Wednesday, September 27, 2017
Kubernetes StatefulSets & DaemonSets Updates
Editor’s note: today’s post is by Janet Kuo and Kenneth Owens, Software Engineers at Google.
This post talks about recent updates to the DaemonSet and StatefulSet API objects for Kubernetes. We explore these features using Apache ZooKeeper and Apache Kafka StatefulSets and a Prometheus node exporter DaemonSet.
In Kubernetes 1.6, we added the RollingUpdate update strategy to the DaemonSet API Object. Configuring your DaemonSets with the RollingUpdate strategy causes the DaemonSet controller to perform automated rolling updates to the Pods in your DaemonSets when their spec.template are updated.
In Kubernetes 1.7, we enhanced the DaemonSet controller to track a history of revisions to the PodTemplateSpecs of DaemonSets. This allows the DaemonSet controller to roll back an update. We also added the RollingUpdate strategy to the StatefulSet API Object, and implemented revision history tracking for the StatefulSet controller. Additionally, we added the Parallel pod management policy to support stateful applications that require Pods with unique identities but not ordered Pod creation and termination.
StatefulSet rolling update and Pod management policy
First, we’re going to demonstrate how to use StatefulSet rolling updates and Pod management policies by deploying a ZooKeeper ensemble and a Kafka cluster.
Prerequisites
To follow along, you’ll need to set up a Kubernetes 1.7 cluster with at least 3 schedulable nodes. Each node needs 1 CPU and 2 GiB of memory available. You will also need either a dynamic provisioner to allow the StatefulSet controller to provision 6 persistent volumes (PVs) with 10 GiB each, or you will need to manually provision the PVs prior to deploying the ZooKeeper ensemble or deploying the Kafka cluster.
Deploying a ZooKeeper ensemble
Apache ZooKeeper is a strongly consistent, distributed system used by other distributed systems for cluster coordination and configuration management.
Note: You can create a ZooKeeper ensemble using this zookeeper_mini.yaml manifest. You can learn more about running a ZooKeeper ensemble on Kubernetes here, as well as a more in-depth explanation of the manifest and its contents.
When you apply the manifest, you will see output like the following.
$ kubectl apply -f zookeeper\_mini.yaml
service "zk-hs" created
service "zk-cs" created
poddisruptionbudget "zk-pdb" created
statefulset "zk" created
The manifest creates an ensemble of three ZooKeeper servers using a StatefulSet, zk; a Headless Service, zk-hs, to control the domain of the ensemble; a Service, zk-cs, that clients can use to connect to the ready ZooKeeper instances; and a PodDisruptionBugdet, zk-pdb, that allows for one planned disruption. (Note that while this ensemble is suitable for demonstration purposes, it isn’t sized correctly for production use.)
If you use kubectl get to watch Pod creation in another terminal you will see that, in contrast to the OrderedReady strategy (the default policy that implements the full version of the StatefulSet guarantees), all of the Pods in the zk StatefulSet are created in parallel.
$ kubectl get po -lapp=zk -w
NAME READY STATUS RESTARTS AGE
zk-0 0/1 Pending 0 0s
zk-0 0/1 Pending 0 0s
zk-1 0/1 Pending 0 0s
zk-1 0/1 Pending 0 0s
zk-0 0/1 ContainerCreating 0 0s
zk-2 0/1 Pending 0 0s
zk-1 0/1 ContainerCreating 0 0s
zk-2 0/1 Pending 0 0s
zk-2 0/1 ContainerCreating 0 0s
zk-0 0/1 Running 0 10s
zk-2 0/1 Running 0 11s
zk-1 0/1 Running 0 19s
zk-0 1/1 Running 0 20s
zk-1 1/1 Running 0 30s
zk-2 1/1 Running 0 30s
This is because the zookeeper_mini.yaml manifest sets the podManagementPolicy of the StatefulSet to Parallel.
apiVersion: apps/v1beta1
kind: StatefulSet
metadata:
name: zk
spec:
serviceName: zk-hs
replicas: 3
updateStrategy:
type: RollingUpdate
podManagementPolicy: Parallel
...
Many distributed systems, like ZooKeeper, do not require ordered creation and termination for their processes. You can use the Parallel Pod management policy to accelerate the creation and deletion of StatefulSets that manage these systems. Note that, when Parallel Pod management is used, the StatefulSet controller will not block when it fails to create a Pod. Ordered, sequential Pod creation and termination is performed when a StatefulSet’s podManagementPolicy is set to OrderedReady.
Deploying a Kafka Cluster
Apache Kafka is a popular distributed streaming platform. Kafka producers write data to partitioned topics which are stored, with a configurable replication factor, on a cluster of brokers. Consumers consume the produced data from the partitions stored on the brokers.
Note: Details of the manifests contents can be found here. You can learn more about running a Kafka cluster on Kubernetes here.
To create a cluster, you only need to download and apply the kafka_mini.yaml manifest. When you apply the manifest, you will see output like the following:
$ kubectl apply -f kafka\_mini.yaml
service "kafka-hs" created
poddisruptionbudget "kafka-pdb" created
statefulset "kafka" created
The manifest creates a three broker cluster using the kafka StatefulSet, a Headless Service, kafka-hs, to control the domain of the brokers; and a PodDisruptionBudget, kafka-pdb, that allows for one planned disruption. The brokers are configured to use the ZooKeeper ensemble we created above by connecting through the zk-cs Service. As with the ZooKeeper ensemble deployed above, this Kafka cluster is fine for demonstration purposes, but it’s probably not sized correctly for production use.
If you watch Pod creation, you will notice that, like the ZooKeeper ensemble created above, the Kafka cluster uses the Parallel podManagementPolicy.
$ kubectl get po -lapp=kafka -w
NAME READY STATUS RESTARTS AGE
kafka-0 0/1 Pending 0 0s
kafka-0 0/1 Pending 0 0s
kafka-1 0/1 Pending 0 0s
kafka-1 0/1 Pending 0 0s
kafka-2 0/1 Pending 0 0s
kafka-0 0/1 ContainerCreating 0 0s
kafka-2 0/1 Pending 0 0s
kafka-1 0/1 ContainerCreating 0 0s
kafka-1 0/1 Running 0 11s
kafka-0 0/1 Running 0 19s
kafka-1 1/1 Running 0 23s
kafka-0 1/1 Running 0 32s
Producing and consuming data
You can use kubectl run to execute the kafka-topics.sh script to create a topic named test.
$ kubectl run -ti --image=gcr.io/google\_containers/kubernetes-kafka:1.0-10.2.1 createtopic --restart=Never --rm -- kafka-topics.sh --create \
\> --topic test \
\> --zookeeper zk-cs.default.svc.cluster.local:2181 \
\> --partitions 1 \
\> --replication-factor 3
Now you can use kubectl run to execute the kafka-console-consumer.sh command to listen for messages.
$ kubectl run -ti --image=gcr.io/google\_containers/kubnetes-kafka:1.0-10.2.1 consume --restart=Never --rm -- kafka-console-consumer.sh --topic test --bootstrap-server kafka-0.kafka-hs.default.svc.cluster.local:9093
In another terminal, you can run the kafka-console-producer.sh command.
$kubectl run -ti --image=gcr.io/google\_containers/kubernetes-kafka:1.0-10.2.1 produce --restart=Never --rm \
\> -- kafka-console-producer.sh --topic test --broker-list kafka-0.kafka-hs.default.svc.cluster.local:9093,kafka-1.kafka-hs.default.svc.cluster.local:9093,kafka-2.kafka-hs.default.svc.cluster.local:9093
Output from the second terminal appears in the first terminal. If you continue to produce and consume messages while updating the cluster, you will notice that no messages are lost. You may see error messages as the leader for the partition changes when individual brokers are updated, but the client retries until the message is committed. This is due to the ordered, sequential nature of StatefulSet rolling updates which we will explore further in the next section.
Updating the Kafka cluster
StatefulSet updates are like DaemonSet updates in that they are both configured by setting the spec.updateStrategy of the corresponding API object. When the update strategy is set to OnDelete, the respective controllers will only create new Pods when a Pod in the StatefulSet or DaemonSet has been deleted. When the update strategy is set to RollingUpdate, the controllers will delete and recreate Pods when a modification is made to the spec.template field of a DaemonSet or StatefulSet. You can use rolling updates to change the configuration (via environment variables or command line parameters), resource requests, resource limits, container images, labels, and/or annotations of the Pods in a StatefulSet or DaemonSet. Note that all updates are destructive, always requiring that each Pod in the DaemonSet or StatefulSet be destroyed and recreated. StatefulSet rolling updates differ from DaemonSet rolling updates in that Pod termination and creation is ordered and sequential.
You can patch the kafka StatefulSet to reduce the CPU resource request to 250m.
$ kubectl patch sts kafka --type='json' -p='[{"op": "replace", "path": "/spec/template/spec/containers/0/resources/requests/cpu", "value":"250m"}]'
statefulset "kafka" patched
If you watch the status of the Pods in the StatefulSet, you will see that each Pod is deleted and recreated in reverse ordinal order (starting with the Pod with the largest ordinal and progressing to the smallest). The controller waits for each updated Pod to be running and ready before updating the subsequent Pod.
$kubectl get po -lapp=kafka -w
NAME READY STATUS RESTARTS AGE
kafka-0 1/1 Running 0 13m
kafka-1 1/1 Running 0 13m
kafka-2 1/1 Running 0 13m
kafka-2 1/1 Terminating 0 14m
kafka-2 0/1 Terminating 0 14m
kafka-2 0/1 Terminating 0 14m
kafka-2 0/1 Terminating 0 14m
kafka-2 0/1 Pending 0 0s
kafka-2 0/1 Pending 0 0s
kafka-2 0/1 ContainerCreating 0 0s
kafka-2 0/1 Running 0 10s
kafka-2 1/1 Running 0 21s
kafka-1 1/1 Terminating 0 14m
kafka-1 0/1 Terminating 0 14m
kafka-1 0/1 Terminating 0 14m
kafka-1 0/1 Terminating 0 14m
kafka-1 0/1 Pending 0 0s
kafka-1 0/1 Pending 0 0s
kafka-1 0/1 ContainerCreating 0 0s
kafka-1 0/1 Running 0 11s
kafka-1 1/1 Running 0 21s
kafka-0 1/1 Terminating 0 14m
kafka-0 0/1 Terminating 0 14m
kafka-0 0/1 Terminating 0 14m
kafka-0 0/1 Terminating 0 14m
kafka-0 0/1 Pending 0 0s
kafka-0 0/1 Pending 0 0s
kafka-0 0/1 ContainerCreating 0 0s
kafka-0 0/1 Running 0 10s
kafka-0 1/1 Running 0 22s
Note that unplanned disruptions will not lead to unintentional updates during the update process. That is, the StatefulSet controller will always recreate the Pod at the correct version to ensure the ordering of the update is preserved. If a Pod is deleted, and if it has already been updated, it will be created from the updated version of the StatefulSet’s spec.template. If the Pod has not already been updated, it will be created from the previous version of the StatefulSet’s spec.template. We will explore this further in the following sections.
Staging an update
Depending on how your organization handles deployments and configuration modifications, you may want or need to stage updates to a StatefulSet prior to allowing the roll out to progress. You can accomplish this by setting a partition for the RollingUpdate. When the StatefulSet controller detects a partition in the updateStrategy of a StatefulSet, it will only apply the updated version of the StatefulSet’s spec.template to Pods whose ordinal is greater than or equal to the value of the partition.
You can patch the kafka StatefulSet to add a partition to the RollingUpdate update strategy. If you set the partition to a number greater than or equal to the StatefulSet’s spec.replicas (as below), any subsequent updates you perform to the StatefulSet’s spec.template will be staged for roll out, but the StatefulSet controller will not start a rolling update.
$ kubectl patch sts kafka -p '{"spec":{"updateStrategy":{"type":"RollingUpdate","rollingUpdate":{"partition":3}}}}'
statefulset "kafka" patched
If you patch the StatefulSet to set the requested CPU to 0.3, you will notice that none of the Pods are updated.
$ kubectl patch sts kafka --type='json' -p='[{"op": "replace", "path": "/spec/template/spec/containers/0/resources/requests/cpu", "value":"0.3"}]'
statefulset "kafka" patched
Even if you delete a Pod and wait for the StatefulSet controller to recreate it, you will notice that the Pod is recreated with current CPU request.
$ kubectl delete po kafka-1
pod "kafka-1" deleted
$ kubectl get po kafka-1 -w
NAME READY STATUS RESTARTS AGE
kafka-1 0/1 ContainerCreating 0 10s
kafka-1 0/1 Running 0 19s
kafka-1 1/1 Running 0 21s
$ kubectl get po kafka-1 -o yaml
apiVersion: v1
kind: Pod
metadata:
...
resources:
requests:
cpu: 250m
memory: 1Gi
Rolling out a canary
Often, we want to verify an image update or configuration change on a single instance of an application before rolling it out globally. If you modify the partition created above to be 2, the StatefulSet controller will roll out a canary that can be used to verify that the update is working as intended.
$ kubectl patch sts kafka -p '{"spec":{"updateStrategy":{"type":"RollingUpdate","rollingUpdate":{"partition":2}}}}'
statefulset "kafka" patched
You can watch the StatefulSet controller update the kafka-2 Pod and pause after the update is complete.
$ kubectl get po -lapp=kafka -w
NAME READY STATUS RESTARTS AGE
kafka-0 1/1 Running 0 50m
kafka-1 1/1 Running 0 10m
kafka-2 1/1 Running 0 29s
kafka-2 1/1 Terminating 0 34s
kafka-2 0/1 Terminating 0 38s
kafka-2 0/1 Terminating 0 39s
kafka-2 0/1 Terminating 0 39s
kafka-2 0/1 Pending 0 0s
kafka-2 0/1 Pending 0 0s
kafka-2 0/1 Terminating 0 20s
kafka-2 0/1 Terminating 0 20s
kafka-2 0/1 Pending 0 0s
kafka-2 0/1 Pending 0 0s
kafka-2 0/1 ContainerCreating 0 0s
kafka-2 0/1 Running 0 19s
kafka-2 1/1 Running 0 22s
Phased roll outs
Similar to rolling out a canary, you can roll out updates based on a phased progression (e.g. linear, geometric, or exponential roll outs).
If you patch the kafka StatefulSet to set the partition to 1, the StatefulSet controller updates one more broker.
$ kubectl patch sts kafka -p '{"spec":{"updateStrategy":{"type":"RollingUpdate","rollingUpdate":{"partition":1}}}}'
statefulset "kafka" patched
If you set it to 0, the StatefulSet controller updates the final broker and completes the update.
$ kubectl patch sts kafka -p '{"spec":{"updateStrategy":{"type":"RollingUpdate","rollingUpdate":{"partition":0}}}}'
statefulset "kafka" patched
Note that you don’t have to decrement the partition by one. For a larger StatefulSet–for example, one with 100 replicas–you might use a progression more like 100, 99, 90, 50, 0. In this case, you would stage your update, deploy a canary, roll out to 10 instances, update fifty percent of the Pods, and then complete the update.
Cleaning up
To delete the API Objects created above, you can use kubectl delete on the two manifests you used to create the ZooKeeper ensemble and the Kafka cluster.
$ kubectl delete -f kafka\_mini.yaml
service "kafka-hs" deleted
poddisruptionbudget "kafka-pdb" deleted
Statefulset “kafka” deleted
$ kubectl delete -f zookeeper\_mini.yaml
service "zk-hs" deleted
service "zk-cs" deleted
poddisruptionbudget "zk-pdb" deleted
statefulset "zk" deleted
By design, the StatefulSet controller does not delete any persistent volume claims (PVCs): the PVCs created for the ZooKeeper ensemble and the Kafka cluster must be manually deleted. Depending on the storage reclamation policy of your cluster, you many also need to manually delete the backing PVs.
DaemonSet rolling update, history, and rollback
In this section, we’re going to show you how to perform a rolling update on a DaemonSet, look at its history, and then perform a rollback after a bad rollout. We will use a DaemonSet to deploy a Prometheus node exporter on each Kubernetes node in the cluster. These node exporters export node metrics to the Prometheus monitoring system. For the sake of simplicity, we’ve omitted the installation of the Prometheus server and the service for communication with DaemonSet pods from this blogpost.
Prerequisites
To follow along with this section of the blog, you need a working Kubernetes 1.7 cluster and kubectl version 1.7 or later. If you followed along with the first section, you can use the same cluster.
DaemonSet rolling upFirst, prepare the node exporter DaemonSet manifest to run a v0.13 Prometheus node exporter on every node in the cluster:
$ cat \>\> node-exporter-v0.13.yaml \<\<EOF
apiVersion: extensions/v1beta1
kind: DaemonSet
metadata:
name: node-exporter
spec:
updateStrategy:
type: RollingUpdate
template:
metadata:
labels:
app: node-exporter
name: node-exporter
spec:
containers:
- image: prom/node-exporter:v0.13.0
name: node-exporter
ports:
- containerPort: 9100
hostPort: 9100
name: scrape
hostNetwork: true
hostPID: true
EOF
Note that you need to enable the DaemonSet rolling update feature by explicitly setting DaemonSet .spec.updateStrategy.type to RollingUpdate.
Apply the manifest to create the node exporter DaemonSet:
$ kubectl apply -f node-exporter-v0.13.yaml --record
daemonset "node-exporter" created
Wait for the first DaemonSet rollout to complete:
$ kubectl rollout status ds node-exporter
daemon set "node-exporter" successfully rolled out
You should see each of your node runs one copy of the node exporter pod:
$ kubectl get pods -l app=node-exporter -o wide
To perform a rolling update on the node exporter DaemonSet, prepare a manifest that includes the v0.14 Prometheus node exporter:
$ cat node-exporter-v0.13.yaml ``` sed "s/v0.13.0/v0.14.0/g" \> node-exporter-v0.14.yaml
Then apply the v0.14 node exporter DaemonSet:
$ kubectl apply -f node-exporter-v0.14.yaml --record
daemonset "node-exporter" configured
Wait for the DaemonSet rolling update to complete:
$ kubectl rollout status ds node-exporter
...
Waiting for rollout to finish: 3 out of 4 new pods have been updated...
Waiting for rollout to finish: 3 of 4 updated pods are available...
daemon set "node-exporter" successfully rolled out
We just triggered a DaemonSet rolling update by updating the DaemonSet template. By default, one old DaemonSet pod will be killed and one new DaemonSet pod will be created at a time.
Now we’ll cause a rollout to fail by updating the image to an invalid value:
$ cat node-exporter-v0.13.yaml | sed "s/v0.13.0/bad/g" \> node-exporter-bad.yaml
$ kubectl apply -f node-exporter-bad.yaml --record
daemonset "node-exporter" configured
Notice that the rollout never finishes:
$ kubectl rollout status ds node-exporter
Waiting for rollout to finish: 0 out of 4 new pods have been updated...
Waiting for rollout to finish: 1 out of 4 new pods have been updated…
# Use ^C to exit
This behavior is expected. We mentioned earlier that a DaemonSet rolling update kills and creates one pod at a time. Because the new pod never becomes available, the rollout is halted, preventing the invalid specification from propagating to more than one node. StatefulSet rolling updates implement the same behavior with respect to failed deployments. Unsuccessful updates are blocked until it corrected via roll back or by rolling forward with a specification.
$ kubectl get pods -l app=node-exporter
NAME READY STATUS RESTARTS AGE
node-exporter-f2n14 0/1 ErrImagePull 0 3m
...
# N = number of nodes
$ kubectl get ds node-exporter
NAME DESIRED CURRENT READY UP-TO-DATE AVAILABLE NODE SELECTOR AGE
node-exporter N N N-1 1 N \<none\> 46m
DaemonSet history, rollbacks, and rolling forward
Next, perform a rollback. Take a look at the node exporter DaemonSet rollout history:
$ kubectl rollout history ds node-exporter
daemonsets "node-exporter"
REVISION CHANGE-CAUSE
1 kubectl apply --filename=node-exporter-v0.13.yaml --record=true
2 kubectl apply --filename=node-exporter-v0.14.yaml --record=true
3 kubectl apply --filename=node-exporter-bad.yaml --record=true
Check the details of the revision you want to roll back to:
$ kubectl rollout history ds node-exporter --revision=2
daemonsets "node-exporter" with revision #2
Pod Template:
Labels: app=node-exporter
Containers:
node-exporter:
Image: prom/node-exporter:v0.14.0
Port: 9100/TCP
Environment: \<none\>
Mounts: \<none\>
Volumes: \<none\>
You can quickly roll back to any DaemonSet revision you found through kubectl rollout history:
# Roll back to the last revision
$ kubectl rollout undo ds node-exporter
daemonset "node-exporter" rolled back
# Or use --to-revision to roll back to a specific revision
$ kubectl rollout undo ds node-exporter --to-revision=2
daemonset "node-exporter" rolled back
A DaemonSet rollback is done by rolling forward. Therefore, after the rollback, DaemonSet revision 2 becomes revision 4 (current revision):
$ kubectl rollout history ds node-exporter
daemonsets "node-exporter"
REVISION CHANGE-CAUSE
1 kubectl apply --filename=node-exporter-v0.13.yaml --record=true
3 kubectl apply --filename=node-exporter-bad.yaml --record=true
4 kubectl apply --filename=node-exporter-v0.14.yaml --record=true
The node exporter DaemonSet is now healthy again:
$ kubectl rollout status ds node-exporter
daemon set "node-exporter" successfully rolled out
# N = number of nodes
$ kubectl get ds node-exporter
NAME DESIRED CURRENT READY UP-TO-DATE AVAILABLE NODE SELECTOR AGE
node-exporter N N N N N \<none\> 46m
If current DaemonSet revision is specified while performing a rollback, the rollback is skipped:
$ kubectl rollout undo ds node-exporter --to-revision=4
daemonset "node-exporter" skipped rollback (current template already matches revision 4)
You will see this complaint from kubectl if the DaemonSet revision is not found:
$ kubectl rollout undo ds node-exporter --to-revision=10
error: unable to find specified revision 10 in history
Note that kubectl rollout history and kubectl rollout status support StatefulSets, too!
Cleaning up
$ kubectl delete ds node-exporter
What’s next for DaemonSet and StatefulSet
Rolling updates and roll backs close an important feature gap for DaemonSets and StatefulSets. As we plan for Kubernetes 1.8, we want to continue to focus on advancing the core controllers to GA. This likely means that some advanced feature requests (e.g. automatic roll back, infant mortality detection) will be deferred in favor of ensuring the consistency, usability, and stability of the core controllers. We welcome feedback and contributions, so please feel free to reach out on Slack, to ask questions on Stack Overflow, or open issues or pull requests on GitHub.
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