---
title: "MySQL sync cluster fault tolerance"
sidebar_label: "Cluster fault tolerance"
sidebar_position: 14
description: "Role of the master node and replicas in a MySQL sync cluster, how to change the number of replicas"
---
import ClusterFaultTolerance from '@site/i18n/en/docusaurus-plugin-content-docs/current/_partials/managed-databases/common/cluster-fault-tolerance.mdx'
import Formbricks from '@theme/MDXComponents/Formbricks'
# MySQL sync cluster fault tolerance
By default, a MySQL sync Managed Database cluster consists of one [master node](#master-node). To provide fault tolerance, you need to add [replicas](#replicas) to the cluster. In multi-node clusters, one node will be a master node, and the others will be replicas.
:::note
For example, if you have created a three-node cluster, the cluster will have one master node and two replicas. If you add more nodes, they will be replicas.
:::
A MySQL sync cluster with replicas has [limitations](/managed-databases/mysql-sync/limitations.mdx).
When adding replicas, consider the [node allocation type](#node-allocation-type) in the cluster.
## Master node \{#master-node}
By default, the cluster consists of one primary node—the master node. When connected to the master node, all operations are available: reading (`SELECT`) and writing (`INSERT`, `UPDATE`, `DELETE`, and others).
All data changes on the master node are replicated to the replicas. The replication process does not affect the performance of the replicas and the master node.
## Replicas \{#replicas}
Replicas are full copies of the master node. They are available for read-only data access (`SELECT`).
Having replicas in a cluster ensures its fault tolerance — if the master node stops working, its role will be transferred to one of the replicas, and the cluster will continue to operate normally. When the master node is restored, it will take on the role of a replica. Note that the address of the master node will change.
An SLA applies to a cluster with replicas [SLA](https://files.selectel.ru/docs/ru/conditions-cloud-platform.pdf) — we guarantee 99.95% write availability and 99.99% read availability.
If a cluster has no replicas, it will be unavailable until the master node is restored. Data will not be lost.
We recommend [creating fault-tolerant clusters](/managed-databases/mysql-sync/create-cluster.mdx) with replicas or [adding replicas](#change-number-of-replicas) to existing clusters.