Swarm 514 mode arrived in Docker 1.12 514 (2016), moving orchestration into the daemon itself (the SwarmKit library) and replacing the older standalone "Docker Swarm" product:
Managers keep the desired state in a replicated Raft log, schedule work and serve the API; one leads.
Workers run containers. Managers are workers too, unless drained.
A service declares what should run (image, replicas, ports, update policy); each container it creates is a task, pinned to one node. A stack is a group of services deployed from a Compose 514 file.
In 2026 Swarm is maintained, not developed. Engine 29.8.0 (3 September 2026) still carried Swarm and overlay fixes, and Mirantis, which bought Docker's enterprise business in 2019, committed in July 2025 to support Swarm in Mirantis Kubernetes 5,150 Engine 3 through 2030. But the ecosystem, the cloud providers and the job market have converged on Kubernetes, which 82% of container users ran in production in the CNCF's 2025 survey. Swarm's concepts (desired state, replicas, rolling updates, overlay networks) carry straight over to Kubernetes.