Docker Swarm

Docker Swarm is a group of either physical or virtual machines that are running the Docker application and that have been configured to join together in a cluster. Once a group of machines have been clustered together, you can still run the Docker commands that you're used to, but they will now be carried out by the machines in your cluster.

The activities of the cluster are controlled by a swarm manager, and machines that have joined the cluster are referred to as nodes.

Docker swarm is a container orchestration tool, meaning that it allows the user to manage multiple containers deployed across multiple host machines.

The cluster management and orchestration features embedded in the Docker Engine are built using Swarmkit. 

A Node is an instance of the Docker engine participating in the swarm. 

Worker Nodes receive and execute tasks dispatched from manager nodes. By default manager nodes also run services as worker nodes, but you can configure them to run manager tasks exclusively and be manager-only nodes. 

Manager nodes  perform the orchestration and cluster management functions required to maintain the desired state of the swarm. 

The swarm manager can automatically assign the service a PublishedPort or you can configure a PublishedPort for the service.

A Service is the definition of tasks to execute on the manager or worker nodes. It is the central structure of the swarm system and the primary root of user interaction with the swarm.

Node Management:

• Initialize a swarm: docker swarm init
• List swarm nodes: docker node ls

Activate a node (after maintenance): 

docker node update --availability active node_name

Service management:

• List services (manager node): docker service ls
• Describe services (manager node): docker service ps service_name
• Inspect a service: docker service inspect service_name
• Scale a service: docker service scale service_name=N
• Remove service: docker service rm service_name

Stack management

• Deploy stack from docker-compose file: docker stack deploy -c docker-compose.yml stack_name
• List stacks: docker stack ls
• List stack services: docker stack services stack_name
• List stack tasks: docker stack ps stack_name
• Remove stack : docker stack rm stack_name

Network management:

• List networks: docker network ls
• Create overlay network: docker network create -d overlay network_name
• Remove network: docker network rm network_name

Monitor services

• Docker stats: docker stats
• Service logs: docker service logs service_name