| --- |
| id: high-availability |
| title: "High availability" |
| --- |
| |
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| |
| |
| Apache ZooKeeper, metadata store, the coordinator, the overlord, and brokers are recommended to set up a high availability environment. |
| |
| - For highly-available ZooKeeper, you will need a cluster of 3 or 5 ZooKeeper nodes. |
| We recommend either installing ZooKeeper on its own hardware, or running 3 or 5 Master servers (where overlords or coordinators are running) |
| and configuring ZooKeeper on them appropriately. See the [ZooKeeper admin guide](https://zookeeper.apache.org/doc/current/zookeeperAdmin) for more details. |
| - For highly-available metadata storage, we recommend MySQL or PostgreSQL with replication and failover enabled. |
| See [MySQL Enterprise High Availability](https://www.mysql.com/products/enterprise/high_availability.html) and [PostgreSQL's High Availability, Load Balancing, and Replication](https://www.postgresql.org/docs/current/high-availability.html) for more information. |
| - For highly-available Apache Druid Coordinators and Overlords, we recommend to run multiple servers. |
| If they are all configured to use the same ZooKeeper cluster and metadata storage, |
| then they will automatically failover between each other as necessary. |
| Only one will be active at a time, but inactive servers will redirect to the currently active server. |
| - Druid Brokers can be scaled out and all running servers will be active and queryable. |
| We recommend placing them behind a load balancer. |
