Running a Container

Running a Container

To execute a container image and create the container — that is, the image instance in memory and the kernel process that runs it — you must execute three separate Docker commands, as follows:

1. docker pull — Downloads the image from the registry.

2. docker create — Defines the container instance and its parameters.

3. docker start — Starts (launches) the container.

For convenience, however, the docker run command combines these commands, which it executes in sequence, and is the typical means of creating and starting a container.

The docker run command has a number of options, and it is important to remember that the command that creates the container instance defines its characteristics for its operational lifetime; while a running container can be stopped and then restarted (not a typical practice in production environments), the aspects of its execution determined by the docker run command cannot be changed. For instance, a storage location can be mounted as a volume within the container with an option in the docker run command (for example, --volume /home/storage:/storage3), but the volumes mounted in this fashion in the command are fixed for that instantiation of the image; they cannot be modified or added to.

When a containerized application is modified — for example, it is upgraded, or components are added — the existing container is removed, and a new container is created and started by instantiating a different image with the docker run command. The new container itself has no association with the previous container, but if the command creating and starting it publishes the same ports, connects to the same network, and mounts the same external storage locations, it effectively replaces the previous container. (For information about upgrading InterSystems IRIS containers, see Upgrading InterSystems IRIS Containers.)