# About Percona XtraDB Cluster

Percona XtraDB [Cluster](https://www.percona.com/software/mysql-database/percona-xtradb-cluster) is a fully open-source high-availability solution for MySQL.  It integrates Percona Server for [MySQL](https://www.mysql.com/) and Percona [XtraBackup](https://www.percona.com/software/mysql-database/percona-xtrabackup) with the [Galera](https://github.com/percona/galera) library to enable synchronous multi-source replication.

A *cluster* consists of *nodes*, where each node contains the same set of data
synchronized accross nodes.  The recommended configuration is to have at least 3
nodes, but you can have 2 nodes as well.  Each node is a regular MySQL Server
instance (for example, Percona Server).  You can convert an existing MySQL
Server instance to a node and run the cluster using this node as a base.  You
can also detach any node from the cluster and use it as a regular MySQL Server
instance.

![image](_static/cluster-diagram1.png)

### Benefits

* When you execute a query, it is executed locally on the node. All data is available locally, no need for remote access.

* No central management. You can loose any node at any point of time, and the cluster will continue to function without any data loss.

* Good solution for scaling a read workload. You can put read queries to any of the nodes.

### Drawbacks

* Overhead of provisioning new node. When you add a new node, it has to copy the full data set from one of existing nodes. If it is *100 GB*, it copies *100 GB*.

* This can’t be used as an effective write scaling solution.  There might be some improvements in write throughput when you run write traffic to 2 nodes versus all traffic to 1 node, but you can’t expect a lot.  All writes still have to go on all nodes.

* You have several duplicates of data: for 3 nodes you have 3 duplicates.

## Components

Percona XtraDB Cluster <https://www.percona.com/software/mysql-database/percona-xtradb-cluster> is based on Percona Server for [MySQL](https://www.mysql.com/) running with the [XtraDB](https://www.percona.com/software/mysql-database/percona-server/xtradb) storage engine.
It uses the [Galera](https://github.com/percona/galera) library, which is an implementation of the write set replication (wsrep) API developed by [Codership Oy](https://www.galeracluster.com/).
The default and recommended data transfer method is via Percona [XtraBackup](https://www.percona.com/software/mysql-database/percona-xtrabackup) .