--- approvers: - bprashanth - enisoc - erictune - foxish - janetkuo - kow3ns - smarterclayton title: Running ZooKeeper, A CP Distributed System --- {% capture overview %} This tutorial demonstrates [Apache Zookeeper](https://zookeeper.apache.org) on Kubernetes using [StatefulSets](/docs/concepts/abstractions/controllers/statefulsets/), [PodDisruptionBudgets](/docs/admin/disruptions/#specifying-a-poddisruptionbudget), and [PodAntiAffinity](/docs/user-guide/node-selection/#inter-pod-affinity-and-anti-affinity-beta-feature). {% endcapture %} {% capture prerequisites %} Before starting this tutorial, you should be familiar with the following Kubernetes concepts. * [Pods](/docs/user-guide/pods/single-container/) * [Cluster DNS](/docs/concepts/services-networking/dns-pod-service/) * [Headless Services](/docs/concepts/services-networking/service/#headless-services) * [PersistentVolumes](/docs/concepts/storage/volumes/) * [PersistentVolume Provisioning](http://releases.k8s.io/{{page.githubbranch}}/examples/persistent-volume-provisioning/) * [ConfigMaps](/docs/tasks/configure-pod-container/configmap/) * [StatefulSets](/docs/concepts/abstractions/controllers/statefulsets/) * [PodDisruptionBudgets](/docs/admin/disruptions/#specifying-a-poddisruptionbudget) * [PodAntiAffinity](/docs/user-guide/node-selection/#inter-pod-affinity-and-anti-affinity-beta-feature) * [kubectl CLI](/docs/user-guide/kubectl) You will require a cluster with at least four nodes, and each node will require at least 2 CPUs and 4 GiB of memory. In this tutorial you will cordon and drain the cluster's nodes. **This means that all Pods on the cluster's nodes will be terminated and evicted, and the nodes will, temporarily, become unschedulable.** You should use a dedicated cluster for this tutorial, or you should ensure that the disruption you cause will not interfere with other tenants. This tutorial assumes that your cluster is configured to dynamically provision PersistentVolumes. If your cluster is not configured to do so, you will have to manually provision three 20 GiB volumes prior to starting this tutorial. {% endcapture %} {% capture objectives %} After this tutorial, you will know the following. * How to deploy a ZooKeeper ensemble using StatefulSet. * How to consistently configure the ensemble using ConfigMaps. * How to spread the deployment of ZooKeeper servers in the ensemble. * How to use PodDisruptionBudgets to ensure service availability during planned maintenance. {% endcapture %} {% capture lessoncontent %} ### ZooKeeper Basics [Apache ZooKeeper](https://zookeeper.apache.org/doc/current/) is a distributed, open-source coordination service for distributed applications. ZooKeeper allows you to read, write, and observe updates to data. Data are organized in a file system like hierarchy and replicated to all ZooKeeper servers in the ensemble (a set of ZooKeeper servers). All operations on data are atomic and sequentially consistent. ZooKeeper ensures this by using the [Zab](https://pdfs.semanticscholar.org/b02c/6b00bd5dbdbd951fddb00b906c82fa80f0b3.pdf) consensus protocol to replicate a state machine across all servers in the ensemble. The ensemble uses the Zab protocol to elect a leader, and data can not be written until a leader is elected. Once a leader is elected, the ensemble uses Zab to ensure that all writes are replicated to a quorum before they are acknowledged and made visible to clients. Without respect to weighted quorums, a quorum is a majority component of the ensemble containing the current leader. For instance, if the ensemble has three servers, a component that contains the leader and one other server constitutes a quorum. If the ensemble can not achieve a quorum, data can not be written. ZooKeeper servers keep their entire state machine in memory, but every mutation is written to a durable WAL (Write Ahead Log) on storage media. When a server crashes, it can recover its previous state by replaying the WAL. In order to prevent the WAL from growing without bound, ZooKeeper servers will periodically snapshot their in memory state to storage media. These snapshots can be loaded directly into memory, and all WAL entries that preceded the snapshot may be safely discarded. ## Creating a ZooKeeper Ensemble The manifest below contains a [Headless Service](/docs/user-guide/services/#headless-services), a [ConfigMap](/docs/tasks/configure-pod-container/configmap/), a [PodDisruptionBudget](/docs/admin/disruptions/#specifying-a-poddisruptionbudget), and a [StatefulSet](/docs/concepts/abstractions/controllers/statefulsets/). {% include code.html language="yaml" file="zookeeper.yaml" ghlink="/docs/tutorials/stateful-application/zookeeper.yaml" %} Open a command terminal, and use [`kubectl create`](/docs/user-guide/kubectl/{{page.version}}/#create) to create the manifest. ```shell kubectl create -f https://k8s.io/docs/tutorials/stateful-application/zookeeper.yaml ``` This creates the `zk-headless` Headless Service, the `zk-config` ConfigMap, the `zk-budget` PodDisruptionBudget, and the `zk` StatefulSet. ```shell service "zk-headless" created configmap "zk-config" created poddisruptionbudget "zk-budget" created statefulset "zk" created ``` Use [`kubectl get`](/docs/user-guide/kubectl/{{page.version}}/#get) to watch the StatefulSet controller create the StatefulSet's Pods. ```shell kubectl get pods -w -l app=zk ``` Once the `zk-2` Pod is Running and Ready, use `CRTL-C` to terminate kubectl. ```shell NAME READY STATUS RESTARTS AGE zk-0 0/1 Pending 0 0s zk-0 0/1 Pending 0 0s zk-0 0/1 ContainerCreating 0 0s zk-0 0/1 Running 0 19s zk-0 1/1 Running 0 40s zk-1 0/1 Pending 0 0s zk-1 0/1 Pending 0 0s zk-1 0/1 ContainerCreating 0 0s zk-1 0/1 Running 0 18s zk-1 1/1 Running 0 40s zk-2 0/1 Pending 0 0s zk-2 0/1 Pending 0 0s zk-2 0/1 ContainerCreating 0 0s zk-2 0/1 Running 0 19s zk-2 1/1 Running 0 40s ``` The StatefulSet controller creates three Pods, and each Pod has a container with a [ZooKeeper 3.4.9](http://www-us.apache.org/dist/zookeeper/zookeeper-3.4.9/) server. ### Facilitating Leader Election As there is no terminating algorithm for electing a leader in an anonymous network, Zab requires explicit membership configuration in order to perform leader election. Each server in the ensemble needs to have a unique identifier, all servers need to know the global set of identifiers, and each identifier needs to be associated with a network address. Use [`kubectl exec`](/docs/user-guide/kubectl/{{page.version}}/#exec) to get the hostnames of the Pods in the `zk` StatefulSet. ```shell for i in 0 1 2; do kubectl exec zk-$i -- hostname; done ``` The StatefulSet controller provides each Pod with a unique hostname based on its ordinal index. The hostnames take the form `-`. As the `replicas` field of the `zk` StatefulSet is set to `3`, the Set's controller creates three Pods with their hostnames set to `zk-0`, `zk-1`, and `zk-2`. ```shell zk-0 zk-1 zk-2 ``` The servers in a ZooKeeper ensemble use natural numbers as unique identifiers, and each server's identifier is stored in a file called `myid` in the server's data directory. Examine the contents of the `myid` file for each server. ```shell for i in 0 1 2; do echo "myid zk-$i";kubectl exec zk-$i -- cat /var/lib/zookeeper/data/myid; done ``` As the identifiers are natural numbers and the ordinal indices are non-negative integers, you can generate an identifier by adding one to the ordinal. ```shell myid zk-0 1 myid zk-1 2 myid zk-2 3 ``` Get the FQDN (Fully Qualified Domain Name) of each Pod in the `zk` StatefulSet. ```shell for i in 0 1 2; do kubectl exec zk-$i -- hostname -f; done ``` The `zk-headless` Service creates a domain for all of the Pods, `zk-headless.default.svc.cluster.local`. ```shell zk-0.zk-headless.default.svc.cluster.local zk-1.zk-headless.default.svc.cluster.local zk-2.zk-headless.default.svc.cluster.local ``` The A records in [Kubernetes DNS](/docs/concepts/services-networking/dns-pod-service/) resolve the FQDNs to the Pods' IP addresses. If the Pods are rescheduled, the A records will be updated with the Pods' new IP addresses, but the A record's names will not change. ZooKeeper stores its application configuration in a file named `zoo.cfg`. Use `kubectl exec` to view the contents of the `zoo.cfg` file in the `zk-0` Pod. ``` kubectl exec zk-0 -- cat /opt/zookeeper/conf/zoo.cfg ``` For the `server.1`, `server.2`, and `server.3` properties at the bottom of the file, the `1`, `2`, and `3` correspond to the identifiers in the ZooKeeper servers' `myid` files. They are set to the FQDNs for the Pods in the `zk` StatefulSet. ```shell clientPort=2181 dataDir=/var/lib/zookeeper/data dataLogDir=/var/lib/zookeeper/log tickTime=2000 initLimit=10 syncLimit=2000 maxClientCnxns=60 minSessionTimeout= 4000 maxSessionTimeout= 40000 autopurge.snapRetainCount=3 autopurge.purgeInteval=0 server.1=zk-0.zk-headless.default.svc.cluster.local:2888:3888 server.2=zk-1.zk-headless.default.svc.cluster.local:2888:3888 server.3=zk-2.zk-headless.default.svc.cluster.local:2888:3888 ``` ### Achieving Consensus Consensus protocols require that the identifiers of each participant be unique. No two participants in the Zab protocol should claim the same unique identifier. This is necessary to allow the processes in the system to agree on which processes have committed which data. If two Pods were launched with the same ordinal, two ZooKeeper servers would both identify themselves as the same server. When you created the `zk` StatefulSet, the StatefulSet's controller created each Pod sequentially, in the order defined by the Pods' ordinal indices, and it waited for each Pod to be Running and Ready before creating the next Pod. ```shell kubectl get pods -w -l app=zk NAME READY STATUS RESTARTS AGE zk-0 0/1 Pending 0 0s zk-0 0/1 Pending 0 0s zk-0 0/1 ContainerCreating 0 0s zk-0 0/1 Running 0 19s zk-0 1/1 Running 0 40s zk-1 0/1 Pending 0 0s zk-1 0/1 Pending 0 0s zk-1 0/1 ContainerCreating 0 0s zk-1 0/1 Running 0 18s zk-1 1/1 Running 0 40s zk-2 0/1 Pending 0 0s zk-2 0/1 Pending 0 0s zk-2 0/1 ContainerCreating 0 0s zk-2 0/1 Running 0 19s zk-2 1/1 Running 0 40s ``` The A records for each Pod are only entered when the Pod becomes Ready. Therefore, the FQDNs of the ZooKeeper servers will only resolve to a single endpoint, and that endpoint will be the unique ZooKeeper server claiming the identity configured in its `myid` file. ```shell zk-0.zk-headless.default.svc.cluster.local zk-1.zk-headless.default.svc.cluster.local zk-2.zk-headless.default.svc.cluster.local ``` This ensures that the `servers` properties in the ZooKeepers' `zoo.cfg` files represents a correctly configured ensemble. ```shell server.1=zk-0.zk-headless.default.svc.cluster.local:2888:3888 server.2=zk-1.zk-headless.default.svc.cluster.local:2888:3888 server.3=zk-2.zk-headless.default.svc.cluster.local:2888:3888 ``` When the servers use the Zab protocol to attempt to commit a value, they will either achieve consensus and commit the value (if leader election has succeeded and at least two of the Pods are Running and Ready), or they will fail to do so (if either of the aforementioned conditions are not met). No state will arise where one server acknowledges a write on behalf of another. ### Sanity Testing the Ensemble The most basic sanity test is to write some data to one ZooKeeper server and to read the data from another. Use the `zkCli.sh` script to write `world` to the path `/hello` on the `zk-0` Pod. ```shell kubectl exec zk-0 zkCli.sh create /hello world ``` This will write `world` to the `/hello` path in the ensemble. ```shell WATCHER:: WatchedEvent state:SyncConnected type:None path:null Created /hello ``` Get the data from the `zk-1` Pod. ```shell kubectl exec zk-1 zkCli.sh get /hello ``` The data that you created on `zk-0` is available on all of the servers in the ensemble. ```shell WATCHER:: WatchedEvent state:SyncConnected type:None path:null world cZxid = 0x100000002 ctime = Thu Dec 08 15:13:30 UTC 2016 mZxid = 0x100000002 mtime = Thu Dec 08 15:13:30 UTC 2016 pZxid = 0x100000002 cversion = 0 dataVersion = 0 aclVersion = 0 ephemeralOwner = 0x0 dataLength = 5 numChildren = 0 ``` ### Providing Durable Storage As mentioned in the [ZooKeeper Basics](#zookeeper-basics) section, ZooKeeper commits all entries to a durable WAL, and periodically writes snapshots in memory state, to storage media. Using WALs to provide durability is a common technique for applications that use consensus protocols to achieve a replicated state machine and for storage applications in general. Use [`kubectl delete`](/docs/user-guide/kubectl/{{page.version}}/#delete) to delete the `zk` StatefulSet. ```shell kubectl delete statefulset zk statefulset "zk" deleted ``` Watch the termination of the Pods in the StatefulSet. ```shell get pods -w -l app=zk ``` When `zk-0` if fully terminated, use `CRTL-C` to terminate kubectl. ```shell zk-2 1/1 Terminating 0 9m zk-0 1/1 Terminating 0 11m zk-1 1/1 Terminating 0 10m zk-2 0/1 Terminating 0 9m zk-2 0/1 Terminating 0 9m zk-2 0/1 Terminating 0 9m zk-1 0/1 Terminating 0 10m zk-1 0/1 Terminating 0 10m zk-1 0/1 Terminating 0 10m zk-0 0/1 Terminating 0 11m zk-0 0/1 Terminating 0 11m zk-0 0/1 Terminating 0 11m ``` Reapply the manifest in `zookeeper.yaml`. ```shell kubectl apply -f https://k8s.io/docs/tutorials/stateful-application/zookeeper.yaml ``` The `zk` StatefulSet will be created, but, as they already exist, the other API Objects in the manifest will not be modified. ```shell statefulset "zk" created Error from server (AlreadyExists): error when creating "zookeeper.yaml": services "zk-headless" already exists Error from server (AlreadyExists): error when creating "zookeeper.yaml": configmaps "zk-config" already exists Error from server (AlreadyExists): error when creating "zookeeper.yaml": poddisruptionbudgets.policy "zk-budget" already exists ``` Watch the StatefulSet controller recreate the StatefulSet's Pods. ```shell kubectl get pods -w -l app=zk ``` Once the `zk-2` Pod is Running and Ready, use `CRTL-C` to terminate kubectl. ```shell NAME READY STATUS RESTARTS AGE zk-0 0/1 Pending 0 0s zk-0 0/1 Pending 0 0s zk-0 0/1 ContainerCreating 0 0s zk-0 0/1 Running 0 19s zk-0 1/1 Running 0 40s zk-1 0/1 Pending 0 0s zk-1 0/1 Pending 0 0s zk-1 0/1 ContainerCreating 0 0s zk-1 0/1 Running 0 18s zk-1 1/1 Running 0 40s zk-2 0/1 Pending 0 0s zk-2 0/1 Pending 0 0s zk-2 0/1 ContainerCreating 0 0s zk-2 0/1 Running 0 19s zk-2 1/1 Running 0 40s ``` Get the value you entered during the [sanity test](#sanity-testing-the-ensemble), from the `zk-2` Pod. ```shell kubectl exec zk-2 zkCli.sh get /hello ``` Even though all of the Pods in the `zk` StatefulSet have been terminated and recreated, the ensemble still serves the original value. ```shell WATCHER:: WatchedEvent state:SyncConnected type:None path:null world cZxid = 0x100000002 ctime = Thu Dec 08 15:13:30 UTC 2016 mZxid = 0x100000002 mtime = Thu Dec 08 15:13:30 UTC 2016 pZxid = 0x100000002 cversion = 0 dataVersion = 0 aclVersion = 0 ephemeralOwner = 0x0 dataLength = 5 numChildren = 0 ``` The `volumeClaimTemplates` field, of the `zk` StatefulSet's `spec`, specifies a PersistentVolume that will be provisioned for each Pod. ```yaml volumeClaimTemplates: - metadata: name: datadir annotations: volume.alpha.kubernetes.io/storage-class: anything spec: accessModes: [ "ReadWriteOnce" ] resources: requests: storage: 20Gi ``` The StatefulSet controller generates a PersistentVolumeClaim for each Pod in the StatefulSet. Get the StatefulSet's PersistentVolumeClaims. ```shell kubectl get pvc -l app=zk ``` When the StatefulSet recreated its Pods, the Pods' PersistentVolumes were remounted. ```shell NAME STATUS VOLUME CAPACITY ACCESSMODES AGE datadir-zk-0 Bound pvc-bed742cd-bcb1-11e6-994f-42010a800002 20Gi RWO 1h datadir-zk-1 Bound pvc-bedd27d2-bcb1-11e6-994f-42010a800002 20Gi RWO 1h datadir-zk-2 Bound pvc-bee0817e-bcb1-11e6-994f-42010a800002 20Gi RWO 1h ``` The `volumeMounts` section of the StatefulSet's container `template` causes the PersistentVolumes to be mounted to the ZooKeeper servers' data directories. ```shell volumeMounts: - name: datadir mountPath: /var/lib/zookeeper ``` When a Pod in the `zk` StatefulSet is (re)scheduled, it will always have the same PersistentVolume mounted to the ZooKeeper server's data directory. Even when the Pods are rescheduled, all of the writes made to the ZooKeeper servers' WALs, and all of their snapshots, remain durable. ## Ensuring Consistent Configuration As noted in the [Facilitating Leader Election](#facilitating-leader-election) and [Achieving Consensus](#achieving-consensus) sections, the servers in a ZooKeeper ensemble require consistent configuration in order to elect a leader and form a quorum. They also require consistent configuration of the Zab protocol in order for the protocol to work correctly over a network. You can use ConfigMaps to achieve this. Get the `zk-config` ConfigMap. ```shell kubectl get cm zk-config -o yaml apiVersion: v1 data: client.cnxns: "60" ensemble: zk-0;zk-1;zk-2 init: "10" jvm.heap: 2G purge.interval: "0" snap.retain: "3" sync: "5" tick: "2000" ``` The `env` field of the `zk` StatefulSet's Pod `template` reads the ConfigMap into environment variables. These variables are injected into the containers environment. ```yaml env: - name : ZK_ENSEMBLE valueFrom: configMapKeyRef: name: zk-config key: ensemble - name : ZK_HEAP_SIZE valueFrom: configMapKeyRef: name: zk-config key: jvm.heap - name : ZK_TICK_TIME valueFrom: configMapKeyRef: name: zk-config key: tick - name : ZK_INIT_LIMIT valueFrom: configMapKeyRef: name: zk-config key: init - name : ZK_SYNC_LIMIT valueFrom: configMapKeyRef: name: zk-config key: tick - name : ZK_MAX_CLIENT_CNXNS valueFrom: configMapKeyRef: name: zk-config key: client.cnxns - name: ZK_SNAP_RETAIN_COUNT valueFrom: configMapKeyRef: name: zk-config key: snap.retain - name: ZK_PURGE_INTERVAL valueFrom: configMapKeyRef: name: zk-config key: purge.interval ``` The entry point of the container invokes a bash script, `zkGenConfig.sh`, prior to launching the ZooKeeper server process. This bash script generates the ZooKeeper configuration files from the supplied environment variables. ```yaml command: - sh - -c - zkGenConfig.sh && zkServer.sh start-foreground ``` Examine the environment of all of the Pods in the `zk` StatefulSet. ```shell for i in 0 1 2; do kubectl exec zk-$i env | grep ZK_*;echo""; done ``` All of the variables populated from `zk-config` contain identical values. This allows the `zkGenConfig.sh` script to create consistent configurations for all of the ZooKeeper servers in the ensemble. ```shell ZK_ENSEMBLE=zk-0;zk-1;zk-2 ZK_HEAP_SIZE=2G ZK_TICK_TIME=2000 ZK_INIT_LIMIT=10 ZK_SYNC_LIMIT=2000 ZK_MAX_CLIENT_CNXNS=60 ZK_SNAP_RETAIN_COUNT=3 ZK_PURGE_INTERVAL=0 ZK_CLIENT_PORT=2181 ZK_SERVER_PORT=2888 ZK_ELECTION_PORT=3888 ZK_USER=zookeeper ZK_DATA_DIR=/var/lib/zookeeper/data ZK_DATA_LOG_DIR=/var/lib/zookeeper/log ZK_LOG_DIR=/var/log/zookeeper ZK_ENSEMBLE=zk-0;zk-1;zk-2 ZK_HEAP_SIZE=2G ZK_TICK_TIME=2000 ZK_INIT_LIMIT=10 ZK_SYNC_LIMIT=2000 ZK_MAX_CLIENT_CNXNS=60 ZK_SNAP_RETAIN_COUNT=3 ZK_PURGE_INTERVAL=0 ZK_CLIENT_PORT=2181 ZK_SERVER_PORT=2888 ZK_ELECTION_PORT=3888 ZK_USER=zookeeper ZK_DATA_DIR=/var/lib/zookeeper/data ZK_DATA_LOG_DIR=/var/lib/zookeeper/log ZK_LOG_DIR=/var/log/zookeeper ZK_ENSEMBLE=zk-0;zk-1;zk-2 ZK_HEAP_SIZE=2G ZK_TICK_TIME=2000 ZK_INIT_LIMIT=10 ZK_SYNC_LIMIT=2000 ZK_MAX_CLIENT_CNXNS=60 ZK_SNAP_RETAIN_COUNT=3 ZK_PURGE_INTERVAL=0 ZK_CLIENT_PORT=2181 ZK_SERVER_PORT=2888 ZK_ELECTION_PORT=3888 ZK_USER=zookeeper ZK_DATA_DIR=/var/lib/zookeeper/data ZK_DATA_LOG_DIR=/var/lib/zookeeper/log ZK_LOG_DIR=/var/log/zookeeper ``` ### Configuring Logging One of the files generated by the `zkGenConfig.sh` script controls ZooKeeper's logging. ZooKeeper uses [Log4j](http://logging.apache.org/log4j/2.x/), and, by default, it uses a time and size based rolling file appender for its logging configuration. Get the logging configuration from one of Pods in the `zk` StatefulSet. ```shell kubectl exec zk-0 cat /usr/etc/zookeeper/log4j.properties ``` The logging configuration below will cause the ZooKeeper process to write all of its logs to the standard output file stream. ```shell zookeeper.root.logger=CONSOLE zookeeper.console.threshold=INFO log4j.rootLogger=${zookeeper.root.logger} log4j.appender.CONSOLE=org.apache.log4j.ConsoleAppender log4j.appender.CONSOLE.Threshold=${zookeeper.console.threshold} log4j.appender.CONSOLE.layout=org.apache.log4j.PatternLayout log4j.appender.CONSOLE.layout.ConversionPattern=%d{ISO8601} [myid:%X{myid}] - %-5p [%t:%C{1}@%L] - %m%n ``` This is the simplest possible way to safely log inside the container. As the application's logs are being written to standard out, Kubernetes will handle log rotation for you. Kubernetes also implements a sane retention policy that ensures application logs written to standard out and standard error do not exhaust local storage media. Use [`kubectl logs`](/docs/user-guide/kubectl/{{page.version}}/#logs) to retrieve the last few log lines from one of the Pods. ```shell kubectl logs zk-0 --tail 20 ``` Application logs that are written to standard out or standard error are viewable using `kubectl logs` and from the Kubernetes Dashboard. ```shell 2016-12-06 19:34:16,236 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxn@827] - Processing ruok command from /127.0.0.1:52740 2016-12-06 19:34:16,237 [myid:1] - INFO [Thread-1136:NIOServerCnxn@1008] - Closed socket connection for client /127.0.0.1:52740 (no session established for client) 2016-12-06 19:34:26,155 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxnFactory@192] - Accepted socket connection from /127.0.0.1:52749 2016-12-06 19:34:26,155 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxn@827] - Processing ruok command from /127.0.0.1:52749 2016-12-06 19:34:26,156 [myid:1] - INFO [Thread-1137:NIOServerCnxn@1008] - Closed socket connection for client /127.0.0.1:52749 (no session established for client) 2016-12-06 19:34:26,222 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxnFactory@192] - Accepted socket connection from /127.0.0.1:52750 2016-12-06 19:34:26,222 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxn@827] - Processing ruok command from /127.0.0.1:52750 2016-12-06 19:34:26,226 [myid:1] - INFO [Thread-1138:NIOServerCnxn@1008] - Closed socket connection for client /127.0.0.1:52750 (no session established for client) 2016-12-06 19:34:36,151 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxnFactory@192] - Accepted socket connection from /127.0.0.1:52760 2016-12-06 19:34:36,152 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxn@827] - Processing ruok command from /127.0.0.1:52760 2016-12-06 19:34:36,152 [myid:1] - INFO [Thread-1139:NIOServerCnxn@1008] - Closed socket connection for client /127.0.0.1:52760 (no session established for client) 2016-12-06 19:34:36,230 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxnFactory@192] - Accepted socket connection from /127.0.0.1:52761 2016-12-06 19:34:36,231 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxn@827] - Processing ruok command from /127.0.0.1:52761 2016-12-06 19:34:36,231 [myid:1] - INFO [Thread-1140:NIOServerCnxn@1008] - Closed socket connection for client /127.0.0.1:52761 (no session established for client) 2016-12-06 19:34:46,149 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxnFactory@192] - Accepted socket connection from /127.0.0.1:52767 2016-12-06 19:34:46,149 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxn@827] - Processing ruok command from /127.0.0.1:52767 2016-12-06 19:34:46,149 [myid:1] - INFO [Thread-1141:NIOServerCnxn@1008] - Closed socket connection for client /127.0.0.1:52767 (no session established for client) 2016-12-06 19:34:46,230 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxnFactory@192] - Accepted socket connection from /127.0.0.1:52768 2016-12-06 19:34:46,230 [myid:1] - INFO [NIOServerCxn.Factory:0.0.0.0/0.0.0.0:2181:NIOServerCnxn@827] - Processing ruok command from /127.0.0.1:52768 2016-12-06 19:34:46,230 [myid:1] - INFO [Thread-1142:NIOServerCnxn@1008] - Closed socket connection for client /127.0.0.1:52768 (no session established for client) ``` Kubernetes also supports more powerful, but more complex, logging integrations with [Logging Using Stackdriver](/docs/tasks/debug-application-cluster/logging-stackdriver/) and [Logging Using Elasticsearch and Kibana](/docs/tasks/debug-application-cluster/logging-elasticsearch-kibana/). For cluster level log shipping and aggregation, you should consider deploying a [sidecar](http://blog.kubernetes.io/2015/06/the-distributed-system-toolkit-patterns.html) container to rotate and ship your logs. ### Configuring a Non-Privileged User The best practices with respect to allowing an application to run as a privileged user inside of a container are a matter of debate. If your organization requires that applications be run as a non-privileged user you can use a [SecurityContext](/docs/tasks/configure-pod-container/security-context/) to control the user that the entry point runs as. The `zk` StatefulSet's Pod `template` contains a SecurityContext. ```yaml securityContext: runAsUser: 1000 fsGroup: 1000 ``` In the Pods' containers, UID 1000 corresponds to the zookeeper user and GID 1000 corresponds to the zookeeper group. Get the ZooKeeper process information from the `zk-0` Pod. ```shell kubectl exec zk-0 -- ps -elf ``` As the `runAsUser` field of the `securityContext` object is set to 1000, instead of running as root, the ZooKeeper process runs as the zookeeper user. ```shell F S UID PID PPID C PRI NI ADDR SZ WCHAN STIME TTY TIME CMD 4 S zookeep+ 1 0 0 80 0 - 1127 - 20:46 ? 00:00:00 sh -c zkGenConfig.sh && zkServer.sh start-foreground 0 S zookeep+ 27 1 0 80 0 - 1155556 - 20:46 ? 00:00:19 /usr/lib/jvm/java-8-openjdk-amd64/bin/java -Dzookeeper.log.dir=/var/log/zookeeper -Dzookeeper.root.logger=INFO,CONSOLE -cp /usr/bin/../build/classes:/usr/bin/../build/lib/*.jar:/usr/bin/../share/zookeeper/zookeeper-3.4.9.jar:/usr/bin/../share/zookeeper/slf4j-log4j12-1.6.1.jar:/usr/bin/../share/zookeeper/slf4j-api-1.6.1.jar:/usr/bin/../share/zookeeper/netty-3.10.5.Final.jar:/usr/bin/../share/zookeeper/log4j-1.2.16.jar:/usr/bin/../share/zookeeper/jline-0.9.94.jar:/usr/bin/../src/java/lib/*.jar:/usr/bin/../etc/zookeeper: -Xmx2G -Xms2G -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.local.only=false org.apache.zookeeper.server.quorum.QuorumPeerMain /usr/bin/../etc/zookeeper/zoo.cfg ``` By default, when the Pod's PersistentVolume is mounted to the ZooKeeper server's data directory, it is only accessible by the root user. This configuration prevents the ZooKeeper process from writing to its WAL and storing its snapshots. Get the file permissions of the ZooKeeper data directory on the `zk-0` Pod. ```shell kubectl exec -ti zk-0 -- ls -ld /var/lib/zookeeper/data ``` As the `fsGroup` field of the `securityContext` object is set to 1000, the ownership of the Pods' PersistentVolumes is set to the zookeeper group, and the ZooKeeper process is able to successfully read and write its data. ```shell drwxr-sr-x 3 zookeeper zookeeper 4096 Dec 5 20:45 /var/lib/zookeeper/data ``` ## Managing the ZooKeeper Process The [ZooKeeper documentation](https://zookeeper.apache.org/doc/current/zookeeperAdmin.html#sc_supervision) indicates that "You will want to have a supervisory process that manages each of your ZooKeeper server processes (JVM)." Utilizing a watchdog (supervisory process) to restart failed processes in a distributed system is a common pattern. When deploying an application in Kubernetes, rather than using an external utility as a supervisory process, you should use Kubernetes as the watchdog for your application. ### Handling Process Failure [Restart Policies](/docs/user-guide/pod-states/#restartpolicy) control how Kubernetes handles process failures for the entry point of the container in a Pod. For Pods in a StatefulSet, the only appropriate RestartPolicy is Always, and this is the default value. For stateful applications you should **never** override the default policy. Examine the process tree for the ZooKeeper server running in the `zk-0` Pod. ```shell kubectl exec zk-0 -- ps -ef ``` The command used as the container's entry point has PID 1, and the ZooKeeper process, a child of the entry point, has PID 23. ``` UID PID PPID C STIME TTY TIME CMD zookeep+ 1 0 0 15:03 ? 00:00:00 sh -c zkGenConfig.sh && zkServer.sh start-foreground zookeep+ 27 1 0 15:03 ? 00:00:03 /usr/lib/jvm/java-8-openjdk-amd64/bin/java -Dzookeeper.log.dir=/var/log/zookeeper -Dzookeeper.root.logger=INFO,CONSOLE -cp /usr/bin/../build/classes:/usr/bin/../build/lib/*.jar:/usr/bin/../share/zookeeper/zookeeper-3.4.9.jar:/usr/bin/../share/zookeeper/slf4j-log4j12-1.6.1.jar:/usr/bin/../share/zookeeper/slf4j-api-1.6.1.jar:/usr/bin/../share/zookeeper/netty-3.10.5.Final.jar:/usr/bin/../share/zookeeper/log4j-1.2.16.jar:/usr/bin/../share/zookeeper/jline-0.9.94.jar:/usr/bin/../src/java/lib/*.jar:/usr/bin/../etc/zookeeper: -Xmx2G -Xms2G -Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.local.only=false org.apache.zookeeper.server.quorum.QuorumPeerMain /usr/bin/../etc/zookeeper/zoo.cfg ``` In one terminal watch the Pods in the `zk` StatefulSet. ```shell kubectl get pod -w -l app=zk ``` In another terminal, kill the ZooKeeper process in Pod `zk-0`. ```shell kubectl exec zk-0 -- pkill java ``` The death of the ZooKeeper process caused its parent process to terminate. As the RestartPolicy of the container is Always, the parent process was relaunched. ```shell NAME READY STATUS RESTARTS AGE zk-0 1/1 Running 0 21m zk-1 1/1 Running 0 20m zk-2 1/1 Running 0 19m NAME READY STATUS RESTARTS AGE zk-0 0/1 Error 0 29m zk-0 0/1 Running 1 29m zk-0 1/1 Running 1 29m ``` If your application uses a script (such as zkServer.sh) to launch the process that implements the application's business logic, the script must terminate with the child process. This ensures that Kubernetes will restart the application's container when the process implementing the application's business logic fails. ### Testing for Liveness Configuring your application to restart failed processes is not sufficient to keep a distributed system healthy. There are many scenarios where a system's processes can be both alive and unresponsive, or otherwise unhealthy. You should use liveness probes in order to notify Kubernetes that your application's processes are unhealthy and should be restarted. The Pod `template` for the `zk` StatefulSet specifies a liveness probe. ```yaml livenessProbe: exec: command: - "zkOk.sh" initialDelaySeconds: 15 timeoutSeconds: 5 ``` The probe calls a simple bash script that uses the ZooKeeper `ruok` four letter word to test the server's health. ```bash ZK_CLIENT_PORT=${ZK_CLIENT_PORT:-2181} OK=$(echo ruok | nc 127.0.0.1 $ZK_CLIENT_PORT) if [ "$OK" == "imok" ]; then exit 0 else exit 1 fi ``` In one terminal window, watch the Pods in the `zk` StatefulSet. ```shell kubectl get pod -w -l app=zk ``` In another window, delete the `zkOk.sh` script from the file system of Pod `zk-0`. ```shell kubectl exec zk-0 -- rm /opt/zookeeper/bin/zkOk.sh ``` When the liveness probe for the ZooKeeper process fails, Kubernetes will automatically restart the process for you, ensuring that unhealthy processes in the ensemble are restarted. ```shell kubectl get pod -w -l app=zk NAME READY STATUS RESTARTS AGE zk-0 1/1 Running 0 1h zk-1 1/1 Running 0 1h zk-2 1/1 Running 0 1h NAME READY STATUS RESTARTS AGE zk-0 0/1 Running 0 1h zk-0 0/1 Running 1 1h zk-0 1/1 Running 1 1h ``` ### Testing for Readiness Readiness is not the same as liveness. If a process is alive, it is scheduled and healthy. If a process is ready, it is able to process input. Liveness is a necessary, but not sufficient, condition for readiness. There are many cases, particularly during initialization and termination, when a process can be alive but not ready. If you specify a readiness probe, Kubernetes will ensure that your application's processes will not receive network traffic until their readiness checks pass. For a ZooKeeper server, liveness implies readiness. Therefore, the readiness probe from the `zookeeper.yaml` manifest is identical to the liveness probe. ```yaml readinessProbe: exec: command: - "zkOk.sh" initialDelaySeconds: 15 timeoutSeconds: 5 ``` Even though the liveness and readiness probes are identical, it is important to specify both. This ensures that only healthy servers in the ZooKeeper ensemble receive network traffic. ## Tolerating Node Failure ZooKeeper needs a quorum of servers in order to successfully commit mutations to data. For a three server ensemble, two servers must be healthy in order for writes to succeed. In quorum based systems, members are deployed across failure domains to ensure availability. In order to avoid an outage, due to the loss of an individual machine, best practices preclude co-locating multiple instances of the application on the same machine. By default, Kubernetes may co-locate Pods in a StatefulSet on the same node. For the three server ensemble you created, if two servers reside on the same node, and that node fails, the clients of your ZooKeeper service will experience an outage until at least one of the Pods can be rescheduled. You should always provision additional capacity to allow the processes of critical systems to be rescheduled in the event of node failures. If you do so, then the outage will only last until the Kubernetes scheduler reschedules one of the ZooKeeper servers. However, if you want your service to tolerate node failures with no downtime, you should set `podAntiAffinity`. Get the nodes for Pods in the `zk` Stateful Set. ```shell{% raw %} for i in 0 1 2; do kubectl get pod zk-$i --template {{.spec.nodeName}}; echo ""; done ``` {% endraw %} All of the Pods in the `zk` StatefulSet are deployed on different nodes. ```shell kubernetes-minion-group-cxpk kubernetes-minion-group-a5aq kubernetes-minion-group-2g2d ``` This is because the Pods in the `zk` StatefulSet have a PodAntiAffinity specified. ```yaml affinity: podAntiAffinity: requiredDuringSchedulingIgnoredDuringExecution: - labelSelector: matchExpressions: - key: "app" operator: In values: - zk-headless topologyKey: "kubernetes.io/hostname" ``` The `requiredDuringSchedulingRequiredDuringExecution` field tells the Kubernetes Scheduler that it should never co-locate two Pods from the `zk-headless` Service in the domain defined by the `topologyKey`. The `topologyKey` `kubernetes.io/hostname` indicates that the domain is an individual node. Using different rules, labels, and selectors, you can extend this technique to spread your ensemble across physical, network, and power failure domains. ## Surviving Maintenance **In this section you will cordon and drain nodes. If you are using this tutorial on a shared cluster, be sure that this will not adversely affect other tenants.** The previous section showed you how to spread your Pods across nodes to survive unplanned node failures, but you also need to plan for temporary node failures that occur due to planned maintenance. Get the nodes in your cluster. ```shell kubectl get nodes ``` Use [`kubectl cordon`](/docs/user-guide/kubectl/{{page.version}}/#cordon) to cordon all but four of the nodes in your cluster. ```shell{% raw %} kubectl cordon < node name > ```{% endraw %} Get the `zk-budget` PodDisruptionBudget. ```shell kubectl get poddisruptionbudget zk-budget ``` The `min-available` field indicates to Kubernetes that at least two Pods from `zk` StatefulSet must be available at any time. ```yaml NAME MIN-AVAILABLE ALLOWED-DISRUPTIONS AGE zk-budget 2 1 1h ``` In one terminal, watch the Pods in the `zk` StatefulSet. ```shell kubectl get pods -w -l app=zk ``` In another terminal, get the nodes that the Pods are currently scheduled on. ```shell{% raw %} for i in 0 1 2; do kubectl get pod zk-$i --template {{.spec.nodeName}}; echo ""; done kubernetes-minion-group-pb41 kubernetes-minion-group-ixsl kubernetes-minion-group-i4c4 {% endraw %} ``` Use [`kubectl drain`](/docs/user-guide/kubectl/{{page.version}}/#drain) to cordon and drain the node on which the `zk-0` Pod is scheduled. ```shell {% raw %} kubectl drain $(kubectl get pod zk-0 --template {{.spec.nodeName}}) --ignore-daemonsets --force --delete-local-data node "kubernetes-minion-group-pb41" cordoned WARNING: Deleting pods not managed by ReplicationController, ReplicaSet, Job, or DaemonSet: fluentd-cloud-logging-kubernetes-minion-group-pb41, kube-proxy-kubernetes-minion-group-pb41; Ignoring DaemonSet-managed pods: node-problem-detector-v0.1-o5elz pod "zk-0" deleted node "kubernetes-minion-group-pb41" drained {% endraw %}``` As there are four nodes in your cluster, `kubectl drain`, succeeds and the `zk-0` is rescheduled to another node. ``` NAME READY STATUS RESTARTS AGE zk-0 1/1 Running 2 1h zk-1 1/1 Running 0 1h zk-2 1/1 Running 0 1h NAME READY STATUS RESTARTS AGE zk-0 1/1 Terminating 2 2h zk-0 0/1 Terminating 2 2h zk-0 0/1 Terminating 2 2h zk-0 0/1 Terminating 2 2h zk-0 0/1 Pending 0 0s zk-0 0/1 Pending 0 0s zk-0 0/1 ContainerCreating 0 0s zk-0 0/1 Running 0 51s zk-0 1/1 Running 0 1m ``` Keep watching the StatefulSet's Pods in the first terminal and drain the node on which `zk-1` is scheduled. ```shell{% raw %} kubectl drain $(kubectl get pod zk-1 --template {{.spec.nodeName}}) --ignore-daemonsets --force --delete-local-data "kubernetes-minion-group-ixsl" cordoned WARNING: Deleting pods not managed by ReplicationController, ReplicaSet, Job, or DaemonSet: fluentd-cloud-logging-kubernetes-minion-group-ixsl, kube-proxy-kubernetes-minion-group-ixsl; Ignoring DaemonSet-managed pods: node-problem-detector-v0.1-voc74 pod "zk-1" deleted node "kubernetes-minion-group-ixsl" drained {% endraw %}``` The `zk-1` Pod can not be scheduled. As the `zk` StatefulSet contains a PodAntiAffinity rule preventing co-location of the Pods, and as only two nodes are schedulable, the Pod will remain in a Pending state. ```shell kubectl get pods -w -l app=zk NAME READY STATUS RESTARTS AGE zk-0 1/1 Running 2 1h zk-1 1/1 Running 0 1h zk-2 1/1 Running 0 1h NAME READY STATUS RESTARTS AGE zk-0 1/1 Terminating 2 2h zk-0 0/1 Terminating 2 2h zk-0 0/1 Terminating 2 2h zk-0 0/1 Terminating 2 2h zk-0 0/1 Pending 0 0s zk-0 0/1 Pending 0 0s zk-0 0/1 ContainerCreating 0 0s zk-0 0/1 Running 0 51s zk-0 1/1 Running 0 1m zk-1 1/1 Terminating 0 2h zk-1 0/1 Terminating 0 2h zk-1 0/1 Terminating 0 2h zk-1 0/1 Terminating 0 2h zk-1 0/1 Pending 0 0s zk-1 0/1 Pending 0 0s ``` Continue to watch the Pods of the stateful set, and drain the node on which `zk-2` is scheduled. ```shell{% raw %} kubectl drain $(kubectl get pod zk-2 --template {{.spec.nodeName}}) --ignore-daemonsets --force --delete-local-data node "kubernetes-minion-group-i4c4" cordoned WARNING: Deleting pods not managed by ReplicationController, ReplicaSet, Job, or DaemonSet: fluentd-cloud-logging-kubernetes-minion-group-i4c4, kube-proxy-kubernetes-minion-group-i4c4; Ignoring DaemonSet-managed pods: node-problem-detector-v0.1-dyrog WARNING: Ignoring DaemonSet-managed pods: node-problem-detector-v0.1-dyrog; Deleting pods not managed by ReplicationController, ReplicaSet, Job, or DaemonSet: fluentd-cloud-logging-kubernetes-minion-group-i4c4, kube-proxy-kubernetes-minion-group-i4c4 There are pending pods when an error occurred: Cannot evict pod as it would violate the pod's disruption budget. pod/zk-2 {% endraw %}``` Use `CRTL-C` to terminate to kubectl. You can not drain the third node because evicting `zk-2` would violate `zk-budget`. However, the node will remain cordoned. Use `zkCli.sh` to retrieve the value you entered during the sanity test from `zk-0`. ```shell kubectl exec zk-0 zkCli.sh get /hello ``` The service is still available because its PodDisruptionBudget is respected. ``` WatchedEvent state:SyncConnected type:None path:null world cZxid = 0x200000002 ctime = Wed Dec 07 00:08:59 UTC 2016 mZxid = 0x200000002 mtime = Wed Dec 07 00:08:59 UTC 2016 pZxid = 0x200000002 cversion = 0 dataVersion = 0 aclVersion = 0 ephemeralOwner = 0x0 dataLength = 5 numChildren = 0 ``` Use [`kubectl uncordon`](/docs/user-guide/kubectl/{{page.version}}/#uncordon) to uncordon the first node. ```shell kubectl uncordon kubernetes-minion-group-pb41 node "kubernetes-minion-group-pb41" uncordoned ``` `zk-1` is rescheduled on this node. Wait until `zk-1` is Running and Ready. ```shell kubectl get pods -w -l app=zk NAME READY STATUS RESTARTS AGE zk-0 1/1 Running 2 1h zk-1 1/1 Running 0 1h zk-2 1/1 Running 0 1h NAME READY STATUS RESTARTS AGE zk-0 1/1 Terminating 2 2h zk-0 0/1 Terminating 2 2h zk-0 0/1 Terminating 2 2h zk-0 0/1 Terminating 2 2h zk-0 0/1 Pending 0 0s zk-0 0/1 Pending 0 0s zk-0 0/1 ContainerCreating 0 0s zk-0 0/1 Running 0 51s zk-0 1/1 Running 0 1m zk-1 1/1 Terminating 0 2h zk-1 0/1 Terminating 0 2h zk-1 0/1 Terminating 0 2h zk-1 0/1 Terminating 0 2h zk-1 0/1 Pending 0 0s zk-1 0/1 Pending 0 0s zk-1 0/1 Pending 0 12m zk-1 0/1 ContainerCreating 0 12m zk-1 0/1 Running 0 13m zk-1 1/1 Running 0 13m ``` Attempt to drain the node on which `zk-2` is scheduled. ```shell{% raw %} kubectl drain $(kubectl get pod zk-2 --template {{.spec.nodeName}}) --ignore-daemonsets --force --delete-local-data node "kubernetes-minion-group-i4c4" already cordoned WARNING: Deleting pods not managed by ReplicationController, ReplicaSet, Job, or DaemonSet: fluentd-cloud-logging-kubernetes-minion-group-i4c4, kube-proxy-kubernetes-minion-group-i4c4; Ignoring DaemonSet-managed pods: node-problem-detector-v0.1-dyrog pod "heapster-v1.2.0-2604621511-wht1r" deleted pod "zk-2" deleted node "kubernetes-minion-group-i4c4" drained {% endraw %}``` This time `kubectl drain` succeeds. Uncordon the second node to allow `zk-2` to be rescheduled. ```shell kubectl uncordon kubernetes-minion-group-ixsl node "kubernetes-minion-group-ixsl" uncordoned ``` You can use `kubectl drain` in conjunction with PodDisruptionBudgets to ensure that your service remains available during maintenance. If drain is used to cordon nodes and evict pods prior to taking the node offline for maintenance, services that express a disruption budget will have that budget respected. You should always allocate additional capacity for critical services so that their Pods can be immediately rescheduled. {% endcapture %} {% capture cleanup %} * Use `kubectl uncordon` to uncordon all the nodes in your cluster. * You will need to delete the persistent storage media for the PersistentVolumes used in this tutorial. Follow the necessary steps, based on your environment, storage configuration, and provisioning method, to ensure that all storage is reclaimed. {% endcapture %} {% include templates/tutorial.md %}