What is the process of rolling updates in Docker Swarm?

In Docker Swarm, rolling updates refer to the strategy used to update a service by gradually replacing running containers with new ones to minimize downtime and maintain application availability. The rolling update process involves the following steps:

1. Create a New Service Version:

   • Create a new version of your Docker service with the desired changes, such as updated application code or configuration. This could involve building a new Docker image or updating the service definition.

2. Update Service with New Version:

   • Use the docker service update command to apply the changes to the service. For example:

       docker service update --image <new-image> <service-name>
     

     This command updates the service to use the new image.

3. Rolling Update Begins:

   • Docker Swarm initiates the rolling update process, starting with a small number of tasks (containers) from the new version while maintaining the existing tasks from the old version.

4. Monitor and Verify:

   • Monitor the update progress using commands like docker service ps <service-name> to view the state of tasks. Verify that the new tasks are running and healthy.

5. Incremental Replacement:

   • Docker Swarm continues to replace containers incrementally, ensuring that the service remains available. The number of containers replaced at a time can be controlled through the --update-parallelism and --update-delay options.

6. Completion and Cleanup:

   • The rolling update completes when all tasks have been replaced with the new version. The old containers are gradually decommissioned.

By using a rolling update strategy, Docker Swarm helps maintain service availability during updates. It avoids abrupt service interruptions by systematically replacing containers in a controlled manner. This is particularly important in production environments where minimizing downtime is a critical consideration.