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MySQL with automatic failover on Kubernetes

This is a galera cluster setup, with plain manifests. We actually use it in production, though with modest loads.

Get started

First create a storage class mysql-data. See exampels in ./configure/. You might also want to edit the volume size request, at the bottom of ./50mariadb.yml.

Then: kubectl apply -f ..

Cluster Health

Readiness and liveness probes will only assert client-level health of individual pods. Watch logs for "sst" or "Quorum results", or run this quick check:

for i in 0 1 2; do kubectl -n mysql exec mariadb-$i -- mysql -e "SHOW STATUS LIKE 'wsrep_cluster_size';" -N; done

Port 9104 exposes plaintext metris in Prometheus scrape format.

# with kubectl -n mysql port-forward mariadb-0 9104:9104
$ curl -s http://localhost:9104/metrics | grep ^mysql_global_status_wsrep_cluster_size
mysql_global_status_wsrep_cluster_size 3

A reasonable alert is on mysql_global_status_wsrep_cluster_size staying below the desired number of replicas.

Cluster un-health

We need to assume a couple of things here. First and foremost: Production clusters are configured so that the statefulset pods do not go down together.

  • Pods are properly spread across nodes.
  • Nodes are spread across multiple availability zones.

Let's also assume that there is monitoring. Any wsrep_cluster_size issue (see above), or absence of wsrep_cluster_size should lead to a human being paged.

Rarity combined with manual attention means that this statefulset can/should avoid attempts at automatic recovery. The reason for that being: we can't test for failure modes properly, as they depend on the Kubernetes setup. Automation may appoint the wrong leader - losing writes - or cause split-brain situations.

We can however support detection in the init script.

It's normal operations to scale down to two instances

  • actually one instance, but nodes should be considered ephemeral so don't do that - and up to any number of replicas.

phpMyAdmin

Carefully consider the security implications before you create this. Note that it uses a non-official image.

kubectl apply -f myadmin/

PhpMyAdmin has a login page where you need a mysql user. To allow login (with full access) create a user with your choice of password:

kubectl -n mysql exec mariadb-0 -- mysql -e "CREATE USER 'phpmyadmin'@'%' IDENTIFIED BY 'my-admin-pw'; GRANT ALL ON *.* TO 'phpmyadmin'@'%' WITH GRANT OPTION;"