[kwlug-disc] Docker Swarm -- placement constraints and why July 8th demo ... had no graphics

[Mikalai Birukou]

Post mortem:

1. In yesterday's demo of Docker Swarm, I destroyed swarm to show that 
running stack requires getting into swarm node, i.e. requires swarm.

2. Destruction of swarm removed node labels (those things that allow 
controlled placement of processes).

3. Two services in the compose file were constraint to be placed only on 
nodes with particular label.

4. When I added labels via "docker node update --add-label key=value" 
docker recognized this as a change to its resources (nodes) and reacted 
accordingly by starting missing processes.


Useful observations:

a. Labels should be recreate on the machine that changes swarms.

b. Docker is a little silent when services are not started due to lack 
of resources.

c. Reaction to "node update" is similar to addition of a node: swarm is 
trying to start processes. This is nice.