<p>I have an Kubernetes environment running multipe applications (services). Now i'm  a little bit confused how to setup the MySQL database instance(s). </p>

<p>According to different sources each microservice should have there own database. Should i create a single MySQL statefulset in HA mode running multiple databases OR should i deploy a separate MySQL instance for each application (service) running one database each.</p>

<p>My first thought would be the first option hence where should HA oterwise be usefull for? Would like to hear some differente views on this.</p>

<p>Slightly subjective question, but here's what we have setup. Hopefully, that will help you build a case. I'm sure someone would have a different opinion, and that might be equally valid too:</p>

<p>We deploy about 70 microservices, each with it's own database ("schema"), and it's own JDBC URL (defined via a service). Each microservice has it's own endpoint and credentials that we do not share between microservices. So in effect, we have kept the design to be completely independent across the microservices as far as the schema is concerned. </p>

<p>Deployment-wise, however, we have opted to go with a single database instance for hosting all databases (or "schemas"). While technically, we could deploy each database on its own database instance, we chose not to do it for few main reasons:</p>

<ol>
<li><strong>Cost overhead</strong>: Running separate database instances for each microservice would add a lot of "fixed" costs. This may not be directly relevant to you if you are simply starting the database as a MySQL Docker container (we use a separate database service, such as RDS or Google Cloud SQL). But even in the case of MySQL as a Docker container, you might end up having a non-trivial cost if you run, for example, 70 separate containers one per microservice.</li>
<li><strong>Administration overhead</strong>: Given that databases are usually quite involved (disk space, IIOPs, backup/archiving, purge, upgrades and other administration activities), having separate database instances -- or Docker container instances -- may put a significant toll on your admin or operations teams, especially if you have a large number of microservices</li>
<li><strong>Security</strong>: Databases are usually also critical when it comes to security as the "truth" usually goes in the DB. Keeping encryption, TLS configuration and strengths of credentials aside (as they should be of utmost importance regardless of your deployment model), security considerations, reviews, audits and logging will bring in significant challenges if your databases instances are too many.</li>
<li><strong>Ease of development</strong>: Relatively less critical in the grand scheme of things, but significant, nonetheless. Unless you are thinking of coming up with a different model for development (and thus breaking the "dev-prod parity"), your developers may have a hard time figuring out the database endpoints for debugging even if they only need that information once-in-a-while.</li>
</ol>

<p>So, my recommendation would be to go with a single database instance (Docker or otherwise), but keep the databases/schemas completely independent and inaccessible by the any microservice but the "owner" microservice. </p>

<p>If you are deploying MySQL as Docker container(s), go with a <code>StatefulSet</code> for persistence. Define an external <code>pvc</code> so that you can always preserve the data, no matter what happens to your pods or even your cluster. Of course, if you run 'active-active', you will need to ensure clustering between your nodes, but we do run it in 'active-passive' mode, so we keep the <code>replica</code> count to 1 given we only use MySQL Docker container alternative for our test environments to save costs of external DBaaS service where it's not required.</p>
