Ordered startup

Learn ordered Pod startup and shutdown.

Ordered start

With the default podManagementPolicy: OrderedReady, pods are created in order: db-1 doesn't start until db-0 is Ready. Scaling down happens in reverse (highest ordinal first). If there's no dependency between replicas, Parallel starts them all at once:

$ kubectl get pods -l app=db -w
db-0   0/1   ContainerCreating   0   2s
db-0   1/1   Running             0   9s
db-1   0/1   Pending             0   0s
db-1   1/1   Running             0   8s
db-2   0/1   Pending             0   0s

Partitioned updates

A StatefulSet update also proceeds in reverse, one by one. partition updates only pods with an ordinal greater than or equal to the partition value; it's used to try the new version on one replica first (canary):

spec:
  updateStrategy:
    type: RollingUpdate
    rollingUpdate:
      partition: 2       # only db-2 is updated

kubectl patch sts db -p '{"spec":{"updateStrategy":{"rollingUpdate":{"partition":0}}}}'   # update them all

Exercise A canary update

Write a StatefulSet's updateStrategy section (starting with updateStrategy:): type: RollingUpdate, rollingUpdate.partition: 2. Then, on a separate line, write the command watching the pods of the db StatefulSet live (-l app=db -w).

Show example solution

updateStrategy:
  type: RollingUpdate
  rollingUpdate:
    partition: 2

kubectl get pods -l app=db -w

Test yourself

Under the OrderedReady policy, what happens if db-0 never passes its readiness check?
- db-0 is deleted
- db-1 becomes the primary
- db-1 and db-2 are never created
- All start at once
Show answer

db-1 and db-2 are never created — The next pod waits for the previous one to be Ready.

Start the pods at the same time.

spec:
  podManagementPolicy: ____

Show answer

spec:
  podManagementPolicy: Parallel

Match.

OrderedReady

In order, waiting for ready
Parallel

At the same time
partition: 2

Only db-2 and above
scale-down

Starts from the highest ordinal

Next lesson: Summary - StatefulSet

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