========================== Galera cluster maintenance ========================== Routine maintenance includes gracefully adding or removing nodes from the cluster without impacting operation and also starting a cluster after gracefully shutting down all nodes. MySQL instances are restarted when creating a cluster, when adding a node, when the service is not running, or when changes are made to the ``/etc/mysql/my.cnf`` configuration file. Verify cluster status ~~~~~~~~~~~~~~~~~~~~~ Compare the output of the following command with the following output. It should give you information about the status of your cluster. .. code-block:: shell-session # ansible galera_container -m shell -a "mysql -h 127.0.0.1 \ -e 'show status like \"%wsrep_cluster_%\";'" node3_galera_container-3ea2cbd3 | FAILED | rc=1 >> ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2) node2_galera_container-49a47d25 | FAILED | rc=1 >> ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2) node4_galera_container-76275635 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 7 wsrep_cluster_size 1 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary In the previous example, all but one node was responding. Gracefully shutting down the MariaDB service on all but one node allows the remaining operational node to continue processing SQL requests. When gracefully shutting down multiple nodes, perform the actions sequentially to retain operation. Start a cluster ~~~~~~~~~~~~~~~ Gracefully shutting down all nodes destroys the cluster. Starting or restarting a cluster from zero nodes requires creating a new cluster on one of the nodes. #. Start a new cluster on the most advanced node. Change to the ``playbooks`` directory and check the ``seqno`` value in the ``grastate.dat`` file on all of the nodes: .. code-block:: shell-session # ansible galera_container -m shell -a "cat /var/lib/mysql/grastate.dat" node2_galera_container-49a47d25 | success | rc=0 >> # GALERA saved state version: 2.1 uuid: 338b06b0-2948-11e4-9d06-bef42f6c52f1 seqno: 31 cert_index: node3_galera_container-3ea2cbd3 | success | rc=0 >> # GALERA saved state version: 2.1 uuid: 338b06b0-2948-11e4-9d06-bef42f6c52f1 seqno: 31 cert_index: node4_galera_container-76275635 | success | rc=0 >> # GALERA saved state version: 2.1 uuid: 338b06b0-2948-11e4-9d06-bef42f6c52f1 seqno: 31 cert_index: In this example, all nodes in the cluster contain the same positive ``seqno`` values as they were synchronized just prior to graceful shutdown. If all ``seqno`` values are equal, any node can start the new cluster. .. code-block:: shell-session ## for init # /etc/init.d/mysql start --wsrep-new-cluster ## for systemd # systemctl start mysql --wsrep-new-cluster Please also have a look at `upstream starting a cluster page `_ This can also be done with the help of ansible using the shell module: .. code-block:: shell-session # ansible galera_container -m shell -a "/etc/init.d/mysql start --wsrep-new-cluster" --limit galera_container[0] This command results in a cluster containing a single node. The ``wsrep_cluster_size`` value shows the number of nodes in the cluster. .. code-block:: shell-session node2_galera_container-49a47d25 | FAILED | rc=1 >> ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111) node3_galera_container-3ea2cbd3 | FAILED | rc=1 >> ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (2) node4_galera_container-76275635 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 1 wsrep_cluster_size 1 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary #. Restart MariaDB on the other nodes (replace [0] from previous ansible command with [1:]) and verify that they rejoin the cluster. .. code-block:: shell-session node2_galera_container-49a47d25 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 3 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary node3_galera_container-3ea2cbd3 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 3 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary node4_galera_container-76275635 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 3 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary Galera cluster recovery ~~~~~~~~~~~~~~~~~~~~~~~ Run the ``galera-install`` playbook using the ``galera-bootstrap`` tag to automatically recover a node or an entire environment. #. Run the following Ansible command to show the failed nodes: .. code-block:: shell-session # openstack-ansible galera-install.yml --tags galera-bootstrap The cluster comes back online after completion of this command. Recover a single-node failure ----------------------------- If a single node fails, the other nodes maintain quorum and continue to process SQL requests. #. Change to the ``playbooks`` directory and run the following Ansible command to determine the failed node: .. code-block:: shell-session # ansible galera_container -m shell -a "mysql -h 127.0.0.1 \ -e 'show status like \"%wsrep_cluster_%\";'" node3_galera_container-3ea2cbd3 | FAILED | rc=1 >> ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111) node2_galera_container-49a47d25 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 17 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary node4_galera_container-76275635 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 17 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary In this example, node 3 has failed. #. Restart MariaDB on the failed node and verify that it rejoins the cluster. #. If MariaDB fails to start, run the ``mysqld`` command and perform further analysis on the output. As a last resort, rebuild the container for the node. Recover a multi-node failure ---------------------------- When all but one node fails, the remaining node cannot achieve quorum and stops processing SQL requests. In this situation, failed nodes that recover cannot join the cluster because it no longer exists. #. Run the following Ansible command to show the failed nodes: .. code-block:: shell-session # ansible galera_container -m shell -a "mysql \ -h 127.0.0.1 -e 'show status like \"%wsrep_cluster_%\";'" node2_galera_container-49a47d25 | FAILED | rc=1 >> ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111) node3_galera_container-3ea2cbd3 | FAILED | rc=1 >> ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111) node4_galera_container-76275635 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 18446744073709551615 wsrep_cluster_size 1 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status non-Primary In this example, nodes 2 and 3 have failed. The remaining operational server indicates ``non-Primary`` because it cannot achieve quorum. #. Run the following command to `rebootstrap `_ the operational node into the cluster: .. code-block:: shell-session # mysql -e "SET GLOBAL wsrep_provider_options='pc.bootstrap=yes';" node4_galera_container-76275635 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 15 wsrep_cluster_size 1 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary node3_galera_container-3ea2cbd3 | FAILED | rc=1 >> ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111) node2_galera_container-49a47d25 | FAILED | rc=1 >> ERROR 2002 (HY000): Can't connect to local MySQL server through socket '/var/run/mysqld/mysqld.sock' (111) The remaining operational node becomes the primary node and begins processing SQL requests. #. Restart MariaDB on the failed nodes and verify that they rejoin the cluster: .. code-block:: shell-session # ansible galera_container -m shell -a "mysql \ -h 127.0.0.1 -e 'show status like \"%wsrep_cluster_%\";'" node3_galera_container-3ea2cbd3 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 17 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary node2_galera_container-49a47d25 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 17 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary node4_galera_container-76275635 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 17 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary #. If MariaDB fails to start on any of the failed nodes, run the ``mysqld`` command and perform further analysis on the output. As a last resort, rebuild the container for the node. Recover a complete environment failure -------------------------------------- Restore from backup if all of the nodes in a Galera cluster fail (do not shutdown gracefully). Change to the ``playbook`` directory and run the following command to determine if all nodes in the cluster have failed: .. code-block:: shell-session # ansible galera_container -m shell -a "cat /var/lib/mysql/grastate.dat" node3_galera_container-3ea2cbd3 | success | rc=0 >> # GALERA saved state version: 2.1 uuid: 338b06b0-2948-11e4-9d06-bef42f6c52f1 seqno: -1 cert_index: node2_galera_container-49a47d25 | success | rc=0 >> # GALERA saved state version: 2.1 uuid: 338b06b0-2948-11e4-9d06-bef42f6c52f1 seqno: -1 cert_index: node4_galera_container-76275635 | success | rc=0 >> # GALERA saved state version: 2.1 uuid: 338b06b0-2948-11e4-9d06-bef42f6c52f1 seqno: -1 cert_index: All the nodes have failed if ``mysqld`` is not running on any of the nodes and all of the nodes contain a ``seqno`` value of -1. If any single node has a positive ``seqno`` value, then that node can be used to restart the cluster. However, because there is no guarantee that each node has an identical copy of the data, we do not recommend to restart the cluster using the ``--wsrep-new-cluster`` command on one node. Rebuild a container ------------------- Recovering from certain failures require rebuilding one or more containers. #. Disable the failed node on the load balancer. .. note:: Do not rely on the load balancer health checks to disable the node. If the node is not disabled, the load balancer sends SQL requests to it before it rejoins the cluster and cause data inconsistencies. #. Destroy the container and remove MariaDB data stored outside of the container: .. code-block:: shell-session # lxc-stop -n node3_galera_container-3ea2cbd3 # lxc-destroy -n node3_galera_container-3ea2cbd3 # rm -rf /openstack/node3_galera_container-3ea2cbd3/* In this example, node 3 failed. #. Run the host setup playbook to rebuild the container on node 3: .. code-block:: shell-session # openstack-ansible setup-hosts.yml -l node3 \ -l node3_galera_container-3ea2cbd3 The playbook restarts all other containers on the node. #. Run the infrastructure playbook to configure the container specifically on node 3: .. code-block:: shell-session # openstack-ansible setup-infrastructure.yml \ --limit node3_galera_container-3ea2cbd3 .. warning:: The new container runs a single-node Galera cluster, which is a dangerous state because the environment contains more than one active database with potentially different data. .. code-block:: shell-session # ansible galera_container -m shell -a "mysql \ -h 127.0.0.1 -e 'show status like \"%wsrep_cluster_%\";'" node3_galera_container-3ea2cbd3 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 1 wsrep_cluster_size 1 wsrep_cluster_state_uuid da078d01-29e5-11e4-a051-03d896dbdb2d wsrep_cluster_status Primary node2_galera_container-49a47d25 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 4 wsrep_cluster_size 2 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary node4_galera_container-76275635 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 4 wsrep_cluster_size 2 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary #. Restart MariaDB in the new container and verify that it rejoins the cluster. .. note:: In larger deployments, it may take some time for the MariaDB daemon to start in the new container. It will be synchronizing data from the other MariaDB servers during this time. You can monitor the status during this process by tailing the ``/var/log/mysql_logs/galera_server_error.log`` log file. Lines starting with ``WSREP_SST`` will appear during the sync process and you should see a line with ``WSREP: SST complete, seqno: `` if the sync was successful. .. code-block:: shell-session # ansible galera_container -m shell -a "mysql \ -h 127.0.0.1 -e 'show status like \"%wsrep_cluster_%\";'" node2_galera_container-49a47d25 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 5 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary node3_galera_container-3ea2cbd3 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 5 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary node4_galera_container-76275635 | success | rc=0 >> Variable_name Value wsrep_cluster_conf_id 5 wsrep_cluster_size 3 wsrep_cluster_state_uuid 338b06b0-2948-11e4-9d06-bef42f6c52f1 wsrep_cluster_status Primary #. Enable the previously failed node on the load balancer.