404 lines
16 KiB
Plaintext
404 lines
16 KiB
Plaintext
[[how-watcher-works]]
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== How Watcher Works
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Once you have <<getting-started, installed watcher>>, you can <<watch-definition, add watches>>
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to automatically perform an action when certain conditions are met. The conditions are generally
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based on data you've loaded into the watch by querying an Elasticsearch index or submitting an
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HTTP request to a web service. For example, you could send an email to the sysadmin when a
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search of your log data indicates that there are errors.
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This topic describes the elements of a watch and how watches operate.
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[[watch-definition]]
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=== Watch Definition
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A watch consists of a trigger, input, condition, and the actions you want to perform when the
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watch condition is met. In addition, you can define transforms to process the watch payload
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before executing the actions.
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<<trigger,Trigger>> :: Determines when the watch is checked.
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A watch must have a trigger.
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<<input,Input>> :: Loads data into the watch payload.
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If no input is specified, an empty payload is loaded.
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<<condition,Condition>> :: Controls whether the watch actions are executed.
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If no condition is specified, the condition defaults to `always`.
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<<transform,Transform>> :: Processes the watch payload to prepare it for the watch actions.
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You can define transforms at the watch level or define action-specific
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transforms. Optional.
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<<actions,Actions>> :: Specify what happens when the watch
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condition is met.
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[[watch-definition-example]]
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For example, the following snippet shows a <<api-rest-put-watch, Put Watch>> request that defines
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a watch that looks for log error events:
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[source,json]
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--------------------------------------------------
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PUT _watcher/watch/log_event_watch
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{
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"metadata" : { <1>
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"color" : "red"
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},
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"trigger" : { <2>
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"schedule" : {
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"interval" : "5m"
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}
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},
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"input" : { <3>
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"search" : {
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"request" : {
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"search_type" : "count",
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"indices" : "log-events",
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"body" : {
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"query" : { "match" : { "status" : "error" } }
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}
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}
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}
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},
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"condition" : { <4>
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"script" : "return ctx.payload.hits.total > 5"
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},
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"transform" : { <5>
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"search" : {
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"request" : {
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"indices" : "log-events",
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"body" : {
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"query" : { "match" : { "status" : "error" } }
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}
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}
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}
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},
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"actions" : { <6>
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"my_webhook" : {
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"webhook" : {
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"method" : "POST",
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"host" : "mylisteninghost",
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"port" : 9200,
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"path" : "/{{watch_id}}",
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"body" : "Encountered {{ctx.payload.hits.total}} errors"
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}
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},
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"email_administrator" : {
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"email" : {
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"to" : "sys.admino@host.domain",
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"subject" : "Encountered {{ctx.payload.hits.total}} errors",
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"body" : "Too many error in the system, see attached data",
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"attach_data" : true,
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"priority" : "high"
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}
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}
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}
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}
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--------------------------------------------------
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// AUTOSENSE
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<1> Metadata - You can attach optional static metadata to a watch.
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<2> Trigger - This schedule trigger executes the watch every 5 minutes
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<3> Input - This input searches for errors in the `log-events` index and loads the response
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into the watch payload.
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<4> Condition - This condition checks to see if there are more than 5 error events (hits in the
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search response). If there are, execution continues.
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<5> Transform - If the watch condition is met, this transform loads all of the errors into
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the watch payload by searching for the errors using the default search type,
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`query_then_fetch`. All of the watch actions have access to this payload.
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<6> Actions - This watch has two actions. The `my_webhook` action notifies a 3rd party system
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about the problem. The `email_administrator` action sends a high priority
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email to the system administrator. The watch payload
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that contains the errors is attached to the email.
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[[watch-execution]]
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=== Watch Execution
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When you add a watch, Watcher immediately registers its trigger with the appropriate trigger
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engine. Watches that have a `schedule` trigger are registered with the `scheduler` trigger engine.
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The trigger engine is responsible for triggering execution of the watch. Trigger engines run on
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the master node and use a separate thread pool from the one used to execute watches.
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When a watch is triggered, Watcher queues it up for execution. A `watch_record` document is
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created and added to the <<watch-history, watch history>> index and the initial status is set to
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`awaits_execution`.
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When execution starts, Watcher creates a watch execution context for the watch. The execution
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context provides scripts and templates access to the watch metadata, payload, watch ID, execution
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time, and trigger information. For more information, see
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<<watch-execution-context, Watch Execution Context>>.
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During the execution process, Watcher:
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. Loads the input data into the payload in the watch execution context. This makes the data
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available to all subsequent steps in the execution process. This step is controlled by the
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input configured for the watch.
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. Evaluates the watch condition to determine whether or not to continue processing the watch.
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If the condition is met (evaluates to `true`), processing advances to the next step. If it
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is not met (evaluates to `false`), execution of the watch stops.
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. Applies transforms to the watch payload (if defined).
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. Executes the watch actions if the condition is met and the watch is not being
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<<watch-acknowledgment-throttling, throttled>>.
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When watch execution finishes, Watcher updates the watch record with the execution results.
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The watch record includes the execution time and duration, whether the watch condition was met,
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and the status of each action that was performed. For more information, see
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<<watch-history, Watch History>>.
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The following diagram shows the watch execution process:
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image::images/watch-execution.jpg[align="center"]
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[[watch-acknowledgment-throttling]]
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=== Watch Acknowledgment and Throttling
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Watcher supports both time-based and acknowledgment-based throttling. This enables you to prevent
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actions from being repeatedly executed for the same event.
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By default, Watcher uses time-based throttling with a throttle period of 5 seconds. This means that
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if a watch is executed every second, its actions are performed a maximum of once every 5 seconds,
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even when the condition is met. You can configure the throttle period on a per-action basis, at the
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watch level, or change the <<configuring-default-throttle-period, default throttle period>> in
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`elasticsearch.yml`.
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Acknowledgment-based throttling enables you to tell Watcher not to send any more notifications
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about a watch as long as its condition remains true. Once the condition evaluates to `false`, the
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acknowledgment is cleared and Watcher resumes executing the watch's actions normally.
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For more information, see <<actions-ack-throttle, action throttling>>.
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[[scripts-templates]]
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=== Scripts and Templates
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You can use scripts and templates when defining a watch. Scripts and templates can reference
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elements in the watch execution context, including the watch payload. The execution context defines
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variables you can use in a script and parameter placeholders you can use in a template. Transforms
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also update the contents of the watch payload.
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Watcher uses the Elasticsearch script infrastructure, which supports <<inline-templates-scripts,inline>>,
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<<indexed-templates-scripts, indexed>>, and <<file-templates-scripts, file-based scripts>>. Scripts
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and templates are compiled and cached by Elasticsearch to optimize recurring execution.
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{ref}/modules-scripting.html#_automatic_script_reloading[Autoloading] is also supported. For more
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information, see {ref}/modules-scripting.html[Scripting] in the Elasticsearch Reference.
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[[watch-execution-context]]
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==== Watch Execution Context
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The following snippet shows the basic elements in a watch's execution context:
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[source,js]
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----------------------------------------------------------------------
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{
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"ctx" : {
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"metadata" : { ... }, <1>
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"payload" : { ... }, <2>
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"watch_id" : "<id>", <3>
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"execution_time" : "20150220T00:00:10Z", <4>
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"trigger" : { <5>
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"triggered_time" : "20150220T00:00:10Z",
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"scheduled_time" : "20150220T00:00:00Z"
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},
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"vars" : { ... } <6>
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}
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----------------------------------------------------------------------
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<1> Any static metadata specified in the watch definition.
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<2> The current watch payload.
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<3> The id of the executing watch.
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<4> A timestamp that shows when the watch execution started.
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<5> Information about the trigger event. For a `schedule` trigger, this
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consists of the `triggered_time` (when the watch was triggered)
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and the `scheduled_time` (when the watch was scheduled to be triggered).
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<6> Dynamic variables that can be set and accessed by different constructs
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during the execution. These variables are scoped to a single execution
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(i.e they're not persisted and can't be used between different executions
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of the same watch)
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[[scripts]]
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==== Using Scripts
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You can use scripts to define <<condition-script, conditions>> and <<transform-script, transforms>>.
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The default scripting language is groovy.
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Scripts can reference any of the values in the watch execution context or values explicitly passed
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through script parameters.
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For example, if the context metadata contains a `color` field, `"metadata" : {"color": "red"}`, you
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can access its value with the variable `ctx.metadata.color`. If you pass in a `color` parameter as
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part of the condition or transform definition, `"params" : {"color": "red"}`, you access its value
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with the variable `color`.
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[[templates]]
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==== Using Templates
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You use templates to define dynamic content for a watch. At execution time, templates pull in data
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from the watch's execution context. For example, you could use a template to populate the `subject`
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field for an `email` action with data stored in the watch payload. Templates can also access values
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explicitly passed through template parameters.
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Watcher supports templates in a variety of places:
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* The <<input-http, `http`>> input supports templates in the `path`, `params`, `headers` and
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`body` fields.
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* The <<actions-email, `email`>> action supports templates in the `from`, `reply_to`, `priority`,
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`to`, `cc`, `bcc`, `subject`, `body.text` and `body.html` fields.
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* The <<actions-webhook, `webhook`>> action supports templates in the `path`, `params`, `headers`
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and `body` fields.
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You specify templates using the https://mustache.github.io[Mustache] scripting language.
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[NOTE]
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===============================
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While Elasticsearch supports Mustache out of the box, Watcher ships with its own version registered
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under `xmustache`. This is because the default Mustache implementation in Elasticsearch 1.5 lacks
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array/list access support. `xmustache` adds this support to enable easy array access. For example,
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to refer to the source of the third search hit in the payload use
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`{{ctx.payload.hits.hits.2._source}}`.
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When this feature is available in Elasticsearch, we expect to remove `xmustache` from Watcher and
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use the version that ships with Elasticsearch.
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===============================
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For example, if the context metadata contains a `color` field, you can access its value with the
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expression `{{ctx.metadata.color}}`. If the context payload contains the results of a search, you
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could access the source of the 3rd search hit in the payload with the following expression
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`{{ctx.payload.hits.hits.2._source}}`.
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If you pass in a parameter as part of the input or action definition, you can reference the
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parameter by name. For example, the following snippet defines and references the `color` parameter.
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[source,js]
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----------------------------------------------------------------------
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{
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"actions" : {
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"email_notification" : {
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"email" : {
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"subject" : {
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"inline" : "{{color}} alert",
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"params" : {
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"color" : "red"
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}
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}
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}
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}
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}
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}
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----------------------------------------------------------------------
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[[inline-templates-scripts]]
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==== Inline Templates and Scripts
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To define an inline template or script, you simply specify it directly in the value of a field.
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For example, the following snippet configures the subject of the `email` action using an inline
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template that references the `color` value in the context metadata.
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[source,js]
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----------------------------------------------------------------------
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"actions" : {
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"email_notification" : {
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"email" : {
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"subject" : "{{ctx.metadata.color}} alert"
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}
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}
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}
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}
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----------------------------------------------------------------------
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For a script, you simply specify the inline script as the value of the `script` field.
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For example:
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[source,js]
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----------------------------------------------------------------------
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"condition" : {
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"script" : "return true"
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}
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----------------------------------------------------------------------
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You can also explicitly specify the inline type by using a formal object definition as the field
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value. For example:
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[source,js]
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----------------------------------------------------------------------
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"actions" : {
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"email_notification" : {
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"email" : {
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"subject" : {
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"inline" : "{{ctx.metadata.color}} alert"
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}
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}
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}
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}
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----------------------------------------------------------------------
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The formal object definition for a script would be:
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[source,js]
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----------------------------------------------------------------------
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"condition" : {
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"script" : {
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"inline": "return true"
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}
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}
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----------------------------------------------------------------------
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[[indexed-templates-scripts]]
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==== Indexed Templates and Scripts
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If you {ref}/modules-scripting.html#_indexed_scripts[index] your templates and scripts, you can
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reference them by id.
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To reference an indexed script or template, you use the formal object definition and specify its
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id in the `id` field. For example, the following snippet references the `email_notification_subject`
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template.
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[source,js]
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----------------------------------------------------------------------
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{
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...
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"actions" : {
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"email_notification" : {
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"email" : {
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"subject" : {
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"id" : "email_notification_subject",
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"params" : {
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"color" : "red"
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}
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}
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}
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}
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}
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}
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----------------------------------------------------------------------
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[[file-templates-scripts]]
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==== File-based Templates and Scripts
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If you store templates or scripts in the `$ES_HOME/config/scripts` directory, you can reference
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them by name. Template files must be saved with the extension `.mustache`. Script files must be
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saved with the appropriate file extension, such as `.groovy`.
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NOTE: The `config/scripts` directory is scanned periodically for changes. New and changed
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templates and scripts are reloaded and deleted templates and scripts are removed from
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the preloaded scripts cache. For more information, see
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{ref}/modules-scripting.html#_automatic_script_reloading[Automatic Script Reloading]
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in the Elasticsearch Reference.
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To reference a file-based index or script, you use the formal object definition and specify its
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name in the `file` field. For example, the following snippet references the script file
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`threshold_hits.groovy`.
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[source,js]
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--------------------------------------------------
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"condition" : {
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"script" : {
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"file" : "threshold_hits",
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"params" : {
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"threshold" : 0
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}
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}
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}
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--------------------------------------------------
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include::how-watcher-works/dynamic-index-names.asciidoc[]
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