Getting Started with MariaDB Galera Cluster

Galera Cluster Support in MariaDB Server

MariaDB Galera Cluster is powered by:

• MariaDB Server.

• The MySQL-wsrep patch for MySQL Server and MariaDB Server. The patch currently supports only Unix-like operating systems.

• The .

The patch has been merged into MariaDB Server. This means that the functionality of MariaDB Galera Cluster can be obtained by installing the standard MariaDB Server packages and the package.

See for more information about how to interpret these version numbers.

See for more information about which specific version is included in each release of MariaDB Server.

In supported builds, Galera Cluster functionality can be enabled by setting some configuration options that are mentioned below. Galera Cluster functionality is not enabled in a standard MariaDB Server installation unless explicitly enabled with these configuration options.

During normal operation, a MariaDB Galera node consumes no more memory than a regular MariaDB server. Additional memory is consumed for the certification index and uncommitted write sets, but normally, this should not be noticeable in a typical application. There is one exception, though:

When a node is receiving a state transfer, it cannot process and apply incoming writesets because it has no state to apply them to yet. Depending on a state transfer mechanism (e.g.) the node that sends the state transfer may not be able to apply writesets as well. Thus, they need to cache those writesets for a catch-up phase. Currently the writesets are cached in memory and, if the system runs out of memory either the state transfer will fail or the cluster will block waiting for the state transfer to end.

To control memory usage for writeset caching, check the : gcs.recv_q_hard_limit, gcs.recv_q_soft_limit, and gcs.max_throttle.

Before using MariaDB Galera Cluster, we would recommend reading through the , so you can be sure that it is appropriate for your application.

To use MariaDB Galera Cluster, there are two primary packages that you need to install:

1. A MariaDB Server version that supports Galera Cluster.

2. The Galera wsrep provider library.

As mentioned in the previous section, Galera Cluster support is actually included in the standard MariaDB Server packages. That means that installing MariaDB Galera Cluster package is the same as installing standard MariaDB Server package in those versions. However, you will also have to install an additional package to obtain the Galera wsrep provider library.

Some methods may also require additional packages to be installed. The SST method is generally the best option for large clusters that expect a lot of loads.

MariaDB Galera Cluster can be installed via a package manager on Linux. In order to do so, your system needs to be configured to install from one of the MariaDB repositories.

You can configure your package manager to install it from MariaDB Corporation's MariaDB Package Repository by using the .

You can also configure your package manager to install it from MariaDB Foundation's MariaDB Repository by using the .

On RHEL, CentOS, Fedora, and other similar Linux distributions, it is highly recommended to install the relevant from MariaDB's repository using or . Starting with RHEL 8 and Fedora 22, yum has been replaced by dnf, which is the next major version of yum. However, yum commands still work on many systems that use dnf.

To install MariaDB Galera Cluster with yum or dnf, follow the instructions at .

On Debian, Ubuntu, and other similar Linux distributions, it is highly recommended to install the relevant from MariaDB's repository using .

To install MariaDB Galera Cluster with apt-get, follow the instructions at .

On SLES, OpenSUSE, and other similar Linux distributions, it is highly recommended to install the relevant from MariaDB's repository using .

To install MariaDB Galera Cluster with zypper, follow the instructions at .

To install MariaDB Galera Cluster with a binary tarball, follow the instructions at .

To make the location of the libgalera_smm.so library in binary tarballs more similar to its location in other packages, the library is now found at lib/galera/libgalera_smm.so in the binary tarballs, and there is a symbolic link in the lib directory that points to it.

To install MariaDB Galera Cluster by compiling it from source, you will have to compile both MariaDB Server and the Galera wsrep provider library. For some information on how to do this, see the pages at . The pages at and Galera Cluster Documentation: Building Galera Cluster for MySQL may also be helpful.

A number of options need to be set in order for Galera Cluster to work when using MariaDB. See for more information.

To first node of a new cluster needs to be bootstrapped by starting on that node with the option option. This option tells the node that there is no existing cluster to connect to. The node will create a new UUID to identify the new cluster.

Do not use the option when connecting to an existing cluster. Restarting the node with this option set will cause the node to create new UUID to identify the cluster again, and the node won't reconnect to the old cluster. See the next section about how to reconnect to an existing cluster.

For example, if you are manually starting mariadbd on a node, you can bootstrap it by executing the following:

However, keep in mind that most users are not going to be starting mariadbd manually. Instead, most users will use a to start mariadbd. See the following sections on how to bootstrap a node with the most common service managers.

On operating systems that use , a node can be bootstrapped in the following way:

This wrapper uses systemd to run mariadbd with the --wsrep-new-cluster option.

If you are using the systemd service that supports the , you can bootstrap a specific instance by specifying the instance name as a suffix:

Systemd support and the galera_new_cluster script were added.

On operating systems that use , a node can be bootstrapped in the following way:

This runs mariadbd with the --wsrep-new-cluster option.

Once you have a cluster running and you want to add/reconnect another node to it, you must supply an address of one or more of the existing cluster members in the option. For example, if the first node of the cluster has the address 192.168.0.1, then you could add a second node to the cluster by setting the following option in a server option group in an option file:

The new node only needs to connect to one of the existing cluster nodes. Once it connects to one of the existing cluster nodes, it will be able to see all of the nodes in the cluster. However, it is generally better to list all nodes of the cluster in , so that any node can join a cluster by connecting to any of the other cluster nodes, even if one or more of the cluster nodes are down. It is even OK to list a node's own IP address in , since Galera Cluster is smart enough to ignore it.

Once all members agree on the membership, the cluster's state will be exchanged. If the new node's state is different from that of the cluster, then it will request an IST or to make itself consistent with the other nodes.

If you shut down all nodes at the same time, then you have effectively terminated the cluster. Of course, the cluster's data still exists, but the running cluster no longer exists. When this happens, you'll need to bootstrap the cluster again.

If the cluster is not bootstrapped and mariadbd on the first node is just started normally, then the node will try to connect to at least one of the nodes listed in the option. If no nodes are currently running, then this will fail. Bootstrapping the first node solves this problem.

In some cases Galera will refuse to bootstrap a node if it detects that it might not be the most advanced node in the cluster. Galera makes this determination if the node was not the last one in the cluster to be shut down or if the node crashed. In those cases, manual intervention is needed.

If you know for sure which node is the most advanced you can edit the grastate.dat file in the datadir. You can set safe_to_bootstrap=1 on the most advanced node.

You can determine which node is the most advanced by checking grastate.dat on each node and looking for the node with the highest seqno. If the node crashed and seqno=-1, then you can find the most advanced node by recovering the seqno on each node with the option. For example:

On operating systems that use systemd, the position of a node can be recovered by running the galera_recovery script:

If you are using the systemd service that supports the , you can recover the position of a specific instance by specifying the instance name as a suffix:

The galera_recovery script recovers the position of a node by running mariadbd with the option.

When the galera_recovery script runs mariadbd, it does not write to the . Instead, it redirects mariadbd log output to a file named with the format /tmp/wsrep_recovery.XXXXXX, where XXXXXX is replaced with random characters.

When Galera is enabled, MariaDB's systemd service automatically runs the galera_recovery script prior to starting MariaDB, so that MariaDB starts with the proper Galera position.

Support for systemd and the galera_recovery script were added.

In a State Snapshot Transfer (SST), the cluster provisions nodes by transferring a full data copy from one node to another. When a new node joins the cluster, the new node initiates a State Snapshot Transfer to synchronize its data with a node that is already part of the cluster.

See for more information.

In an Incremental State Transfer (SST), the cluster provisions nodes by transferring a node's missing writesets from one node to another. When a new node joins the cluster, the new node initiates a Incremental State Transfer to synchronize its data with a node that is already part of the cluster.

If a node has only been out of a cluster for a little while, then an IST is generally faster than an SST.

MariaDB Galera Cluster supports . See for some disclaimers on how SSTs are affected when encryption is configured.

can be queried with the standard statement:

The cluster nodes can be configured to invoke a command when cluster membership or node status changes. This mechanism can also be used to communicate the event to some external monitoring agent. This is configured by setting .

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