---
id: zookeeper
title: "ZooKeeper"
---

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Apache Druid uses [Apache ZooKeeper](http://zookeeper.apache.org/) (ZK) for management of current cluster state.

## Minimum ZooKeeper versions

Apache Druid supports ZooKeeper versions 3.5.x and above.

:::info
 Note: Starting with Apache Druid 0.22.0, support for ZooKeeper 3.4.x has been removed
 Starting with Apache Druid 31.0.0, support for Zookeeper-based segment loading has been removed.
:::

## ZooKeeper Operations

The operations that happen over ZK are

1.  [Coordinator](../design/coordinator.md) leader election
2.  Segment "publishing" protocol from [Historical](../design/historical.md)
3.  [Overlord](../design/overlord.md) leader election
4.  [Overlord](../design/overlord.md) and [MiddleManager](../design/middlemanager.md) task management

## Coordinator Leader Election

We use the Curator [LeaderLatch](https://curator.apache.org/curator-recipes/leader-latch.html) recipe to perform leader election at path

```
${druid.zk.paths.coordinatorPath}/_COORDINATOR
```

## Segment "publishing" protocol from Historical and Realtime

The `announcementsPath` and `servedSegmentsPath` are used for this.

All [Historical](../design/historical.md) processes publish themselves on the `announcementsPath`, specifically, they will create an ephemeral znode at

```
${druid.zk.paths.announcementsPath}/${druid.host}
```

Which signifies that they exist. They will also subsequently create a permanent znode at

```
${druid.zk.paths.servedSegmentsPath}/${druid.host}
```

And as they load up segments, they will attach ephemeral znodes that look like

```
${druid.zk.paths.servedSegmentsPath}/${druid.host}/_segment_identifier_
```

Processes like the [Coordinator](../design/coordinator.md) and [Broker](../design/broker.md) can then watch these paths to see which processes are currently serving which segments.