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Kubernetes StatefulSet

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Article from ADMIN 73/2023

By Andreas Stolzenberger

Legacy databases are regarded as stateful applications and, theoretically, not a good fit for containers. We reveal how classic SQL can still work well on Kubernetes and the database options available to SMEs for scale-out environments.

Many enterprises are looking to migrate legacy monolithic applications to scale-out architectures with containers. Unfortunately, this step turns out to be far more complicated than many IT departments would like. A cloud-native architecture requires that various components of the applications communicate with each other on a message bus to allow for distribution and scaling to different nodes. The big challenge for the developer is: Where does the application back up its data files?

Up to now, SQL databases have mainly handled this task. However, very few SQL implementations can run as clusters in a scale-out architecture. Application developers now have a choice: Either run the existing database technology in a container or switch to a scale-out database. However, this choice is only available to large companies with their own software developers. Small and medium-sized enterprises (SMEs), on the other hand, tend to work with off-the-shelf tools and are forced to use one of the databases that their application supports.

Despite the steadily increasing number of NoSQL databases for scale-out architectures, many web applications still only support one or more classic SQL databases. The most popular candidates include MariaDB (MySQL), PostgreSQL, and Microsoft SQL. In this article, I discuss how to run these SQL classics reliably in container environments with Docker, Podman, or Kubernetes and present a few interesting NoSQL approaches.

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In a genuine scale-out application, the failure of a single container should not affect the application as such. Therefore, early implementations had no function to provide persistent (i.e., non-volatile) mass storage for a single container. Unfortunately, the SQL classics run the database server in a single container: If the container fails, the database is lost, and it doesn't help that the container platform can restart the

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