3. Swarms

Swarm clusters

Swarm = enjambre

A group of machines (nodes) that are running Docker and joined into a cluster.

Nodes: the machines in a swarm, can be physical or virtual.

Docker can either run in single-host mode or in swarm mode (enables the use of swarms).

Enabling swarm mode instantly makes the current machine a swarm manager. From then on, Docker runs the commands you execute on the swarm you’re managing, rather than just on the current machine.

Swarm managers can use several strategies to run containers (specified in the Compose file):

• emptiest node: fills the least utilized machines with containers

• global: ensures that each machine gets exactly one instance of the specified container

Swarm managers are the only machines in a swarm that can execute your commands, or authorize other machines to join the swarm as workers. Workers are just there to provide capacity and do not have the authority to tell any other machine what it can and cannot do.

Set up your swarm

A swarm is made up of multiple nodes, which can be either physical or virtual machines.

docker swarm init to enable swarm mode and make your current machine a swarm manager, then run docker swarm join on other machines to have them join the swarm as workers.