Container Placement

Controlling Container Placement

• By default a Swarm Service spreads its tasks out over Nodes

  • Tries to also use the least-used node for a task

• Swarm has multiple ways to control which node a container runs on

• Can be used together for complex requirements

  • #1 Node Labels plus Service Constraints (<key>=<value>)

    • Hard requirement. Only schedules tasks if Swarm matches constraint

    • Add labels to nodes first, based on location, hardware, or purpose

    • Then use constraints when creating services

  • #2 Service Modes (replicated|global)

    • Global is good for monitoring/logging/proxy/security tools

    • Only set at service create time

  • #3 (new in 17.04+) Placement Preferences (spread)

    • Soft requirement. For now only used to spread across availability zones

  • #4 Node Availability (active|pause|drain)

  • #5 Resource Requirements (cpu|memory)

Requirements for the sub-section examples

• Created a 3-node (or more) Swarm

• Not all nodes are managers (1 manager and 2 workers is good)

• Created the swarm Visualizer service

  • docker run -it -d -p 5000:8080 -v /var/run/docker.sock:/var/run/docker.sock dockersamples/visualizer

• Visualizer looks like this:

Here manager2 was originally manager but I demoted it to worker for examples purposes.