When deploying a new version of a Docker Swarm service without causing any downtime for users, what specific update strategy is employed?

The specific update strategy employed by Docker Swarm for deploying a new version of a service without causing any downtime for users is called a rolling update. A rolling update works by incrementally replacing old versions of service tasks, which are individual containers, with new versions. When a user initiates a service update, for example, by changing the image tag of a service definition, the Docker Swarm orchestrator begins the update process. Swarm selects a subset of the currently running tasks to update. For each task selected for an update, Swarm first creates a new container running the updated image and configuration. A critical step for ensuring zero downtime involves health checks. These are predefined checks within the service specification that verify if the application inside the new container is fully started, responsive, and ready to serve traffic. Once the new container successfully passes its health checks and is confirmed to be healthy, Swarm's internal load balancer, known as the routing mesh, automatically directs incoming user requests to this newly available, healthy container. Only after the new container is fully operational and receiving traffic is the corresponding old container terminated. This process continues in batches until all old containers have been replaced by new ones. Key parameters control this process. The `update-parallelism` parameter determines how many tasks are updated concurrently. For instance, setting `update-parallelism` to 1 means only one container is updated at a time, providing maximum stability. The `update-delay` parameter introduces a pause between the updating of batches of tasks, allowing time for new tasks to stabilize before the next batch begins. The `update-monitor` parameter defines the duration Swarm waits after a new task starts for it to become healthy. If a new task does not become healthy within this timeframe, the update for that specific task is considered failed. In such a scenario, the `update-failure-action` parameter dictates Swarm's response. Common actions include `pause`, which stops the update, or `rollback`, which automatically reverts the entire service to its previous stable version. This automatic rollback capability is vital for mitigating the impact of failed deployments. By systematically bringing new, healthy instances online and integrating them into the service's traffic flow before decommissioning old instances, rolling updates ensure that the service always has a sufficient number of healthy containers available to handle user requests, effectively achieving a zero-downtime deployment.