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Management

When you finish installing and configuring the OpenStack database, you can initialize the Galera Cluster.

Prerequisites

Database hosts with Galera Cluster installed
A minimum of three hosts
No firewalls between the hosts
SELinux and AppArmor set to permit access to mysqld
The correct path to libgalera_smm.so given to the wsrep_provider parameter

Initializing the cluster

In the Galera Cluster, the Primary Component is the cluster of database servers that replicate into each other. In the event that a cluster node loses connectivity with the Primary Component, it defaults into a non-operational state, to avoid creating or serving inconsistent data.

By default, cluster nodes do not start as part of a Primary Component. In the Primary Component, replication and state transfers bring all databases to the same state.

To start the cluster, complete the following steps:

Initialize the Primary Component on one cluster node. For servers that use init, run the following command:
```
# service mysql start --wsrep-new-cluster
```
For servers that use systemd, run the following command:
```
# systemctl start mariadb --wsrep-new-cluster
```

Once the database server starts, check the cluster status using the wsrep_cluster_size status variable. From the database client, run the following command:

SHOW STATUS LIKE 'wsrep_cluster_size';

+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| wsrep_cluster_size | 1     |
+--------------------+-------+

Start the database server on all other cluster nodes. For servers that use init, run the following command:
```
# service mysql start
```
For servers that use systemd, run the following command:
```
# systemctl start mariadb
```

When you have all cluster nodes started, log into the database client of any cluster node and check the wsrep_cluster_size status variable again:

SHOW STATUS LIKE 'wsrep_cluster_size';

+--------------------+-------+
| Variable_name      | Value |
+--------------------+-------+
| wsrep_cluster_size | 3     |
+--------------------+-------+

When each cluster node starts, it checks the IP addresses given to the wsrep_cluster_address parameter. It then attempts to establish network connectivity with a database server running there. Once it establishes a connection, it attempts to join the Primary Component, requesting a state transfer as needed to bring itself into sync with the cluster.

Note

In the event that you need to restart any cluster node, you can do so. When the database server comes back it, it establishes connectivity with the Primary Component and updates itself to any changes it may have missed while down.

Restarting the cluster

Individual cluster nodes can stop and be restarted without issue. When a database loses its connection or restarts, the Galera Cluster brings it back into sync once it reestablishes connection with the Primary Component. In the event that you need to restart the entire cluster, identify the most advanced cluster node and initialize the Primary Component on that node.

To find the most advanced cluster node, you need to check the sequence numbers, or the seqnos, on the last committed transaction for each. You can find this by viewing grastate.dat file in database directory:

$ cat /path/to/datadir/grastate.dat

# Galera saved state
version: 3.8
uuid:    5ee99582-bb8d-11e2-b8e3-23de375c1d30
seqno:   8204503945773

Alternatively, if the database server is running, use the wsrep_last_committed status variable:

SHOW STATUS LIKE 'wsrep_last_committed';

+----------------------+--------+
| Variable_name        | Value  |
+----------------------+--------+
| wsrep_last_committed | 409745 |
+----------------------+--------+

This value increments with each transaction, so the most advanced node has the highest sequence number and therefore is the most up to date.

Configuration tips

Deployment strategies

Galera can be configured using one of the following strategies:

Each instance has its own IP address:

OpenStack services are configured with the list of these IP addresses so they can select one of the addresses from those available.
Galera runs behind HAProxy:

HAProxy load balances incoming requests and exposes just one IP address for all the clients.

Galera synchronous replication guarantees a zero slave lag. The failover procedure completes once HAProxy detects that the active back end has gone down and switches to the backup one, which is then marked as UP. If no back ends are UP, the failover procedure finishes only when the Galera Cluster has been successfully reassembled. The SLA is normally no more than 5 minutes.
Use MySQL/Galera in active/passive mode to avoid deadlocks on SELECT ... FOR UPDATE type queries (used, for example, by nova and neutron). This issue is discussed in the following:
- IMPORTANT: MySQL Galera does *not* support SELECT ... FOR UPDATE
- Understanding reservations, concurrency, and locking in Nova

Configuring HAProxy

If you use HAProxy as a load-balancing client to provide access to the Galera Cluster, as described in the controller-ha-haproxy, you can use the clustercheck utility to improve health checks.

Create a configuration file for clustercheck at /etc/sysconfig/clustercheck:

MYSQL_USERNAME="clustercheck_user"
MYSQL_PASSWORD="my_clustercheck_password"
MYSQL_HOST="localhost"
MYSQL_PORT="3306"