A runtime base image is needed because Mendix is Java based. It is possible to define both the JDK and image type for a runtime base image:

• The image type can be either a universal base image (UBI) or a hardened image (HI).
  • Mendix supports only specific versions of UBI.
• The JDK version is based off of the Java version for the application.
  • As in, if the Mendix application is using Java 21, the runtime base image should also use Java 21.

With Mendix Operator 2.15.0 (and above), the Java version is now automatically found. This means the Operator will automatically try to find the runtime base image that is compatible with the Java version used in the application. However, there is a common misconception that this means the image type will also automatically be updated to accommodate. This is not true.

The image type for a runtime base image will not change automatically. Whatever image type was chosen during installation, or manually changed later, will be used for all app environments in that namespace. As an aside, switching image types does not impact running applications. It is only applied to future builds.

This means it is possible for a JDK version and an image type to clash. For example, Red Hat no longer supports universal base image (UBI) 8 for Java 25. If an application is using Java 25, but the environment is configured to use UBI 8, the installation will fail. This is because the Operator will attempt to find a UBI-8-Java-25 runtime base image, which does not exist. Upgrading to UBI 9 would resolve this specific example, as that is a supported combination.

It is therefore important to ensure that the image type and Java type being used for a namespace match and are compatible. Otherwise, the runtime base image will fail to download, as it might not exist.