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For each table, the server will create a file with the .frm extension containing the table definition (for all storage engines).


An acronym for Atomicity, Consistency, Isolation, Durability.

Asynchronous replication

Asynchronous replication is a technique where data is first written to the primary node. After the primary acknowledges the write, the data is written to secondary nodes.


This property guarantees that all updates of a transaction occur in the database or no updates occur. This guarantee also applies with a server exit. If a transaction fails, the entire operation rolls back.

Cluster replication

Normal replication path for cluster members. Can be encrypted (not by default) and unicast or multicast (unicast by default). Runs on tcp port 4567 by default.


This property guarantees that each transaction that modifies the database takes it from one consistent state to another. Consistency is implied with Isolation.


The directory in which the database server stores its databases. Most Linux distribution use /var/lib/mysql by default.

donor node

The node elected to provide a state transfer (SST or IST).


Once a transaction is committed, it will remain so and is resistant to a server exit.

Foreign Key

A referential constraint between two tables. Example: A purchase order in the purchase_orders table must have been made by a customer that exists in the customers table.

General availability (GA)

A finalized version of the product which is made available to the general public. It is the final stage in the software release cycle.


Global Transaction ID, in Percona XtraDB Cluster it consists of UUID and an ordinal sequence number which denotes the position of the change in the sequence.


HAProxy is a free, very fast and reliable solution offering high availability, load balancing, and proxying for TCP and HTTP-based applications. It is particularly suited for web sites crawling under very high loads while needing persistence or Layer7 processing. Supporting tens of thousands of connections is clearly realistic with todays hardware. Its mode of operation makes its integration into existing architectures very easy and riskless, while still offering the possibility not to expose fragile web servers to the net.


Default prefix for tablespace files, e.g., ibdata1 is a 10MB auto-extendable file that MySQL creates for the shared tablespace by default.


The Isolation guarantee means that no transaction can interfere with another. When transactions access data in a session, they also lock that data to prevent other operations on that data by other transaction.


Incremental State Transfer. Functionality which instead of whole state snapshot can catch up with the group by receiving the missing writesets, but only if the writeset is still in the donor’s writeset cache.


Storage Engine for MySQL and derivatives (Percona Server, MariaDB) originally written by Innobase Oy, since acquired by Oracle. It provides ACID compliant storage engine with foreign key support. InnoDB is the default storage engine on all platforms.


Jenkins is a continuous integration system that we use to help ensure the continued quality of the software we produce. It helps us achieve the aims of: * no failed tests in trunk on any platform * aid developers in ensuring merge requests build and test on all platforms * no known performance regressions (without a damn good explanation)

joiner node

The node joining the cluster, usually a state transfer target.


Log Serial Number. A term used in relation to the InnoDB or XtraDB storage engines. There are System-level LSNs and Page-level LSNs. The System LSN represents the most recent LSN value assigned to page changes. Each InnoDB page contains a Page LSN which is the max LSN for that page for changes that reside on the disk. This LSN is updated when the page is flushed to disk.


A fork of MySQL that is maintained primarily by Monty Program AB. It aims to add features, fix bugs while maintaining 100% backwards compatibility with MySQL.


This file refers to the database server’s main configuration file. Most Linux distributions place it as /etc/mysql/my.cnf or /etc/my.cnf, but the location and name depends on the particular installation. Note that this is not the only way of configuring the server, some systems does not have one even and rely on the command options to start the server and its defaults values.


A MySQL Storage Engine that was the default until MySQL 5.5. It doesn’t fully support transactions but in some scenarios may be faster than InnoDB. Each table is stored on disk in 3 files: .frm,i .MYD, .MYI.


An open source database that has spawned several distributions and forks. MySQL AB was the primary maintainer and distributor until bought by Sun Microsystems, which was then acquired by Oracle. As Oracle owns the MySQL trademark, the term MySQL is often used for the Oracle distribution of MySQL as distinct from the drop-in replacements such as MariaDB and Percona Server.


This user is used by the SST process to run the SQL commands needed for SST, such as creating the mysql.pxc.sst.user and assigning it the role mysql.pxc.sst.role.


This role has all the privileges needed to run xtrabackup to create a backup on the donor node.


This user (set up on the donor node) is assigned the mysql.pxc.sst.role and runs the XtraBackup to make backups. The password for this is randomly generated for each SST. The password is generated automatically for each SST.


A cluster node – a single mysql instance that is in the cluster.


Non-Uniform Memory Access (NUMA) is a computer memory design used in multiprocessing, where the memory access time depends on the memory location relative to a processor. Under NUMA, a processor can access its own local memory faster than non-local memory, that is, memory local to another processor or memory shared between processors. The whole system may still operate as one unit, and all memory is basically accessible from everywhere, but at a potentially higher latency and lower performance.

Percona Server for MySQL

Percona’s branch of MySQL with performance and management improvements.

Percona XtraDB Cluster

Percona XtraDB Cluster (PXC) is a high availability solution for MySQL.

primary cluster

A cluster with quorum. A non-primary cluster will not allow any operations and will give Unknown command errors on any clients attempting to read or write from the database.


A majority (> 50%) of nodes. In the event of a network partition, only the cluster partition that retains a quorum (if any) will remain Primary by default.

split brain

Split brain occurs when two parts of a computer cluster are disconnected, each part believing that the other is no longer running. This problem can lead to data inconsistency.


State Snapshot Transfer is the full copy of data from one node to another. It’s used when a new node joins the cluster, it has to transfer data from an existing node. Percona XtraDB Cluster: uses the xtrabackup program for this purpose. xtrabackup does not require READ LOCK for the entire syncing process - only for syncing the MySQL system tables and writing the information about the binlog, galera and replica information (same as the regular Percona XtraBackup backup).

The SST method is configured with the wsrep_sst_method variable.

In PXC 8.0, the mysql-upgrade command is now run automatically as part of SST. You do not have to run it manually when upgrading your system from an older version.

Storage Engine

A Storage Engine is a piece of software that implements the details of data storage and retrieval for a database system. This term is primarily used within the MySQL ecosystem due to it being the first widely used relational database to have an abstraction layer around storage. It is analogous to a Virtual File System layer in an Operating System. A VFS layer allows an operating system to read and write multiple file systems (for example, FAT, NTFS, XFS, ext3) and a Storage Engine layer allows a database server to access tables stored in different engines (e.g. MyISAM, InnoDB).

Tech preview

A tech preview item can be a feature, a variable, or a value within a variable. The term designates that the item is not yet ready for production use and is not included in support by SLA. A tech preview item is included in a release so that users can provide feedback. The item is either updated and released as general availability(GA) or removed if not useful. The item’s functionality can change from tech preview to GA.


Universally Unique IDentifier which uniquely identifies the state and the sequence of changes node undergoes. 128-bit UUID is a classic DCE UUID Version 1 (based on current time and MAC address). Although in theory this UUID could be generated based on the real MAC-address, in the Galera it is always (without exception) based on the generated pseudo-random addresses (“locally administered” bit in the node address (in the UUID structure) is always equal to unity).

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Last update: 2023-03-14