使用 dockerfile 在多个 docker 实例之间共享 tomcat 配置？

Question

I am running multiple Docker containers with bitnami/tomcat image and I want to share the configuration read only across all instances.

Every instance will contain this same configuration like specific libraries, datasources and so on. I already have proved and tested configuration that is being used in physical tomcat servers. I see this configuration can be ready before any instance is run.

The application can be deployed to every instance (copied once actually since configuration is shared) and it can be done by copying the WAR file to host folder ${persistence_path}/tomcat/data that points to /bitnami folder in container, provided I use

-v ${persistence_path}:/bitnami

when running the instance .

The question

I am looking for an reproducible way of creating instances so it would be by script or dockerfile rather than commiting a new image , since it does not provide a reliable record of how the image was created. By using the former I avoid the chance of images getting corrupt or lost in production.

Can I even share the configuration in multiple instances, as described?

Is this the best approach given I will not be modifying any tomcat settings after instances are started and if so I can re-run the instances with the new configuration as to change once and reflect in every instance?

Can it be done with a dockerfile instead of a set of commands ? I don't think -v is fully interchangeable with VOLUME .

Answer 1

You can mount a directory as read-only using the :ro option of -v

docker run -v /your/host/dir:/bitnami:ro YOUR_IMAGE

You can create your own tomcat image starting from bitnami/tomcat including copying your artifacts and configuration at image build time. Then you can override them at runtime using the -v options.

If you want immutable containers (build time data locked in) you should build an image for every service you have. Otherwise you can use the volume binding approach, as described in the image description .

Answer 2

The bitnami/tomcat image uses a simple mechanism in order to decide whether restore data from a volume or initialize a volume.

On startup the container will check if a /bitnami/tomcat/.initialized file exists.

• If this file does not exist : The container will configure Tomcat and will move the /opt/bitnami/tomcat/{conf,data} folders to the volume (this happens at first run). Then, it will link the original folders to the ones in the volume.

Tomcat folder after first run:

root@2346ab1f5c88:/# ls -la /opt/bitnami/tomcat/{conf,data}
lrwxrwxrwx 1 root root 20 Sep 28 13:49 /opt/bitnami/tomcat/conf -> /bitnami/tomcat/conf
lrwxrwxrwx 1 root root 20 Sep 28 13:49 /opt/bitnami/tomcat/data -> /bitnami/tomcat/data

• If this file does exist : The container will only link the Tomcat conf and data folder back to the folders in the volume (and maybe fix permissions).

Therefore, the volume structure follows the following schema.

root@2346ab1f5c88:/# tree -L 1 -a /bitnami/tomcat
/bitnami/tomcat
├── conf
├── data
└── .initialized

2 directories, 1 file

If you want to add your own configuration you should either override the default configuration in /bitnami/tomcat/conf after the Tomcat initialization takes place (at runtime) or override the default configuration in /opt/bitnami/tomcat/conf at build time.

This means you could either:

• Extend from bitnami/tomcat and copy your configuration to /opt/bitnami/tomcat/conf .
• Mount your configuration to the container at runtime (to a known folder) and modify the app-entrypoint.sh file to override the /bitnami/tomcat/conf folder with it.
• Copy your configuration to the container at runtime (same as above, but copying with docker cp )
• Run the Tomcat container at least once, then override the conf folder in the local volume with your configuration.