[role="xpack"]
[[es-monitoring-collectors]]
=== Collectors

Collectors, as their name implies, collect things. In {monitoring} for {es},
collectors have a few rules that define them.

There is only one collector per data type gathered. In other words, for any
monitoring document that is created, it comes from a single collector rather
than being merged from multiple collectors. {monitoring} for {es} currently has
a few collectors because the goal is to minimize overlap between them for
optimal performance.

Each collector can create zero or more monitoring documents. For example,
the `index_stats` collector collects all index statistics at the same time to
avoid many unnecessary calls.

[options="header"]
|=======================
| Collector       | Data Types | Description
| Cluster Stats   | `cluster_stats`
| Gathers details about the cluster state, including parts of
the actual cluster state (for example `GET /_cluster/state`) and statistics
about it (for example, `GET /_cluster/stats`). This produces a single document
type. In versions prior to X-Pack 5.5, this was actually three separate collectors
that resulted in three separate types: `cluster_stats`, `cluster_state`, and
`cluster_info`. In 5.5 and later, all three are combined into `cluster_stats`.
+
This only runs on the _elected_ master node and the data collected
(`cluster_stats`) largely controls the UI. When this data is not present, it
indicates either a misconfiguration on the elected master node, timeouts related
to the collection of the data, or issues with storing the data. Only a single
document is produced per collection.
| Index Stats     | `indices_stats`, `index_stats`
| Gathers details about the indices in the cluster, both in summary and
individually. This creates many documents that represent parts of the index
statistics output (for example, `GET /_stats`).
+
This information only needs to be collected once, so it is collected on the
_elected_ master node. The most common failure for this collector relates to an
extreme number of indices -- and therefore time to gather them -- resulting in
timeouts. One summary `indices_stats` document is produced per collection and one
`index_stats` document is produced per index, per collection.
| Index Recovery  | `index_recovery`
| Gathers details about index recovery in the cluster. Index recovery represents
the assignment of _shards_ at the cluster level. If an index is not recovered,
it is not usable. This also corresponds to shard restoration via snapshots.
+
This information only needs to be collected once, so it is collected on the
_elected_ master node. The most common failure for this collector relates to an
extreme number of shards -- and therefore time to gather them -- resulting in
timeouts. This creates a single document that contains all recoveries by default,
which can be quite large, but it gives the most accurate picture of recovery in
the production cluster.
| Shards          | `shards`
| Gathers details about all _allocated_ shards for all indices, particularly
including what node the shard is allocated to.
+
This information only needs to be collected once, so it is collected on the
_elected_ master node. The collector uses the local cluster state to get the
routing table without any network timeout issues unlike most other collectors.
Each shard is represented by a separate monitoring document.
| Jobs            | `job_stats`
| Gathers details about all machine learning job statistics (for example,
`GET /_xpack/ml/anomaly_detectors/_stats`).
+
This information only needs to be collected once, so it is collected on the
_elected_ master node. However, for the master node to be able to perform the
collection, the master node must have `xpack.ml.enabled` set to true (default)
and a license level that supports {ml}. 
| Node Stats      | `node_stats`
| Gathers details about the running node, such as memory utilization and CPU
usage (for example, `GET /_nodes/_local/stats`).
+
This runs on _every_ node with {monitoring} enabled. One common failure
results in the timeout of the node stats request due to too many segment files.
As a result, the collector spends too much time waiting for the file system
stats to be calculated until it finally times out. A single `node_stats`
document is created per collection. This is collected per node to help to
discover issues with nodes communicating with each other, but not with the
monitoring cluster (for example, intermittent network issues or memory pressure).
|=======================

Fundamentally, each collector works on the same principle. Per collection
interval, which defaults to 10 seconds (`10s`), each collector is checked to
see whether it should run and then the appropriate collectors run. The failure
of an individual collector does not impact any other collector.

Once collection has completed, all of the monitoring data is passed to the
exporters to route the monitoring data to the monitoring clusters.

The collection interval can be configured dynamically and you can also disable
data collection. This can be very useful when you are using a separate
monitoring cluster to automatically take advantage of the cleaner service.

If gaps exist in the monitoring charts in {kib}, it is typically because either
a collector failed or the monitoring cluster did not receive the data (for
example, it was being restarted). In the event that a collector fails, a logged
error should exist on the node that attempted to perform the collection.

NOTE: Collection is currently done serially, rather than in parallel, to avoid
      extra overhead on the elected master node. The downside to this approach
      is that collectors might observe a different version of the cluster state
      within the same collection period. In practice, this does not make a
      significant difference and running the collectors in parallel would not
      prevent such a possibility.

For more infomration about the configuration options for the collectors, see
<<monitoring-collection-settings>>.
