What is the process of rolling updates in Docker Swarm?

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.

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