ZooKeeper HA Services

Flink’s ZooKeeper HA services use ZooKeeper for high availability services.

Flink leverages ZooKeeper for distributed coordination between all running JobManager instances. ZooKeeper is a separate service from Flink, which provides highly reliable distributed coordination via leader election and light-weight consistent state storage. Check out ZooKeeper’s Getting Started Guide for more information about ZooKeeper. Flink includes scripts to bootstrap a simple ZooKeeper installation.

Configuration

In order to start an HA-cluster you have to configure the following configuration keys:

  • high-availability (required): The high-availability option has to be set to zookeeper.

    1. high-availability: zookeeper

  • high-availability.storageDir (required): JobManager metadata is persisted in the file system high-availability.storageDir and only a pointer to this state is stored in ZooKeeper.

    1. high-availability.storageDir: hdfs:///flink/recovery

    The storageDir stores all metadata needed to recover a JobManager failure.

  • high-availability.zookeeper.quorum (required): A ZooKeeper quorum is a replicated group of ZooKeeper servers, which provide the distributed coordination service.

    1. high-availability.zookeeper.quorum: address1:2181[,...],addressX:2181

    Each addressX:port refers to a ZooKeeper server, which is reachable by Flink at the given address and port.

  • high-availability.zookeeper.path.root (recommended): The root ZooKeeper node, under which all cluster nodes are placed.

    1. high-availability.zookeeper.path.root: /flink

  • high-availability.cluster-id (recommended): The cluster-id ZooKeeper node, under which all required coordination data for a cluster is placed.

    1. high-availability.cluster-id: /default_ns # important: customize per cluster

    Important: You should not set this value manually when running on YARN, native Kubernetes or on another cluster manager. In those cases a cluster-id is being automatically generated. If you are running multiple Flink HA clusters on bare metal, you have to manually configure separate cluster-ids for each cluster.

Example configuration

Configure high availability mode and ZooKeeper quorum in conf/flink-conf.yaml:

  1. high-availability: zookeeper
  2. high-availability.zookeeper.quorum: localhost:2181
  3. high-availability.zookeeper.path.root: /flink
  4. high-availability.cluster-id: /cluster_one # important: customize per cluster
  5. high-availability.storageDir: hdfs:///flink/recovery

Configuring for ZooKeeper Security

If ZooKeeper is running in secure mode with Kerberos, you can override the following configurations in flink-conf.yaml as necessary:

  1. # default is "zookeeper". If the ZooKeeper quorum is configured
  2. # with a different service name then it can be supplied here.
  4. zookeeper.sasl.service-name: zookeeper 
  6. # default is "Client". The value needs to match one of the values
  7. # configured in "security.kerberos.login.contexts".   
  8. zookeeper.sasl.login-context-name: Client

For more information on Flink configuration for Kerberos security, please refer to the security section of the Flink configuration page. You can also find further details on how Flink sets up Kerberos-based security internally.

Advanced Configuration

Tolerating Suspended ZooKeeper Connections

Per default, Flink’s ZooKeeper client treats suspended ZooKeeper connections as an error. This means that Flink will invalidate all leaderships of its components and thereby triggering a failover if a connection is suspended.

This behaviour might be too disruptive in some cases (e.g., unstable network environment). If you are willing to take a more aggressive approach, then you can tolerate suspended ZooKeeper connections and only treat lost connections as an error via high-availability.zookeeper.client.tolerate-suspended-connections. Enabling this feature will make Flink more resilient against temporary connection problems but also increase the risk of running into ZooKeeper timing problems.

For more information take a look at Curator’s error handling.

ZooKeeper Versions

Flink ships with separate ZooKeeper clients for 3.4 and 3.5, with 3.4 being in the lib directory of the distribution and thus used by default, whereas 3.5 is placed in the opt directory.

The 3.5 client allows you to secure the ZooKeeper connection via SSL, but may not work with 3.4- ZooKeeper installations.

You can control which version is used by Flink by placing either jar in the lib directory.

Bootstrap ZooKeeper

If you don’t have a running ZooKeeper installation, you can use the helper scripts, which ship with Flink.

There is a ZooKeeper configuration template in conf/zoo.cfg. You can configure the hosts to run ZooKeeper on with the server.X entries, where X is a unique ID of each server:

  1. server.X=addressX:peerPort:leaderPort
  2. [...]
  3. server.Y=addressY:peerPort:leaderPort

The script bin/start-zookeeper-quorum.sh will start a ZooKeeper server on each of the configured hosts. The started processes start ZooKeeper servers via a Flink wrapper, which reads the configuration from conf/zoo.cfg and makes sure to set some required configuration values for convenience. In production setups, it is recommended to manage your own ZooKeeper installation.