Deployment Affinities

When creating services, you can direct the Node(s) to which the containers should be launched based on scheduling rules.

Affinity

An affinity condition happens when Kontena is trying to find a field that matches (==) a given value. An anti-affinity condition happens when Kontena is trying to find a field that does not match (!=) a given value.

affinity:
  - "<condition 1>"
  - "<condition N>"

Kontena has the ability to compare values against node name, node labels, service name and container name.

For example:

• node==node-1 will match node with name node-1
• node!=node-1 will match all nodes, except node-1
• label==az=1a will match all nodes with label az=1a
• label!=az=1a will match all nodes, except nodes with label az=1a
• service==mysql will match all nodes that have instance of service mysql deployed
• service!=mysql will match all nodes, except those that have instance of service mysql deployed
• container==mysql.db-1 will match all nodes that have container mysql.db-1
• container!=mysql.db-1 will match all nodes, except those that have container mysql.db-1

Soft Affinity

By default affinities are hard-enforced. If an affinity is not met, the service won't be scheduled. With soft affinity the scheduler tries to meet the rule. If rule is not met, the scheduler will discard the filter and schedule the service according to other filters / deployment strategy.

Soft affinities are expressed with ~.

For example:

• label==~node=node-1 tries to match nodes with name node-1. Affinity is discarded if none of the nodes have matching name.
• service!=~mysql tries to match nodes which don't have instance of mysql service deployed. Affinity is discarded if all nodes have instance of mysql service deployed.