Spring Boot & Docker using VOLUME /tmp

Question

Is anyone able to explain why the Spring Boot Docker Guide recommends adding the /tmp volume as follows:

VOLUME /tmp

It explains:

We added a VOLUME pointing to "/tmp" because that is where a Spring Boot application creates working directories for Tomcat by default. The effect is to create a temporary file on your host under "/var/lib/docker" and link it to the container under "/tmp".

however, I'm not really clear why you'd want/need to do this.

If the java application creates any files in the /tmp directory of the container I don't see what the need is to configure this as a volume since, as far as I understand, it would result in files bypassing the union filesystem and being written persistently in the Docker host to /var/lib/docker. Could this also result in the "temporary" files created in the container not being automatically deleted when the container is stopped?

My spring boot app (which does create temporary files) seems to run ok without this VOLUME, but I wonder if there are any negative consequences to not following this advice.

Thanks.

Answer 1

As said in the comments, because of the union file system, writing in the container is probably slower than writing in a volume.

Also, if you want to be able to reuse temporary files between container restart, using a volume is necessary but if this is not the case it would be probably better to use a tmpfs mount as described in the Docker documentation.