[[xpack-alerting]]
= Alerting on Cluster and Index Events

[partintro]
--
{xpack} alerting is a set of administrative features that enable you to watch
for changes or anomalies in your data and perform the necessary actions in
response. For example, you might want to:

* Monitor social media as another way to detect failures in user-facing
  automated systems like ATMs or ticketing systems. When the number of tweets
  and posts in an area exceeds a threshold of significance, notify a service
  technician.

* Monitor your infrastructure, tracking disk usage over time. Open a helpdesk
  ticket when any servers are likely to run out of free space in the next few
  days.

* Track network activity to detect malicious activity, and proactively change
  firewall configuration to reject the malicious user.

* Monitor Elasticsearch, and send immediate notification to the system
  administrator if nodes leave the cluster or query throughput exceeds an
  expected range.

* Track application response times and if page-load time exceeds SLAs for more
  than 5 minutes, open a helpdesk ticket. If SLAs are exceeded for an hour,
  page the administrator on duty.

All of these use-cases share a few key properties:

* The relevant data or changes in data can be identified with a periodic
  Elasticsearch query.

* The results of the query can be checked against a condition.

* One or more actions are taken if the condition is true -- an email is sent, a
  3rd party system is notified, or the query results are stored.

[float]
=== How Watches Work

{xpack} provides an API for creating, managing and testing _watches_. A watch
describes a single alert and can contain multiple notification actions.

A watch is constructed from four simple building blocks:

Schedule::  A schedule for running a query and checking the condition.

Query::     The query to run as input to the condition. Watches
            support the full Elasticsearch query language, including
            aggregations.

Condition:: A condition that determines whether or not to execute the actions.
            You can use simple conditions (always true), or use scripting for
            more sophisticated scenarios.

Actions::   One or more actions, such as sending email, pushing data to
            3rd party systems through a webhook, or indexing the results of
            the query.

A full history of all watches is maintained in an Elasticsearch index. This
history keeps track of each time a watch is triggered and records the results
from the query, whether the condition was met, and what actions were taken.

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