hadoop/hadoop-mapreduce-project/hadoop-yarn/hadoop-yarn-site/src/site/apt/HDFSHighAvailability.apt.vm

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  ---
  Hadoop Distributed File System-${project.version} - High Availability
  ---
  ---
  ${maven.build.timestamp}

HDFS High Availability

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%{toc|section=1|fromDepth=0}

* {Purpose}

  This guide provides an overview of the HDFS High Availability (HA) feature and
  how to configure and manage an HA HDFS cluster.
 
  This document assumes that the reader has a general understanding of
  general components and node types in an HDFS cluster. Please refer to the
  HDFS Architecture guide for details.

* {Background}

  Prior to Hadoop 2.0.0, the NameNode was a single point of failure (SPOF) in
  an HDFS cluster. Each cluster had a single NameNode, and if that machine or
  process became unavailable, the cluster as a whole would be unavailable
  until the NameNode was either restarted or brought up on a separate machine.
  
  This impacted the total availability of the HDFS cluster in two major ways:

    * In the case of an unplanned event such as a machine crash, the cluster would
      be unavailable until an operator restarted the NameNode.

    * Planned maintenance events such as software or hardware upgrades on the
      NameNode machine would result in windows of cluster downtime.
  
  The HDFS High Availability feature addresses the above problems by providing
  the option of running two redundant NameNodes in the same cluster in an
  Active/Passive configuration with a hot standby. This allows a fast failover to
  a new NameNode in the case that a machine crashes, or a graceful
  administrator-initiated failover for the purpose of planned maintenance.

* {Architecture}

  In a typical HA cluster, two separate machines are configured as NameNodes.
  At any point in time, exactly one of the NameNodes is in an <Active> state,
  and the other is in a <Standby> state. The Active NameNode is responsible
  for all client operations in the cluster, while the Standby is simply acting
  as a slave, maintaining enough state to provide a fast failover if
  necessary.
  
  In order for the Standby node to keep its state synchronized with the Active
  node, the current implementation requires that the two nodes both have access
  to a directory on a shared storage device (eg an NFS mount from a NAS). This
  restriction will likely be relaxed in future versions.

  When any namespace modification is performed by the Active node, it durably
  logs a record of the modification to an edit log file stored in the shared
  directory.  The Standby node is constantly watching this directory for edits,
  and as it sees the edits, it applies them to its own namespace. In the event of
  a failover, the Standby will ensure that it has read all of the edits from the
  shared storage before promoting itself to the Active state. This ensures that
  the namespace state is fully synchronized before a failover occurs.
  
  In order to provide a fast failover, it is also necessary that the Standby node
  have up-to-date information regarding the location of blocks in the cluster.
  In order to achieve this, the DataNodes are configured with the location of
  both NameNodes, and send block location information and heartbeats to both.
  
  It is vital for the correct operation of an HA cluster that only one of the
  NameNodes be Active at a time. Otherwise, the namespace state would quickly
  diverge between the two, risking data loss or other incorrect results.  In
  order to ensure this property and prevent the so-called "split-brain scenario,"
  the administrator must configure at least one <fencing method> for the shared
  storage. During a failover, if it cannot be verified that the previous Active
  node has relinquished its Active state, the fencing process is responsible for
  cutting off the previous Active's access to the shared edits storage. This
  prevents it from making any further edits to the namespace, allowing the new
  Active to safely proceed with failover.

* {Hardware resources}

  In order to deploy an HA cluster, you should prepare the following:

    * <<NameNode machines>> - the machines on which you run the Active and
    Standby NameNodes should have equivalent hardware to each other, and
    equivalent hardware to what would be used in a non-HA cluster.

    * <<Shared storage>> - you will need to have a shared directory which both
    NameNode machines can have read/write access to. Typically this is a remote
    filer which supports NFS and is mounted on each of the NameNode machines.
    Currently only a single shared edits directory is supported. Thus, the
    availability of the system is limited by the availability of this shared edits
    directory, and therefore in order to remove all single points of failure there
    needs to be redundancy for the shared edits directory. Specifically, multiple
    network paths to the storage, and redundancy in the storage itself (disk,
    network, and power). Beacuse of this, it is recommended that the shared storage
    server be a high-quality dedicated NAS appliance rather than a simple Linux
    server.
  
  Note that, in an HA cluster, the Standby NameNode also performs checkpoints of
  the namespace state, and thus it is not necessary to run a Secondary NameNode,
  CheckpointNode, or BackupNode in an HA cluster. In fact, to do so would be an
  error. This also allows one who is reconfiguring a non-HA-enabled HDFS cluster
  to be HA-enabled to reuse the hardware which they had previously dedicated to
  the Secondary NameNode.

* {Deployment}

** Configuration overview

  Similar to Federation configuration, HA configuration is backward compatible
  and allows existing single NameNode configurations to work without change.
  The new configuration is designed such that all the nodes in the cluster may
  have the same configuration without the need for deploying different
  configuration files to different machines based on the type of the node.
 
  Like HDFS Federation, HA clusters reuse the <<<nameservice ID>>> to identify a
  single HDFS instance that may in fact consist of multiple HA NameNodes. In
  addition, a new abstraction called <<<NameNode ID>>> is added with HA. Each
  distinct NameNode in the cluster has a different NameNode ID to distinguish it.
  To support a single configuration file for all of the NameNodes, the relevant
  configuration parameters are suffixed with the <<nameservice ID>> as well as
  the <<NameNode ID>>.

** Configuration details

  To configure HA NameNodes, you must add several configuration options to your
  <<hdfs-site.xml>> configuration file.

  The order in which you set these configurations is unimportant, but the values
  you choose for <<dfs.federation.nameservices>> and
  <<dfs.ha.namenodes.[nameservice ID]>> will determine the keys of those that
  follow. Thus, you should decide on these values before setting the rest of the
  configuration options.

  * <<dfs.federation.nameservices>> - the logical name for this new nameservice

    Choose a logical name for this nameservice, for example "mycluster", and use
    this logical name for the value of this config option. The name you choose is
    arbitrary. It will be used both for configuration and as the authority
    component of absolute HDFS paths in the cluster.

    <<Note:>> If you are also using HDFS Federation, this configuration setting
    should also include the list of other nameservices, HA or otherwise, as a
    comma-separated list.

----
<property>
  <name>dfs.federation.nameservices</name>
  <value>mycluster</value>
</property>
----

  * <<dfs.ha.namenodes.[nameservice ID]>> - unique identifiers for each NameNode in the nameservice

    Configure with a list of comma-separated NameNode IDs. This will be used by
    DataNodes to determine all the NameNodes in the cluster. For example, if you
    used "mycluster" as the nameservice ID previously, and you wanted to use "nn1"
    and "nn2" as the individual IDs of the NameNodes, you would configure this as
    such:

----
<property>
  <name>dfs.ha.namenodes.mycluster</name>
  <value>nn1,nn2</value>
</property>
----

    <<Note:>> Currently, only a maximum of two NameNodes may be configured per
    nameservice.

  * <<dfs.namenode.rpc-address.[nameservice ID].[name node ID]>> - the fully-qualified RPC address for each NameNode to listen on

    For both of the previously-configured NameNode IDs, set the full address and
    IPC port of the NameNode processs. Note that this results in two separate
    configuration options. For example:

----
<property>
  <name>dfs.namenode.rpc-address.mycluster.nn1</name>
  <value>machine1.example.com:8020</value>
</property>
<property>
  <name>dfs.namenode.rpc-address.mycluster.nn2</name>
  <value>machine2.example.com:8020</value>
</property>
----

    <<Note:>> You may similarly configure the "<<servicerpc-address>>" setting if
    you so desire.

  * <<dfs.namenode.http-address.[nameservice ID].[name node ID]>> - the fully-qualified HTTP address for each NameNode to listen on

    Similarly to <rpc-address> above, set the addresses for both NameNodes' HTTP
    servers to listen on. For example:

----
<property>
  <name>dfs.namenode.http-address.mycluster.nn1</name>
  <value>machine1.example.com:50070</value>
</property>
<property>
  <name>dfs.namenode.http-address.mycluster.nn2</name>
  <value>machine2.example.com:50070</value>
</property>
----

    <<Note:>> If you have Hadoop's security features enabled, you should also set
    the <https-address> similarly for each NameNode.

  * <<dfs.namenode.shared.edits.dir>> - the location of the shared storage directory

    This is where one configures the path to the remote shared edits directory
    which the Standby NameNode uses to stay up-to-date with all the file system
    changes the Active NameNode makes. <<You should only configure one of these
    directories.>> This directory should be mounted r/w on both NameNode machines.
    The value of this setting should be the absolute path to this directory on the
    NameNode machines. For example:

----
<property>
  <name>dfs.namenode.shared.edits.dir</name>
  <value>file:///mnt/filer1/dfs/ha-name-dir-shared</value>
</property>
----

  * <<dfs.client.failover.proxy.provider.[nameservice ID]>> - the Java class that HDFS clients use to contact the Active NameNode

    Configure the name of the Java class which will be used by the DFS Client to
    determine which NameNode is the current Active, and therefore which NameNode is
    currently serving client requests. The only implementation which currently
    ships with Hadoop is the <<ConfiguredFailoverProxyProvider>>, so use this
    unless you are using a custom one. For example:

----
<property>
  <name>dfs.client.failover.proxy.provider.mycluster</name>
  <value>org.apache.hadoop.hdfs.server.namenode.ha.ConfiguredFailoverProxyProvider</value>
</property>
----

  * <<dfs.ha.fencing.methods>> - a list of scripts or Java classes which will be used to fence the Active NameNode during a failover

    It is critical for correctness of the system that only one NameNode be in the
    Active state at any given time. Thus, during a failover, we first ensure that
    the Active NameNode is either in the Standby state, or the process has
    terminated, before transitioning the other NameNode to the Active state. In
    order to do this, you must configure at least one <<fencing method.>> These are
    configured as a carriage-return-separated list, which will be attempted in order
    until one indicates that fencing has succeeded. There are two methods which
    ship with Hadoop: <shell> and <sshfence>. For information on implementing
    your own custom fencing method, see the <org.apache.hadoop.ha.NodeFencer> class.

    * <<sshfence>> - SSH to the Active NameNode and kill the process

      The <sshfence> option SSHes to the target node and uses <fuser> to kill the
      process listening on the service's TCP port. In order for this fencing option
      to work, it must be able to SSH to the target node without providing a
      passphrase. Thus, one must also configure the
      <<dfs.ha.fencing.ssh.private-key-files>> option, which is a
      comma-separated list of SSH private key files. For example:

---
<property>
  <name>dfs.ha.fencing.methods</name>
  <value>sshfence</value>
</property>

<property>
  <name>dfs.ha.fencing.ssh.private-key-files</name>
  <value>/home/exampleuser/.ssh/id_rsa</value>
</property>
---

      Optionally, one may configure a non-standard username or port to perform the
      SSH. One may also configure a timeout, in milliseconds, for the SSH, after
      which this fencing method will be considered to have failed. It may be
      configured like so:

---
<property>
  <name>dfs.ha.fencing.methods</name>
  <value>sshfence([[username][:port]])</value>
</property>
<property>
  <name>dfs.ha.fencing.ssh.connect-timeout</name>
  <value>
</property>
---

    * <<shell>> - run an arbitrary shell command to fence the Active NameNode

      The <shell> fencing method runs an arbitrary shell command. It may be
      configured like so:

---
<property>
  <name>dfs.ha.fencing.methods</name>
  <value>shell(/path/to/my/script.sh arg1 arg2 ...)</value>
</property>
---

      The string between '(' and ')' is passed directly to a bash shell and may not
      include any closing parentheses.

      The shell command will be run with an environment set up to contain all of the
      current Hadoop configuration variables, with the '_' character replacing any
      '.' characters in the configuration keys. The configuration used has already had
      any namenode-specific configurations promoted to their generic forms -- for example
      <<dfs_namenode_rpc-address>> will contain the RPC address of the target node, even
      though the configuration may specify that variable as
      <<dfs.namenode.rpc-address.ns1.nn1>>.
      
      Additionally, the following variables referring to the target node to be fenced
      are also available:

*-----------------------:-----------------------------------+
| $target_host          | hostname of the node to be fenced |
*-----------------------:-----------------------------------+
| $target_port          | IPC port of the node to be fenced |
*-----------------------:-----------------------------------+
| $target_address       | the above two, combined as host:port |
*-----------------------:-----------------------------------+
| $target_nameserviceid | the nameservice ID of the NN to be fenced |
*-----------------------:-----------------------------------+
| $target_namenodeid    | the namenode ID of the NN to be fenced |
*-----------------------:-----------------------------------+
      
      These environment variables may also be used as substitutions in the shell
      command itself. For example:

---
<property>
  <name>dfs.ha.fencing.methods</name>
  <value>shell(/path/to/my/script.sh --nameservice=$target_nameserviceid $target_host:$target_port)</value>
</property>
---
      
      If the shell command returns an exit
      code of 0, the fencing is determined to be successful. If it returns any other
      exit code, the fencing was not successful and the next fencing method in the
      list will be attempted.

      <<Note:>> This fencing method does not implement any timeout. If timeouts are
      necessary, they should be implemented in the shell script itself (eg by forking
      a subshell to kill its parent in some number of seconds).

  * <<fs.defaultFS>> - the default path prefix used by the Hadoop FS client when none is given

    Optionally, you may now configure the default path for Hadoop clients to use
    the new HA-enabled logical URI. If you used "mycluster" as the nameservice ID
    earlier, this will be the value of the authority portion of all of your HDFS
    paths. This may be configured like so, in your <<core-site.xml>> file:

---
<property>
  <name>fs.defaultFS</name>
  <value>hdfs://mycluster</value>
</property>
---

** Deployment details

  After all of the necessary configuration options have been set, one must
  initially synchronize the two HA NameNodes' on-disk metadata. If you are
  setting up a fresh HDFS cluster, you should first run the format command (<hdfs
  namenode -format>) on one of NameNodes. If you have already formatted the
  NameNode, or are converting a non-HA-enabled cluster to be HA-enabled, you
  should now copy over the contents of your NameNode metadata directories to
  the other, unformatted NameNode using <scp> or a similar utility. The location
  of the directories containing the NameNode metadata are configured via the
  configuration options <<dfs.namenode.name.dir>> and/or
  <<dfs.namenode.edits.dir>>. At this time, you should also ensure that the
  shared edits dir (as configured by <<dfs.namenode.shared.edits.dir>>) includes
  all recent edits files which are in your NameNode metadata directories.

  At this point you may start both of your HA NameNodes as you normally would
  start a NameNode.

  You can visit each of the NameNodes' web pages separately by browsing to their
  configured HTTP addresses. You should notice that next to the configured
  address will be the HA state of the NameNode (either "standby" or "active".)
  Whenever an HA NameNode starts, it is initially in the Standby state.

** Administrative commands

  Now that your HA NameNodes are configured and started, you will have access
  to some additional commands to administer your HA HDFS cluster. Specifically,
  you should familiarize yourself with all of the subcommands of the "<hdfs
  haadmin>" command. Running this command without any additional arguments will
  display the following usage information:

---
Usage: DFSHAAdmin [-ns <nameserviceId>]
    [-transitionToActive <serviceId>]
    [-transitionToStandby <serviceId>]
    [-failover [--forcefence] [--forceactive] <serviceId> <serviceId>]
    [-getServiceState <serviceId>]
    [-checkHealth <serviceId>]
    [-help <command>]
---

  This guide describes high-level uses of each of these subcommands. For
  specific usage information of each subcommand, you should run "<hdfs haadmin
  -help <command>>".

  * <<transitionToActive>> and <<transitionToStandby>> - transition the state of the given NameNode to Active or Standby

    These subcommands cause a given NameNode to transition to the Active or Standby
    state, respectively. <<These commands do not attempt to perform any fencing,
    and thus should rarely be used.>> Instead, one should almost always prefer to
    use the "<hdfs haadmin -failover>" subcommand.

  * <<failover>> - initiate a failover between two NameNodes

    This subcommand causes a failover from the first provided NameNode to the
    second. If the first NameNode is in the Standby state, this command simply
    transitions the second to the Active state without error. If the first NameNode
    is in the Active state, an attempt will be made to gracefully transition it to
    the Standby state. If this fails, the fencing methods (as configured by
    <<dfs.ha.fencing.methods>>) will be attempted in order until one
    succeeds. Only after this process will the second NameNode be transitioned to
    the Active state. If no fencing method succeeds, the second NameNode will not
    be transitioned to the Active state, and an error will be returned.

  * <<getServiceState>> - determine whether the given NameNode is Active or Standby

    Connect to the provided NameNode to determine its current state, printing
    either "standby" or "active" to STDOUT appropriately. This subcommand might be
    used by cron jobs or monitoring scripts which need to behave differently based
    on whether the NameNode is currently Active or Standby.

  * <<checkHealth>> - check the health of the given NameNode

    Connect to the provided NameNode to check its health. The NameNode is capable
    of performing some diagnostics on itself, including checking if internal
    services are running as expected. This command will return 0 if the NameNode is
    healthy, non-zero otherwise. One might use this command for monitoring
    purposes.

    <<Note:>> This is not yet implemented, and at present will always return
    success, unless the given NameNode is completely down.

* {Automatic Failover}

** Introduction

  The above sections describe how to configure manual failover. In that mode,
  the system will not automatically trigger a failover from the active to the
  standby NameNode, even if the active node has failed. This section describes
  how to configure and deploy automatic failover.

** Components

  Automatic failover adds two new components to an HDFS deployment: a ZooKeeper
  quorum, and the ZKFailoverController process (abbreviated as ZKFC).

  Apache ZooKeeper is a highly available service for maintaining small amounts
  of coordination data, notifying clients of changes in that data, and
  monitoring clients for failures. The implementation of automatic HDFS failover
  relies on ZooKeeper for the following things:
  
    * <<Failure detection>> - each of the NameNode machines in the cluster
    maintains a persistent session in ZooKeeper. If the machine crashes, the
    ZooKeeper session will expire, notifying the other NameNode that a failover
    should be triggered.

    * <<Active NameNode election>> - ZooKeeper provides a simple mechanism to
    exclusively elect a node as active. If the current active NameNode crashes,
    another node may take a special exclusive lock in ZooKeeper indicating that
    it should become the next active.

  The ZKFailoverController (ZKFC) is a new component which is a ZooKeeper client
  which also monitors and manages the state of the NameNode.  Each of the
  machines which runs a NameNode also runs a ZKFC, and that ZKFC is responsible
  for:

    * <<Health monitoring>> - the ZKFC pings its local NameNode on a periodic
    basis with a health-check command. So long as the NameNode responds in a
    timely fashion with a healthy status, the ZKFC considers the node
    healthy. If the node has crashed, frozen, or otherwise entered an unhealthy
    state, the health monitor will mark it as unhealthy.

    * <<ZooKeeper session management>> - when the local NameNode is healthy, the
    ZKFC holds a session open in ZooKeeper. If the local NameNode is active, it
    also holds a special "lock" znode. This lock uses ZooKeeper's support for
    "ephemeral" nodes; if the session expires, the lock node will be
    automatically deleted.

    * <<ZooKeeper-based election>> - if the local NameNode is healthy, and the
    ZKFC sees that no other node currently holds the lock znode, it will itself
    try to acquire the lock. If it succeeds, then it has "won the election", and
    is responsible for running a failover to make its local NameNode active. The
    failover process is similar to the manual failover described above: first,
    the previous active is fenced if necessary, and then the local NameNode
    transitions to active state.

  For more details on the design of automatic failover, refer to the design
  document attached to HDFS-2185 on the Apache HDFS JIRA.

** Deploying ZooKeeper

  In a typical deployment, ZooKeeper daemons are configured to run on three or
  five nodes. Since ZooKeeper itself has light resource requirements, it is
  acceptable to collocate the ZooKeeper nodes on the same hardware as the HDFS
  NameNode and Standby Node. Many operators choose to deploy the third ZooKeeper
  process on the same node as the YARN ResourceManager. It is advisable to
  configure the ZooKeeper nodes to store their data on separate disk drives from
  the HDFS metadata for best performance and isolation.

  The setup of ZooKeeper is out of scope for this document. We will assume that
  you have set up a ZooKeeper cluster running on three or more nodes, and have
  verified its correct operation by connecting using the ZK CLI.

** Before you begin

  Before you begin configuring automatic failover, you should shut down your
  cluster. It is not currently possible to transition from a manual failover
  setup to an automatic failover setup while the cluster is running.

** Configuring automatic failover

  The configuration of automatic failover requires the addition of two new
  parameters to your configuration. In your <<<hdfs-site.xml>>> file, add:

----
 <property>
   <name>dfs.ha.automatic-failover.enabled</name>
   <value>true</value>
 </property>
----

  This specifies that the cluster should be set up for automatic failover.
  In your <<<core-site.xml>>> file, add:

----
 <property>
   <name>ha.zookeeper.quorum</name>
   <value>zk1.example.com:2181,zk2.example.com:2181,zk3.example.com:2181</value>
 </property>
----

  This lists the host-port pairs running the ZooKeeper service.

  As with the parameters described earlier in the document, these settings may
  be configured on a per-nameservice basis by suffixing the configuration key
  with the nameservice ID. For example, in a cluster with federation enabled,
  you can explicitly enable automatic failover for only one of the nameservices
  by setting <<<dfs.ha.automatic-failover.enabled.my-nameservice-id>>>.

  There are also several other configuration parameters which may be set to
  control the behavior of automatic failover; however, they are not necessary
  for most installations. Please refer to the configuration key specific
  documentation for details.

** Initializing HA state in ZooKeeper

  After the configuration keys have been added, the next step is to initialize
  required state in ZooKeeper. You can do so by running the following command
  from one of the NameNode hosts.

----
$ hdfs zkfc -formatZK
----

  This will create a znode in ZooKeeper inside of which the automatic failover
  system stores its data.

** Starting the cluster with <<<start-dfs.sh>>>

  Since automatic failover has been enabled in the configuration, the
  <<<start-dfs.sh>>> script will now automatically start a ZKFC daemon on any
  machine that runs a NameNode. When the ZKFCs start, they will automatically
  select one of the NameNodes to become active.

** Starting the cluster manually

  If you manually manage the services on your cluster, you will need to manually
  start the <<<zkfc>>> daemon on each of the machines that runs a NameNode. You
  can start the daemon by running:

----
$ hadoop-daemon.sh start zkfc
----

** Securing access to ZooKeeper

  If you are running a secure cluster, you will likely want to ensure that the
  information stored in ZooKeeper is also secured. This prevents malicious
  clients from modifying the metadata in ZooKeeper or potentially triggering a
  false failover.

  In order to secure the information in ZooKeeper, first add the following to
  your <<<core-site.xml>>> file:

----
 <property>
   <name>ha.zookeeper.auth</name>
   <value>@/path/to/zk-auth.txt</value>
 </property>
 <property>
   <name>ha.zookeeper.acl</name>
   <value>@/path/to/zk-acl.txt</value>
 </property>
----

  Please note the '@' character in these values -- this specifies that the
  configurations are not inline, but rather point to a file on disk.

  The first configured file specifies a list of ZooKeeper authentications, in
  the same format as used by the ZK CLI. For example, you may specify something
  like:

----
digest:hdfs-zkfcs:mypassword
----
  ...where <<<hdfs-zkfcs>>> is a unique username for ZooKeeper, and
  <<<mypassword>>> is some unique string used as a password.

  Next, generate a ZooKeeper ACL that corresponds to this authentication, using
  a command like the following:

----
$ java -cp $ZK_HOME/lib/*:$ZK_HOME/zookeeper-3.4.2.jar org.apache.zookeeper.server.auth.DigestAuthenticationProvider hdfs-zkfcs:mypassword
output: hdfs-zkfcs:mypassword->hdfs-zkfcs:P/OQvnYyU/nF/mGYvB/xurX8dYs=
----

  Copy and paste the section of this output after the '->' string into the file
  <<<zk-acls.txt>>>, prefixed by the string "<<<digest:>>>". For example:

----
digest:hdfs-zkfcs:vlUvLnd8MlacsE80rDuu6ONESbM=:rwcda
----

  In order for these ACLs to take effect, you should then rerun the
  <<<zkfc -formatZK>>> command as described above.

  After doing so, you may verify the ACLs from the ZK CLI as follows:

----
[zk: localhost:2181(CONNECTED) 1] getAcl /hadoop-ha
'digest,'hdfs-zkfcs:vlUvLnd8MlacsE80rDuu6ONESbM=
: cdrwa
----

** Verifying automatic failover

  Once automatic failover has been set up, you should test its operation. To do
  so, first locate the active NameNode. You can tell which node is active by
  visiting the NameNode web interfaces -- each node reports its HA state at the
  top of the page.

  Once you have located your active NameNode, you may cause a failure on that
  node.  For example, you can use <<<kill -9 <pid of NN>>>> to simulate a JVM
  crash. Or, you could power cycle the machine or unplug its network interface
  to simulate a different kind of outage.  After triggering the outage you wish
  to test, the other NameNode should automatically become active within several
  seconds. The amount of time required to detect a failure and trigger a
  fail-over depends on the configuration of
  <<<ha.zookeeper.session-timeout.ms>>>, but defaults to 5 seconds.

  If the test does not succeed, you may have a misconfiguration. Check the logs
  for the <<<zkfc>>> daemons as well as the NameNode daemons in order to further
  diagnose the issue.


* Automatic Failover FAQ

  * <<Is it important that I start the ZKFC and NameNode daemons in any
    particular order?>>

  No. On any given node you may start the ZKFC before or after its corresponding
  NameNode.

  * <<What additional monitoring should I put in place?>>

  You should add monitoring on each host that runs a NameNode to ensure that the
  ZKFC remains running. In some types of ZooKeeper failures, for example, the
  ZKFC may unexpectedly exit, and should be restarted to ensure that the system
  is ready for automatic failover.

  Additionally, you should monitor each of the servers in the ZooKeeper
  quorum. If ZooKeeper crashes, then automatic failover will not function.

  * <<What happens if ZooKeeper goes down?>>

  If the ZooKeeper cluster crashes, no automatic failovers will be triggered.
  However, HDFS will continue to run without any impact. When ZooKeeper is
  restarted, HDFS will reconnect with no issues.

  * <<Can I designate one of my NameNodes as primary/preferred?>>

  No. Currently, this is not supported. Whichever NameNode is started first will
  become active. You may choose to start the cluster in a specific order such
  that your preferred node starts first.

  * <<How can I initiate a manual failover when automatic failover is
    configured?>>

  Currently, this facility is not yet implemented. Instead, you may simply stop
  the active NameNode daemon. This will trigger an automatic failover. This
  process will be improved in future versions.