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)

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.

Container Placement