Kubernetes Scheduler

I. Concepts

Node labels

When create a node, we can mark some labels for it.
On AWS nodes are assigned by default some labels such as:

node.kubernetes.io/instance-type=t3.xlarge
kubernetes.io/arch=amd64
node.kubernetes.io/instance-type=t3.xlarge

Node taints

A node taint lets you mark a node to prevent using it for Pods.

A taint consists of a key, value, and effect.

Available effect values:

NoSchedule
- New pods that do not match the taint are not scheduled onto that node.
- Existing pods on the node remain.
PreferNoSchedule
- New pods that do not match the taint might be scheduled onto that node, but the scheduler tries not to.
- Existing pods on the node remain.
NoExecute
- New pods that do not match the taint cannot be scheduled onto that node.
- Existing pods on the node that do not have a matching toleration are removed.
- Pods that tolerate the taint and specifies tolerationSeconds remain bound for the specified amount of time. After that time elapses, this pod will be evicted.

II. Assigning Pods to Nodes

There are 2 main ways to schedule a pod into a specific node:

1/ nodeSelector (pod) - node labales (node)

spec.nodeSelector is a map that specific the node labels that this pod want to be deployed on.

2/ toleration (pod) - taint (node)

Pods that have suitable toleration with the node's taint will be scheduled to the node. The matching rule includes the matching key, value, and the valid rule of effect described above.