container.training/slides/k8s/rollout.md

Rolling updates

• By default (without rolling updates), when a scaled resource is updated:

  • new pods are created

  • old pods are terminated

  • ... all at the same time

  • if something goes wrong, ¯\_(ツ)_/¯

---

Rolling updates

• With rolling updates, when a resource is updated, it happens progressively

• Two parameters determine the pace of the rollout: maxUnavailable and maxSurge

• They can be specified in absolute number of pods, or percentage of the replicas count

• At any given time ...

  • there will always be at least replicas-maxUnavailable pods available

  • there will never be more than replicas+maxSurge pods in total

  • there will therefore be up to maxUnavailable+maxSurge pods being updated

• We have the possibility to rollback to the previous version
  (if the update fails or is unsatisfactory in any way)

---

Checking current rollout parameters

• Recall how we build custom reports with kubectl and jq:

.exercise[

• Show the rollout plan for our deployments:

    kubectl get deploy -o json |
            jq ".items[] | {name:.metadata.name} + .spec.strategy.rollingUpdate"
  

]

---

Rolling updates in practice

• As of Kubernetes 1.8, we can do rolling updates with:

  deployments, daemonsets, statefulsets

• Editing one of these resources will automatically result in a rolling update

• Rolling updates can be monitored with the kubectl rollout subcommand

---

Building a new version of the worker service

.exercise[

• Go to the stack directory:

  cd ~/container.training/stacks
  

• Edit dockercoins/worker/worker.py; update the first sleep line to sleep 1 second

• Build a new tag and push it to the registry:

  #export REGISTRY=localhost:3xxxx
  export TAG=v0.2
  docker-compose -f dockercoins.yml build
  docker-compose -f dockercoins.yml push
  

]

---

Rolling out the new worker service

.exercise[

• Let's monitor what's going on by opening a few terminals, and run:

  kubectl get pods -w
  kubectl get replicasets -w
  kubectl get deployments -w
  

• Update worker either with kubectl edit, or by running:

  kubectl set image deploy worker worker=$REGISTRY/worker:$TAG
  

]

--

That rollout should be pretty quick. What shows in the web UI?

---

Give it some time

• At first, it looks like nothing is happening (the graph remains at the same level)

• According to kubectl get deploy -w, the deployment was updated really quickly

• But kubectl get pods -w tells a different story

• The old pods are still here, and they stay in Terminating state for a while

• Eventually, they are terminated; and then the graph decreases significantly

• This delay is due to the fact that our worker doesn't handle signals

• Kubernetes sends a "polite" shutdown request to the worker, which ignores it

• After a grace period, Kubernetes gets impatient and kills the container

  (The grace period is 30 seconds, but can be changed if needed)

---

Rolling out something invalid

• What happens if we make a mistake?

.exercise[

• Update worker by specifying a non-existent image:

  export TAG=v0.3
  kubectl set image deploy worker worker=$REGISTRY/worker:$TAG
  

• Check what's going on:

  kubectl rollout status deploy worker
  

]

--

Our rollout is stuck. However, the app is not dead.

(After a minute, it will stabilize to be 20-25% slower.)

---

What's going on with our rollout?

• Why is our app a bit slower?

• Because MaxUnavailable=25%

  ... So the rollout terminated 2 replicas out of 10 available

• Okay, but why do we see 5 new replicas being rolled out?

• Because MaxSurge=25%

  ... So in addition to replacing 2 replicas, the rollout is also starting 3 more

• It rounded down the number of MaxUnavailable pods conservatively,
  but the total number of pods being rolled out is allowed to be 25+25=50%

---

class: extra-details

The nitty-gritty details

• We start with 10 pods running for the worker deployment

• Current settings: MaxUnavailable=25% and MaxSurge=25%

• When we start the rollout:

  • two replicas are taken down (as per MaxUnavailable=25%)
  • two others are created (with the new version) to replace them
  • three others are created (with the new version) per MaxSurge=25%)

• Now we have 8 replicas up and running, and 5 being deployed

• Our rollout is stuck at this point!

---

Checking the dashboard during the bad rollout

If you haven't deployed the Kubernetes dashboard earlier, just skip this slide.

.exercise[

• Check which port the dashboard is on:

  kubectl -n kube-system get svc socat
  

]

Note the 3xxxx port.

.exercise[

• Connect to http://oneofournodes:3xxxx/

]

--

• We have failures in Deployments, Pods, and Replica Sets

---

Recovering from a bad rollout

• We could push some v0.3 image

  (the pod retry logic will eventually catch it and the rollout will proceed)

• Or we could invoke a manual rollback

.exercise[

• Cancel the deployment and wait for the dust to settle down:

  kubectl rollout undo deploy worker
  kubectl rollout status deploy worker
  

]

---

Changing rollout parameters

• We want to:

  • revert to v0.1
  • be conservative on availability (always have desired number of available workers)
  • go slow on rollout speed (update only one pod at a time)
  • give some time to our workers to "warm up" before starting more

The corresponding changes can be expressed in the following YAML snippet:

.small[

spec:
  template:
    spec:
      containers:
      - name: worker
        image: $REGISTRY/worker:v0.1
  strategy:
    rollingUpdate:
      maxUnavailable: 0
      maxSurge: 1
  minReadySeconds: 10

]

---

Applying changes through a YAML patch

• We could use kubectl edit deployment worker

• But we could also use kubectl patch with the exact YAML shown before

.exercise[

.small[

• Apply all our changes and wait for them to take effect:

  kubectl patch deployment worker -p "
    spec:
      template:
        spec:
          containers:
          - name: worker
            image: $REGISTRY/worker:v0.1
      strategy:
        rollingUpdate:
          maxUnavailable: 0
          maxSurge: 1
      minReadySeconds: 10
    "
  kubectl rollout status deployment worker
  kubectl get deploy -o json worker |
          jq "{name:.metadata.name} + .spec.strategy.rollingUpdate"
  

  ]

]