What is Docker Swarm?

Docker Swarm is Docker's native clustering and orchestration solution, built into the Docker engine. It transforms a group of Docker hosts into a single virtual Docker host, enabling you to deploy and manage multi-container applications across multiple machines. Key concepts: (1) Swarm: the cluster of Docker hosts; (2) Node: a Docker host in the swarm — either a Manager (manages cluster state, schedules tasks) or Worker (executes containers); (3) Service: the definition of tasks to execute on Swarm — like a docker-compose.yml but for clusters. Includes the image, replicas count, update config, resource limits; (4) Task: a running container that is part of a service; (5) Stack: a group of services sharing a network, deployed with a Compose file (docker stack deploy -c compose.yml mystack). Features: load balancing (built-in mesh routing), service discovery, rolling updates with rollback, scaling (docker service scale web=5), secrets management, health checks, and self-healing (restarts failed containers). Initialize: docker swarm init. Add worker: docker swarm join --token TOKEN manager-ip:2377. Docker Swarm is simpler than Kubernetes but has less ecosystem support. Most new deployments use Kubernetes instead.