reference a stored script or template, you use the formal object definition
and specify its id in the `id` field. For example, the following snippet
references the `email_notification_subject` template:

[source,js]
----------------------------------------------------------------------
{
  ...
  "actions" : {
    "email_notification" : {
      "email" : {
        "subject" : {
          "id" : "email_notification_subject",
          "params" : {
            "color" : "red"
          }
        }
      }
    }
  }
}
----------------------------------------------------------------------
