Questions
The ones that come up, answered without marketing.
Can I write to all three machines at once?
Yes, and that is what multi master means. The only reason not to is if two of them write the same rows at the same time, which produces certification conflicts. If your application does that, sending writes for those tables to one node avoids it.
Does adding machines make it faster?
For reads, yes: reads are served locally and never cross the cluster, so another node is another place to read from. For writes, no, and slightly the opposite: every node applies every write, so a fourth machine is a fourth doing all the work.
What happens if two machines die at once?
The survivor loses quorum and stops taking writes. It keeps answering and refuses to change anything, which is correct: it cannot tell whether the others are dead or whether it is the one that got cut off, and guessing wrong makes two databases. If you know they are gone, there is a control to make it carry on alone, and it says what that costs.
Can I run this on two machines?
It will run and it will not survive anything. Either failure leaves one node holding half, and half is not a majority. Add an arbitrator on any third machine and you have the resilience of three without a third database.
Do I still need backups?
Yes, and this is the question worth being blunt about. Three identical copies protect against a machine dying. They do not protect against a dropped table, a bad migration, or an UPDATE without a WHERE, because those reach every node in milliseconds.
Does it work with MySQL rather than MariaDB?
Yes. There are two installers and you pick one: MariaDB with Galera, compiled from source, or Percona XtraDB Cluster, from packages. Percona is what MySQL means here, because plain MySQL has no Galera in it at all and so has nothing to cluster. The interface asks the server which it is and manages either the same way. The one rule is that a cluster has to be all of one or all of the other, and a join of the wrong one is refused.
How long does adding a machine take?
Installing and building the server is twenty minutes or so, mostly compiling. Joining is seconds on an empty cluster, and on a full one it is however long copying your data takes.
Is there a paid version?
No. There is no paid tier, no signup and nothing held back. It is MIT licensed and it was written for a non profit that could not buy a commercial one.
What happens if I lose the machine the cluster started on?
Nothing special. There is no primary and no founder: once a node has joined, every node is the same as every other. The one thing that machine held alone is the cluster's certificate authority, which matters only when a new node joins.
Can I put the interface on the internet?
You can, and the allow list, the second factor and the rate limiting exist partly for that. Put a real certificate in front of it and restrict it to the addresses your staff use. What must never be on the internet is port 4567.
How do I know it is actually working?
Write a row on one machine and read it on another. It is the only test that proves the whole path, and it takes ten seconds. The getting started page has the commands.
What does it cost to run across two sites?
About a third of the write throughput, measured: 20,968 operations a second in one data center against 14,613 with one node 2,000 miles away. It buys surviving the loss of a whole site.