---
title: "Limitations of MySQL sync clusters with replicas"
sidebar_label: "Cluster limitations"
sidebar_position: 5
description: "Limitations of MySQL sync clusters with replicas"
---

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# Limitations of MySQL sync clusters with replicas

MySQL sync clusters with replicas are subject to the following limitations:

* replication only works with the [InnoDB](https://ru.wikipedia.org/wiki/InnoDB) storage engine ([MyISAM](https://ru.wikipedia.org/wiki/MyISAM) is not supported);
* `ALTER TABLE` can cause cluster performance limitations — we recommend using the `pt-online-schema-change` utility. Learn more in the [Changing Table Structure in a MySQL sync Cluster with Replicas](/managed-databases/mysql-sync/change-table-schema.mdx);
* `LOCK/UNLOCK` and `GET_LOCK()/RELEASE_LOCK()` requests are not supported;
* all tables must have a primary key, otherwise writes (`INSERT`) will not be possible;
* XA transactions are not supported due to a potential `ROLLBACK` at the `COMMIT;` stage;
* a transaction may roll back even after a `COMMIT` — if two transactions modify the same record on different nodes, only one will be successfully completed, and the other will be aborted at the cluster level;
* when naming temporary tables, do not use names that match the names of permanent tables — such a match will stop replication of the permanent table.

Read more about limitations [in the official Percona XtraDB Cluster documentation](https://docs.percona.com/percona-xtradb-cluster/8.0/limitation.html).

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