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<chapter id="Java-Broker-High-Availability">
  <title>High Availability</title>

  <section role="h3" id="Java-Broker-High-Availability-GeneralIntroduction">
    <title>General Introduction</title>
    <para>The term High Availability (HA) usually refers to having a number of instances of a service such as a Message Broker
      available so that should a service unexpectedly fail, or requires to be shutdown for maintenance, users may quickly connect
      to another instance and continue their work with minimal interuption. HA is one way to make a overall system more resilient
      by eliminating a single point of failure from a system.</para>
    <para>HA offerings are usually categorised as <emphasis role="bold">Active/Active</emphasis> or <emphasis role="bold">Active/Passive</emphasis>.
      An Active/Active system is one where all nodes within the cluster are usuaully available for use by clients all of the time.  In an
      Active/Passive system, one only node within the cluster is available for use by clients at any one time, whilst the others are in
      some kind of standby state, awaiting to quickly step-in in the event the active node becomes unavailable.
    </para>
  </section>

  <section role="h3" id="Java-Broker-High-Availability-OfferingsOfJavaBroker">
    <title>HA offerings of the Java Broker</title>
    <para>The Java Broker's HA offering became available at release <emphasis role="bold">0.18</emphasis>.  HA is provided by way of the HA
      features built into the <ulink url="&oracleBdbProductOverviewUrl;">Java Edition of the Berkley Database (BDB JE)</ulink> and as such
      is currently only available to Java Broker users who use the optional BDB JE based persistence store. This
      <emphasis role="bold">optional</emphasis> store requires the use of BDB JE which is licensed under the Sleepycat Licence, which is
      not compatible with the Apache Licence and thus BDB JE is not distributed with Qpid. Users who elect to use this optional store for
      the broker have to provide this dependency.</para>
    <para>HA in the Java Broker provides an <emphasis role="bold">Active/Passive</emphasis> mode of operation with Virtual hosts being
      the unit of replication.  The Active node (referred to as the <emphasis role="bold">Master</emphasis>) accepts all work from all the clients.
       The Passive nodes (referred to as <emphasis role="bold">Replicas</emphasis>) are unavailable for work: the only task they must perform is
       to remain in synch with the Master node by consuming a replication stream containing all data and state.</para>
    <para>If the Master node fails, a Replica node is elected to become the new Master node.  All clients automatically failover
      <footnote><para>The automatic failover feature is available only for AMQP connections from the Java client.  Management connections (JMX)
        do not current offer this feature.</para></footnote> to the new Master and continue their work.</para>
    <para>The Java Broker HA solution is incompatible with the HA solution offered by the CPP Broker.  It is not possible to co-locate Java and CPP
       Brokers within the same cluster.</para>
    <para>HA is not currently available for those using the the <emphasis role="bold">Derby Store</emphasis> or <emphasis role="bold">Memory
      Message Store</emphasis>.</para>
  </section>

  <section role="h3" id="Java-Broker-High-Availability-TwoNodeCluster">
    <title>Two Node Cluster</title>
    <section role="h4">
      <title>Overview</title>
      <para>In this HA solution, a cluster is formed with two nodes. one node serves as
        <emphasis role="bold">master</emphasis> and the other is a <emphasis role="bold">replica</emphasis>.
      </para>
      <para>All data and state required for the operation of the virtual host is automatically sent from the
        master to the replica. This is called the replication stream. The master virtual host confirms each
        message is on the replica before the client transaction completes. The exact way the client awaits
        for the master and replica is gorverned by the <link linkend="Java-Broker-High-Availability-DurabilityGuarantee">durability</link>
        configuration, which is discussed later. In this way, the replica remains ready to take over the
        role of the master if the master becomes unavailable.
      </para>
      <para>It is important to note that there is an inherent limitation of two node clusters is that
        the replica node cannot make itself master automatically in the event of master failure.  This
        is because the replica has no way to distinguish between a network partition (with potentially
        the master still alive on the other side of the partition) and the case of genuine master failure.
        (If the replica were to elect itself as master, the cluster would run the risk of a
        <ulink url="http://en.wikipedia.org/wiki/Split-brain_(computing)">split-brain</ulink> scenario).
        In the event of a master failure, a third party must designate the replica as primary.  This process
        is described in more detail later.
      </para>
      <para>Clients connect to the cluster using a <link linkend="Java-Broker-High-Availability-ClientFailover">failover url</link>.
        This allows the client to maintain a connection to the master in a way that is transparent
        to the client application.</para>
    </section>
    <section role="h4">
      <title>Depictions of cluster operation</title>
      <para>In this section, the operation of the cluster is depicted through a series of figures
        supported by explanatory text.</para>
      <figure>
        <title>Key for figures</title>
        <mediaobject>
          <imageobject>
            <imagedata fileref="images/HA-2N-Key.png" format="PNG" scalefit="1"/>
          </imageobject>
          <textobject>
            <phrase>Key to figures</phrase>
          </textobject>
        </mediaobject>
      </figure>
      <section role="h5" id="Java-Broker-High-Availability-TwoNodeNormalOperation">
        <title>Normal Operation</title>
        <para>The figure below illustrates normal operation.  Clients connecting to the cluster by way
	  of the failover URL achieve a connection to the master. As clients perform work (message
	  production, consumption, queue creation etc), the master additionally sends this data to the
	  replica over the network.</para>
        <figure>
          <title>Normal operation of a two-node cluster</title>
          <mediaobject>
            <imageobject>
              <imagedata fileref="images/HA-2N-Normal.png" format="PNG" scalefit="1"/>
            </imageobject>
            <textobject>
              <phrase>Normal operation</phrase>
            </textobject>
          </mediaobject>
        </figure>
      </section>
      <section role="h5" id="Java-Broker-High-Availability-TwoNodeMasterFailure">
        <title>Master Failure and Recovery</title>
        <para>The figure below illustrates a sequence of events whereby the master suffers a failure
	  and the replica is made the master to allow the clients to continue to work. Later the
	  old master is repaired and comes back on-line in replica role.</para>
        <para>The item numbers in this list apply to the numbered boxes in the figure below.</para>
        <orderedlist>
          <listitem>
            <para>System operating normally</para>
          </listitem>
          <listitem>
            <para>Master suffers a failure and disconnects all clients. Replica realises that it is no
	      longer in contact with master. Clients begin to try to reconnect to the cluster, although these
	      connection attempts will fail at this point.</para>
          </listitem>
          <listitem>
            <para>A third-party (an operator, a script or a combination of the two) verifies that the master has truely
           failed <emphasis role="bold">and is no longer running</emphasis>. If it has truely failed, the decision is made
           to designate the replica as primary, allowing it to assume the role of master despite the other node being down.
           This primary designation is performed using <link linkend="Java-Broker-High-Availability-JMXAPI">JMX</link>.</para>
          </listitem>
          <listitem>
            <para>Client connections to the new master succeed and the <emphasis role="bold">service is restored
	      </emphasis>, albeit without a replica.</para>
          </listitem>
          <listitem>
            <para>The old master is repaired and brought back on-line.  It automatically rejoins the cluster
	       in the <emphasis role="bold">replica</emphasis> role.</para>
          </listitem>
        </orderedlist>
        <figure>
          <title>Failure of master and recovery sequence</title>
          <mediaobject>
            <imageobject>
              <imagedata fileref="images/HA-2N-MasterFail.png" format="PNG" scalefit="1"/>
            </imageobject>
            <textobject>
              <phrase>Failure of master and subsequent recovery sequence</phrase>
            </textobject>
          </mediaobject>
        </figure>
      </section>
      <section role="h5" id="Java-Broker-High-Availability-TwoNodeReplicaFailure">
        <title>Replica Failure and Recovery</title>
        <para>The figure that follows illustrates a sequence of events whereby the replica suffers a failure
	   leaving the master to continue processing alone.  Later the replica is repaired and is restarted.
	   It rejoins the cluster so that it is once again ready to take over in the event of master failure.</para>
        <para>The behavior of the replica failure case is governed by the <varname>designatedPrimary</varname>
        configuration item. If set true on the master, the master will continue to operate solo without outside
        intervention when the replica fails. If false, a third-party must designate the master as primary in order
        for it to continue solo.</para>
        <para>The item numbers in this list apply to the numbered boxes in the figure below. This example assumes
	   that <varname>designatedPrimary</varname> is true on the original master node.</para>
        <orderedlist>
          <listitem>
            <para>System operating normally</para>
          </listitem>
          <listitem>
            <para>Replica suffers a failure. Master realises that replica longer in contact but as
	      <varname>designatedPrimary</varname> is true, master continues processing solo and thus client
	      connections are uninterrupted by the loss of the replica. System continues operating normally, albeit
          with a single node.</para>
          </listitem>
          <listitem>
            <para>Replica is repaired.</para>
          </listitem>
          <listitem>
            <para>After catching up with missed work, replica is once again ready to take over in the event of master failure.</para>
          </listitem>
        </orderedlist>
        <figure>
          <title>Failure of replica and subsequent recovery sequence</title>
          <mediaobject>
            <imageobject>
              <imagedata fileref="images/HA-2N-ReplicaFail.png" format="PNG" scalefit="1"/>
            </imageobject>
            <textobject>
              <phrase>Failure of replica and subsequent recovery sequence</phrase>
            </textobject>
          </mediaobject>
        </figure>
      </section>
      <section role="h5" id="Java-Broker-High-Availability-TwoNodeNetworkPartition">
        <title>Network Partition and Recovery</title>
        <para>The figure below illustrates the sequence of events that would occur if the network between
	  master and replica were to suffer a partition, and the nodes were out of contact with one and other.</para>
        <para>As with <link linkend="Java-Broker-High-Availability-TwoNodeReplicaFailure">Replica Failure and Recovery</link>, the
	  behaviour is governed by the <varname>designatedPrimary</varname>.
	  Only if <varname>designatedPrimary</varname> is true on the master, will the master continue solo.</para>
        <para>The item numbers in this list apply to the numbered boxes in the figure below. This example assumes
	   that <varname>designatedPrimary</varname> is true on the original master node.</para>
        <orderedlist>
          <listitem>
            <para>System operating normally</para>
          </listitem>
          <listitem>
            <para>Network suffers a failure. Master realises that replica longer in contact but as
	      <varname>designatedPrimary</varname> is true, master continues processing solo and thus client
	      connections are uninterrupted by the network partition between master and replica.</para>
          </listitem>
          <listitem>
            <para>Network is repaired.</para>
          </listitem>
          <listitem>
            <para>After catching up with missed work, replica is once again ready to take over in the event of master failure.
	    System operating normally again.</para>
          </listitem>
        </orderedlist>
        <figure>
          <title>Partition of the network separating master and replica</title>
          <mediaobject>
            <imageobject>
              <imagedata fileref="images/HA-2N-NetworkPartition.png" format="PNG" scalefit="1"/>
            </imageobject>
            <textobject>
              <phrase>Network Partition and Recovery</phrase>
            </textobject>
          </mediaobject>
        </figure>
      </section>
      <section role="h5" id="Java-Broker-High-Availability-TwoNodeSplitBrain">
        <title>Split Brain</title>
        <para>A <ulink url="http://en.wikipedia.org/wiki/Split-brain_(computing)">split-brain</ulink>
          is a situation where the two node cluster has two masters. BDB normally strives to prevent
	  this situation arising by preventing two nodes in a cluster being master at the same time.
	  However, if the network suffers a partition, and the third-party intervenes incorrectly
	  and makes the replica a second master a split-brain will be formed and both masters will
	  proceed to perform work  <emphasis role="bold">independently</emphasis> of one and other.</para>
        <para>There is no automatic recovery from a split-brain.</para>
        <para>Manual intervention will be required to choose which store will be retained as master
	  and which will be discarded.  Manual intervention will be required to identify and repeat the
          lost business transactions.</para>
        <para>The item numbers in this list apply to the numbered boxes in the figure below.</para>
        <orderedlist>
          <listitem>
            <para>System operating normally</para>
          </listitem>
          <listitem>
            <para>Network suffers a failure. Master realises that replica longer in contact but as
	      <varname>designatedPrimary</varname> is true, master continues processing solo.  Client
	      connections are uninterrupted by the network partition.</para>
            <para>A third-party <emphasis role="bold">erroneously</emphasis> designates the replica as primary while the
            original master continues running (now solo).</para>
          </listitem>
          <listitem>
            <para>As the nodes cannot see one and other, both behave as masters. Clients may perform work against
	      both master nodes.</para>
          </listitem>
        </orderedlist>
        <figure>
          <title>Split Brain</title>
          <mediaobject>
            <imageobject>
              <imagedata fileref="images/HA-2N-SplitBrain.png" format="PNG" scalefit="1"/>
            </imageobject>
            <textobject>
              <phrase>Split Brain</phrase>
            </textobject>
          </mediaobject>
        </figure>
      </section>
    </section>
  </section>

  <section role="h3" id="Java-Broker-High-Availability-MultiNodeCluster">
    <title>Multi Node Cluster</title>
    <para>Multi node clusters, that is clusters where the number of nodes is three or more, are not yet
         ready for use.</para>
  </section>

  <section role="h3" id="Java-Broker-High-Availability-Configuration">
    <title>Configuring a Virtual Host to be a node</title>
    <para>To configure a virtualhost as a cluster node, configure the virtualhost.xml in the following manner:</para>
    <para>

    <example>
      <title>Configuring a VirtualHost to use the BDBHAMessageStore</title>
      <programlisting language="xml"><![CDATA[
<virtualhost>
  <name>myhost</name>
  <myvhost>
    <store>
      <class>org.apache.qpid.server.store.berkeleydb.BDBHAMessageStore</class>
      <environment-path>${work}/bdbhastore</environment-path>
      <highAvailability>
        <groupName>myclustername</groupName>
        <nodeName>mynode1</nodeName>
        <nodeHostPort>node1host:port</nodeHostPort>
        <helperHostPort>node1host:port</helperHostPort>
        <durability>NO_SYNC\,NO_SYNC\,SIMPLE_MAJORITY</durability>
        <coalescingSync>true|false</coalescingSync>
        <designatedPrimary>true|false</designatedPrimary>
      </highAvailability>
    </store>
    ...
 </myvhost>
</virtualhost>]]></programlisting>
    </example>
    </para>
    <para>The <varname>groupName</varname> is the name of logical name of the cluster.  All nodes within the
      cluster must use the same <varname>groupName</varname> in order to be considered part of the cluster.</para>
    <para>The <varname>nodeName</varname> is the logical name of the node.  All nodes within the cluster must have a
      unique name.  It is recommended that the node name should be chosen from a different nomenclature from that of
      the servers on which they are hosted, in case the need arises to move node to a new server in the future.</para>
    <para>The <varname>nodeHostPort</varname> is the hostname and port number used by this node to communicate with the
      the other nodes in the cluster. For the hostname, an IP address, hostname or fully qualified hostname may be used.
      For the port number, any free port can be used.  It is important that this address is stable over time, as BDB
      records and uses this address internally.</para>
    <para>The <varname>helperHostPort</varname> is the hostname and port number that new nodes use to discover other
      nodes within the cluster when they are newly introduced to the cluster.  When configuring the first node, set the
      <varname>helperHostPort</varname> to its own <varname>nodeHostPort</varname>.  For the second and subsequent nodes,
      set their <varname>helperHostPort</varname> to that of the first node.</para>
    <para><varname>durability</varname> controls the <link linkend="Java-Broker-High-Availability-DurabilityGuarantee">durability</link>
      guarantees made by the cluster. It is important that all nodes use the same value for this property. The default value is
      NO_SYNC\,NO_SYNC\,SIMPLE_MAJORITY. Owing to the internal use of Apache Commons Config, it is currently necessary
      to escape the commas within the durability string.</para>
    <para><varname>coalescingSync</varname> controls the <link linkend="Java-Broker-High-Availability-DurabilityGuarantee_CoalescingSync">coalescing-sync</link>
      mode of Qpid. It is important that all nodes use the same value. If omitted, it defaults to true.</para>
    <para>The <varname>designatedPrimary</varname> is applicable only to the <link linkend="Java-Broker-High-Availability-TwoNodeCluster">two-node
     case.</link>  It governs the behaviour of a node when the other node fails or becomes uncontactable.  If true,
     the node will be designated as primary at startup and will be able to continue operating as a single node master.
     If false, the node will transition to an unavailable state until a third-party manually designates the node as
     primary or the other node is restored. It is suggested that the node that normally fulfils the role of master is
     set true in config file and the node that is normally replica is set false.  Be aware that setting both nodes to
     true will lead to a failure to start up, as both cannot be designated at the point of contact. Designating both
     nodes as primary at runtime (using the JMX interface) will lead to a <link linkend="Java-Broker-High-Availability-TwoNodeSplitBrain">split-brain</link>
     in the case of network partition and must be avoided.</para>
     <note><para>Usage of domain names in  <varname>helperHostPort</varname> and <varname>nodeHostPort</varname> is more preferebale
     over IP addresses due to the tendency of more frequent changes of the last over the former.
     If server IP address changes but domain name remains the same the HA cluster can continue working as normal
     in case when domain names are used in cluster configuration. In case when IP addresses are used and they are changed with the time
     than Qpid <link linkend="Java-Broker-High-Availability-JMXAPI">JMX API for HA</link> can be used to change the addresses or remove the nodes from the cluster.</para></note>

    <section role="h4" id="Java-Broker-High-Availability-Configuration_BDBEnvVars">
      <title>Passing BDB environment and replication configuration options</title>
      <para>It is possible to pass BDB <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/EnvironmentConfig.html">
         environment</ulink> and <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/rep/ReplicationConfig.html">
         replication</ulink> configuration options from the virtualhost.xml. Environment configuration options are passed using
         the <varname>envConfig</varname> element, and replication config using <varname>repConfig</varname>.</para>
      <para>For example, to override the BDB environment configuration options <varname>je.cleaner.threads</varname> and
        <varname>je.txn.timeout</varname></para>
      <programlisting language="xml"><![CDATA[
         ...
      </highAvailability>
      <envConfig>
        <name>je.cleaner.threads</name>
        <value>2</value>
      </envConfig>
      <envConfig>
        <name>je.txn.timeout</name>
        <value>15 min</value>
      </envConfig>
      ...
    </store>]]></programlisting>
      <para>And to override the BDB replication configuration options <varname>je.rep.electionsPrimaryRetries</varname>.</para>
      <programlisting language="xml"><![CDATA[
         ...
      </highAvailability>
      ...
      <repConfig>
        <name>je.rep.electionsPrimaryRetries</name>
        <value>3</value>
      </repConfig>
      ...
    </store>]]></programlisting>
    </section>
  </section>

  <section role="h3" id="Java-Broker-High-Availability-DurabilityGuarantee">
    <title>Durability Guarantees</title>
    <para>The term <ulink url="http://en.wikipedia.org/wiki/ACID#Durability">durability</ulink> is used to mean that once a
      transaction is committed, it remains committed regardless of subsequent failures. A highly durable system is one where
      loss of a committed transaction is extermely unlikely, whereas with a less durable system loss of a transaction is likely
      in a greater number of scenarios.  Typically, the more highly durable a system the slower and more costly it will be.</para>
    <para>Qpid exposes the all the
      <ulink url="&oracleBdbRepGuideUrl;txn-management.html#durabilitycontrols">durability controls</ulink>
      offered by by BDB JE JA and a Qpid specific optimisation called <emphasis role="bold">coalescing-sync</emphasis> which defaults
      to enabled.</para>
    <section role="h4" id="Java-Broker-High-Availability-DurabilityGuarantee_BDBControls">
      <title>BDB Durability Controls</title>
      <para>BDB expresses durability as a triplet with the following form:</para>
      <programlisting><![CDATA[<master sync policy>,<replica sync policy>,<replica acknowledgement policy>]]></programlisting>
      <para>The sync polices controls whether the thread performing the committing thread awaits the successful completion of the
        write, or the write and sync before continuing. The master sync policy and replica sync policy need not be the same.</para>
      <para>For master and replic sync policies, the available values are:
        <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/Durability.SyncPolicy.html#SYNC">SYNC</ulink>,
        <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/Durability.SyncPolicy.html#WRITE_NO_SYNC">WRITE_NO_SYNC</ulink>,
        <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/Durability.SyncPolicy.html#NO_SYNC">NO_SYNC</ulink>. SYNC
        is offers the highest durability whereas NO_SYNC the lowest.</para>
      <para>Note: the combination of a master sync policy of SYNC and <link linkend="Java-Broker-High-Availability-DurabilityGuarantee_CoalescingSync">coalescing-sync</link>
        true would result in poor performance with no corresponding increase in durability guarantee.  It cannot not be used.</para>
      <para>The acknowledgement policy defines whether when a master commits a transaction, it also awaits for the replica(s) to
         commit the same transaction before continuing.  For the two-node case, ALL and SIMPLE_MAJORITY are equal.</para>
      <para>For acknowledgement policy, the available value are:
         <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/Durability.ReplicaAckPolicy.html#ALL">ALL</ulink>,
         <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/Durability.ReplicaAckPolicy.html#SIMPLE_MAJORITY">SIMPLE_MAJORITY</ulink>
         <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/Durability.ReplicaAckPolicy.html#NONE">NONE</ulink>.</para>
    </section>
    <section role="h4" id="Java-Broker-High-Availability-DurabilityGuarantee_CoalescingSync">
      <title>Coalescing-sync</title>
      <para>If enabled (the default) Qpid works to reduce the number of separate
        <ulink url="&oracleJdkDocUrl;java/io/FileDescriptor.html#sync()">file-system sync</ulink> operations
        performed by the <emphasis role="bold">master</emphasis> on the underlying storage device thus improving performance.  It does
        this coalescing separate sync operations arising from the different client commits operations occuring at approximately the same time.
        It does this in such a manner not to reduce the ACID guarantees of the system.</para>
      <para>Coalescing-sync has no effect on the behaviour of the replicas.</para>
    </section>
    <section role="h4" id="Java-Broker-High-Availability-DurabilityGuarantee_Default">
      <title>Default</title>
      <para>The default durability guarantee is <constant>NO_SYNC, NO_SYNC, SIMPLE_MAJORITY</constant> with coalescing-sync enabled. The effect
         of this combination is described in the table below. It offers a good compromise between durability guarantee and performance
         with writes being guaranteed on the master and the additional guarantee that a majority of replicas have received the
         transaction.</para>
    </section>
    <section role="h4" id="Java-Broker-High-Availability-DurabilityGuarantee_Examples">
      <title>Examples</title>
      <para>Here are some examples illustrating the effects of the durability and coalescing-sync settings.</para>
      <para>
        <table>
          <title>Effect of different durability guarantees</title>
          <tgroup cols="4">
            <thead>
              <row>
                <entry/>
                <entry>Durability</entry>
                <entry>Coalescing-sync</entry>
                <entry>Description</entry>
              </row>
            </thead>
            <tbody>
              <row>
                <entry>1</entry>
                <entry>NO_SYNC, NO_SYNC, SIMPLE_MAJORITY</entry>
                <entry>true</entry>
                <entry>Before the commit returns to the client, the transaction will be written/sync'd to the Master's disk (effect of
                   coalescing-sync) and a majority of the replica(s) will have acknowledged the <emphasis role="bold">receipt</emphasis>
                   of the transaction.  The replicas will write and sync the transaction to their disk at a point in the future governed by
                   <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/rep/ReplicationMutableConfig.html#LOG_FLUSH_TASK_INTERVAL">ReplicationMutableConfig#LOG_FLUSH_INTERVAL</ulink>.
                </entry>
              </row>
              <row>
                <entry>2</entry>
                <entry>NO_SYNC, WRITE_NO_SYNC, SIMPLE_MAJORITY</entry>
                <entry>true</entry>
                <entry>Before the commit returns to the client, the transaction will be written/sync'd to the Master's disk (effect of
                  coalescing-sync and a majority of the replica(s) will have acknowledged the <emphasis role="bold">write</emphasis> of
                  the transaction to their disk.  The replicas will sync the transaction to disk at a point in the future with an upper bound governed by
                  ReplicationMutableConfig#LOG_FLUSH_INTERVAL.</entry>
              </row>
              <row>
                <entry>3</entry>
                <entry>NO_SYNC, NO_SYNC, NONE</entry>
                <entry>false</entry>
                <entry>After the commit returns to the client, the transaction is neither guaranteed to be written to the disk of the master
                   nor received by any of the replicas. The master and replicas will write and sync the transaction to their disk at a point
                   in the future with an upper bound governed by ReplicationMutableConfig#LOG_FLUSH_INTERVAL. This offers the weakest durability guarantee.</entry>
              </row>
            </tbody>
          </tgroup>
        </table>
      </para>
    </section>
  </section>

  <section id="Java-Broker-High-Availability-ClientFailover">
    <title>Client failover configuration</title>
    <para>The details about format of Qpid connection URLs can be found at section
        <ulink url="../../Programming-In-Apache-Qpid/html/QpidJNDI.html">Connection URLs</ulink>
        of book <ulink url="../../Programming-In-Apache-Qpid/html/">Programming In Apache Qpid</ulink>.</para>
    <para>The failover policy option in the connection URL for the HA Cluster should be set to <emphasis>roundrobin</emphasis>.
      The Master broker should be put into a first place in <emphasis>brokerlist</emphasis> URL option.
      The recommended value for <emphasis>connectdelay</emphasis> option in broker URL should be set to
      the value greater than 1000 milliseconds. If it is desired that clients re-connect automatically after a
      master to replica failure, <varname>cyclecount</varname> should be tuned so that the retry period is longer than
      the expected length of time to perform the failover.</para>
    <example><title>Example of connection URL for the HA Cluster</title><![CDATA[
amqp://guest:guest@clientid/test?brokerlist='tcp://localhost:5672?connectdelay='2000'&retries='3';tcp://localhost:5671?connectdelay='2000'&retries='3';tcp://localhost:5673?connectdelay='2000'&retries='3''&failover='roundrobin?cyclecount='30''
        ]]></example>
  </section>


  <section role="h3" id="Java-Broker-High-Availability-JMXAPI">
    <title>Qpid JMX API for HA</title>
    <para>Qpid exposes the BDB HA store information via its JMX interface and provides APIs to remove a Node from
     the group, update a Node IP address, and assign a Node as the designated primary.</para>
    <para>An instance of the <classname>BDBHAMessageStore</classname> MBean is instantiated by the broker for the each virtualhost using the HA store.</para>
    <para>The reference to this MBean can be obtained via JMX API using an ObjectName like <emphasis>org.apache.qpid:type=BDBHAMessageStore,name=&quot;&lt;virtualhost name&gt;&quot;</emphasis>
                 where &lt;virtualhost name&gt; is the name of a specific virtualhost on the broker.</para>
    <table border="1">
      <title>Mbean <classname>BDBHAMessageStore</classname> attributes</title>
      <thead>
        <tr>
          <td>Name</td>
          <td>Type</td>
          <td>Accessibility</td>
          <td>Description</td>
        </tr>
      </thead>
      <tbody>
        <tr>
          <td>GroupName</td>
          <td>String</td>
          <td>Read only</td>
          <td>Name identifying the group</td>
        </tr>
        <tr>
          <td>NodeName</td>
          <td>String</td>
          <td>Read only</td>
          <td>Unique name identifying the node within the group</td>
        </tr>
        <tr>
          <td>NodeHostPort</td>
          <td>String</td>
          <td>Read only</td>
          <td>Host/port used to replicate data between this node and others in the group</td>
        </tr>
        <tr>
          <td>HelperHostPort</td>
          <td>String</td>
          <td>Read only</td>
          <td>Host/port used to allow a new node to discover other group members</td>
        </tr>
        <tr>
          <td>NodeState</td>
          <td>String</td>
          <td>Read only</td>
          <td>Current state of the node</td>
        </tr>
        <tr>
          <td>ReplicationPolicy</td>
          <td>String</td>
          <td>Read only</td>
          <td>Node replication durability</td>
        </tr>
        <tr id="JMXDesignatedPrimary">
          <td>DesignatedPrimary</td>
          <td>boolean</td>
          <td>Read/Write</td>
          <td>Designated primary flag. Applicable to the two node case.</td>
        </tr>
        <tr>
          <td>CoalescingSync</td>
          <td>boolean</td>
          <td>Read only</td>
          <td>Coalescing sync flag. Applicable to the master sync policies NO_SYNC and WRITE_NO_SYNC only.</td>
        </tr>
       <tr>
          <td>getAllNodesInGroup</td>
          <td>TabularData</td>
          <td>Read only</td>
          <td>Get all nodes within the group, regardless of whether currently attached or not</td>
        </tr>
      </tbody>
    </table>

    <table border="1">
      <title>Mbean <classname>BDBHAMessageStore</classname> operations</title>
      <thead>
        <tr>
          <td>Operation</td>
          <td>Parameters</td>
          <td>Returns</td>
          <td>Description</td>
        </tr>
      </thead>
      <tbody>
        <tr>
          <td>removeNodeFromGroup</td>
          <td>
            <para><emphasis>nodeName</emphasis>, name of node, string</para>
          </td>
          <td>void</td>
          <td>Remove an existing node from the group</td>
        </tr>
        <tr>
          <td>updateAddress</td>
          <td>
            <itemizedlist>
              <listitem>
                <para><emphasis>nodeName</emphasis>, name of node, string</para>
              </listitem>
              <listitem>
                <para><emphasis>newHostName</emphasis>, new host name, string</para>
              </listitem>
              <listitem>
                <para><emphasis>newPort</emphasis>, new port number, int</para>
              </listitem>
            </itemizedlist>
          </td>
          <td>void</td>
          <td>Update the address of another node. The node must be in a STOPPED state.</td>
        </tr>
      </tbody>
    </table>
    <figure>
      <title>BDBHAMessageStore view from jconsole.</title>
      <graphic fileref="images/HA-BDBHAMessageStore-MBean-jconsole.png"/>
    </figure>
    <example>
      <title>Example of java code to get the node state value</title>
      <programlisting language="java"><![CDATA[
Map<String, Object> environment = new HashMap<String, Object>();

// credentials: user name and password
environment.put(JMXConnector.CREDENTIALS, new String[] {"admin","admin"});
JMXServiceURL url =  new JMXServiceURL("service:jmx:rmi:///jndi/rmi://localhost:9001/jmxrmi");
JMXConnector jmxConnector = JMXConnectorFactory.connect(url, environment);
MBeanServerConnection mbsc =  jmxConnector.getMBeanServerConnection();

ObjectName queueObjectName = new ObjectName("org.apache.qpid:type=BDBHAMessageStore,name=\"test\"");
String state = (String)mbsc.getAttribute(queueObjectName, "NodeState");

System.out.println("Node state:" + state);
        ]]></programlisting>
      <para>Example system output:</para>
      <screen><![CDATA[Node state:MASTER]]></screen>
    </example>
  </section>

  <section id="Java-Broker-High-Availability-Monitoring-cluster">
    <title>Monitoring cluster</title>
    <para>In order to discover potential issues with HA Cluster early, all nodes in the Cluster should be monitored on regular basis
    using the following techniques:</para>
    <itemizedlist>
      <listitem>
        <para>Broker log files scrapping for WARN or ERROR entries and operational log entries like:</para>
        <itemizedlist>
          <listitem>
            <para><emphasis>MST-1007 :</emphasis> Store Passivated. It can indicate that Master virtual host has gone down.</para>
          </listitem>
          <listitem>
            <para><emphasis>MST-1006 :</emphasis> Recovery Complete. It can indicate that a former Replica virtual host is up and became the Master.</para>
          </listitem>
        </itemizedlist>
      </listitem>
      <listitem>
        <para>Disk space usage and system load using system tools.</para>
      </listitem>
      <listitem>
        <para>Berkeley HA node status using <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/rep/util/DbPing.html"><classname>DbPing</classname></ulink> utility.</para>
        <example><title>Using <classname>DbPing</classname> utility for monitoring HA nodes.</title><command>
java -jar je-&oracleBdbProductVersion;.jar DbPing -groupName TestClusterGroup -nodeName Node-5001 -nodeHost localhost:5001 -socketTimeout 10000
</command><screen>
Current state of node: Node-5001 from group: TestClusterGroup
  Current state: MASTER
  Current master: Node-5001
  Current JE version: &oracleBdbProductVersion;
  Current log version: 8
  Current transaction end (abort or commit) VLSN: 165
  Current master transaction end (abort or commit) VLSN: 0
  Current active feeders on node: 0
  Current system load average: 0.35
</screen></example>
        <para>In the example above <classname>DbPing</classname> utility requested status of Cluster node with name
            <emphasis>Node-5001</emphasis> from replication group <emphasis>TestClusterGroup</emphasis> running on host <emphasis>localhost:5001</emphasis>.
            The state of the node was reported into a system output.
            </para>
      </listitem>
      <listitem>
        <para>Using Qpid broker JMX interfaces.</para>
        <para>Mbean <classname>BDBHAMessageStore</classname> can be used to request the following node information:</para>
        <itemizedlist>
          <listitem>
            <para><emphasis>NodeState</emphasis> indicates whether node is a Master or Replica.</para>
          </listitem>
          <listitem>
            <para><emphasis>Durability</emphasis> replication durability.</para>
          </listitem>
          <listitem>
            <para><emphasis>DesignatedPrimary</emphasis> indicates whether Master node is designated primary.</para>
          </listitem>
          <listitem>
            <para><emphasis>GroupName</emphasis> replication group name.</para>
          </listitem>
          <listitem>
            <para><emphasis>NodeName</emphasis> node name.</para>
          </listitem>
          <listitem>
            <para><emphasis>NodeHostPort</emphasis> node host and port.</para>
          </listitem>
          <listitem>
            <para><emphasis>HelperHostPort</emphasis> helper host and port.</para>
          </listitem>
          <listitem>
            <para><emphasis>AllNodesInGroup</emphasis> lists of all nodes in the replication group including their names, hosts and ports.</para>
          </listitem>
        </itemizedlist>
        <para>For more details about <classname>BDBHAMessageStore</classname> MBean please refer section <link linkend="Java-Broker-High-Availability-JMXAPI">Qpid JMX API for HA</link></para>
      </listitem>
    </itemizedlist>
  </section>

  <section id="Java-Broker-High-Availability-DiskSpace">
    <title>Disk space requirements</title>
    <para>Disk space is a critical resource for the HA Qpid broker.</para>
    <para>In case when a Replica goes down (or falls behind the Master in 2 node cluster where the Master is designated primary)
    and the Master continues running, the non-replicated store files are kept on the Masters disk for the period of time
    as specified in <emphasis>je.rep.repStreamTimeout</emphasis> JE setting in order to replicate this data later
    when the Replica is back. This setting is set to 1 hour by default by the broker. The setting can be overridden as described in
    <xref linkend="Java-Broker-High-Availability-Configuration_BDBEnvVars"/>.</para>
    <para>Depending from the application publishing/consuming rates and message sizes,
    the disk space might become overfull during this period of time due to preserved logs.
    Please, make sure to allocate enough space on your disk to avoid this from happening.
    </para>
  </section>

  <section id="Java-Broker-High-Availability-Network-Requirements">
    <title>Network Requirements</title>
    <para>The HA Cluster performance depends on the network bandwidth, its use by existing traffic, and quality of service.</para>
    <para>In order to achieve the best performance it is recommended to use a separate network infrastructure for the Qpid HA Nodes
     which might include installation of dedicated network hardware on Broker hosts, assigning a higher priority to replication ports,
     installing a cluster in a separate network not impacted by any other traffic.</para>
  </section>

  <section id="Java-Broker-High-Availability-Security">
    <title>Security</title>
    <para>At the moment Berkeley replication API supports only TCP/IP protocol to transfer replication data between Master and Replicas.</para>
    <para>As result, the replicated data is unprotected and can be intercepted by anyone having access to the replication network.</para>
    <para>Also, anyone who can access to this network can introduce a new node and therefore receive a copy of the data.</para>
    <para>In order to reduce the security risks the entire HA cluster is recommended to run in a separate network protected from general access.</para>
  </section>

  <section id="Java-Broker-High-Availability-Backup">
    <title>Backups</title>
    <para>In order to protect the entire cluster from some cataclysms which might destroy all cluster nodes,
    backups of the Master store should be taken on a regular basis.</para>
    <para>Qpid Broker distribution includes the "hot" backup utility <emphasis>backup.sh</emphasis> which can be found at broker bin folder.
         This utility can perform the backup when broker is running.</para>
    <para><emphasis>backup.sh</emphasis> script invokes <classname>org.apache.qpid.server.store.berkeleydb.BDBBackup</classname> to do the job.</para>
    <para>You can also run this class from command line like in an example below:</para>
    <example><title>Performing store backup by using <classname>BDBBackup</classname> class directly</title><command>
        java -cp qpid-bdbstore-0.18.jar org.apache.qpid.server.store.berkeleydb.BDBBackup -fromdir path/to/store/folder -todir path/to/backup/foldeAr</command>
    </example>
    <para>In the example above BDBBackup utility is called from qpid-bdbstore-0.18.jar to backup the store at <emphasis>path/to/store/folder</emphasis> and copy store logs into <emphasis>path/to/backup/folder</emphasis>.</para>
    <para>Linux and Unix users can take advantage of <emphasis>backup.sh</emphasis> bash script by running this script in a similar way.</para>
    <example><title>Performing store backup by using <classname>backup.sh</classname> bash script</title>
        <command>backup.sh -fromdir path/to/store/folder -todir path/to/backup/folder</command>
    </example>
    <note>
      <para>Do not forget to ensure that the Master store is being backed up, in the event the Node elected Master changes during
      the lifecycle of the cluster.</para>
    </note>
  </section>

  <section id="Java-Broker-High-Availability-MigrationFromNonHA">
    <title>Migration of a non-HA store to HA</title>
    <para>Non HA stores starting from schema version 4 (0.14 Qpid release) can be automatically converted into HA store on broker startup if replication is first enabled with the <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/rep/util/DbEnableReplication.html"><classname>DbEnableReplication</classname></ulink> utility from the BDB JE jar.</para>
    <para>DbEnableReplication converts a non HA store into an HA store and can be used as follows:</para>
    <example><title>Enabling replication</title><command>
java -jar je-&oracleBdbProductVersion;.jar DbEnableReplication -h /path/to/store -groupName MyReplicationGroup -nodeName MyNode1 -nodeHostPort  localhost:5001
        </command></example>
    <para>In the examples above, je jar of version &oracleBdbProductVersion; is used to convert store at <emphasis>/path/to/store</emphasis> into HA store having replication group name <emphasis>MyReplicationGroup</emphasis>, node name <emphasis>MyNode1</emphasis> and running on host <emphasis>localhost</emphasis> and port <emphasis>5001</emphasis>.</para>
    <para>After running DbEnableReplication and updating the virtual host store to configuration to be an HA message store, like in example below,
    on broker start up the store schema will be upgraded to the most recent version and the broker can be used as normal.</para>
    <example>
      <title>Example of XML configuration for HA message store</title>
      <programlisting language="xml"><![CDATA[
<store>
    <class>org.apache.qpid.server.store.berkeleydb.BDBHAMessageStore</class>
    <environment-path>/path/to/store</environment-path>
    <highAvailability>
        <groupName>MyReplicationGroup</groupName>
        <nodeName>MyNode1</nodeName>
        <nodeHostPort>localhost:5001</nodeHostPort>
        <helperHostPort>localhost:5001</helperHostPort>
    </highAvailability>
</store>]]></programlisting>
    </example>
    <para>The Replica nodes can be started with empty stores. The data will be automatically copied from Master to Replica on Replica start-up.
      This will take a period of time determined by the size of the Masters store and the network bandwidth between the nodes.</para>
    <note>
      <para>Due to existing caveats in Berkeley JE with copying of data from Master into Replica it is recommended to restart the Master node after store schema upgrade is finished before starting the Replica nodes.</para>
    </note>
  </section>

  <section id="Java-Broker-High-Availability-DisasterRecovery">
    <title>Disaster Recovery</title>
    <para>This section describes the steps required to restore HA broker cluster from backup.</para>
    <para>The detailed instructions how to perform backup on replicated environment can be found <link linkend="Java-Broker-High-Availability-Backup">here</link>.</para>
    <para>At this point we assume that backups are collected on regular basis from Master node.</para>
    <para>Replication configuration of a cluster is stored internally in HA message store.
    This information includes IP addresses of the nodes.
    In case when HA message store needs to be restored on a different host with a different IP address
    the cluster replication configuration should be reseted in this case</para>
    <para>Oracle provides a command line utility <ulink url="&oracleBdbJavaDocUrl;com/sleepycat/je/rep/util/DbResetRepGroup.html"><classname>DbResetRepGroup</classname></ulink>
    to reset the members of a replication group and replace the group with a new group consisting of a single new member
    as described by the arguments supplied to the utility</para>
    <para>Cluster can be restored with the following steps:</para>
    <itemizedlist>
         <listitem><para>Copy log files into the store folder from backup</para></listitem>
         <listitem>
            <para>Use <classname>DbResetRepGroup</classname> to reset an existing environment. See an example below</para>
            <example>
                <title>Reseting of replication group with <classname>DbResetRepGroup</classname></title><command>
java -cp je-&oracleBdbProductVersion;.jar com.sleepycat.je.rep.util.DbResetRepGroup -h ha-work/Node-5001/bdbstore -groupName TestClusterGroup -nodeName Node-5001 -nodeHostPort localhost:5001</command>
            </example>
            <para>In the example above <classname>DbResetRepGroup</classname> utility from Berkeley JE of version &oracleBdbProductVersion; is used to reset the store
            at location <emphasis>ha-work/Node-5001/bdbstore</emphasis> and set a replication group to <emphasis>TestClusterGroup</emphasis>
            having a node <emphasis>Node-5001</emphasis> which runs at <emphasis>localhost:5001</emphasis>.</para>
         </listitem>
         <listitem><para>Start a broker with HA store configured as specified on running of <classname>DbResetRepGroup</classname> utility.</para></listitem>
         <listitem><para>Start replica nodes having the same replication group and a helper host port pointing to a new master. The store content will be copied into Replicas from Master on their start up.</para></listitem>
    </itemizedlist>
  </section>

  <section id="Java-Broker-High-Availability-Performance">
    <title>Performance</title>
    <para>The aim of this section is not to provide exact performance metrics relating to HA, as this depends heavily on the test
    environment, but rather showing an impact of HA on Qpid broker performance in comparison with the Non HA case.</para>
    <para>For testing of impact of HA on a broker performance a special test script was written using Qpid performance test framework.
    The script opened a number of connections to the Qpid broker, created producers and consumers on separate connections,
    and published test messages with concurrent producers into a test queue and consumed them with concurrent consumers.
    The table below shows the number of producers/consumers used in the tests.
    The overall throughput was collected for each configuration.
    </para>
    <table border="1">
      <title>Number of producers/consumers in performance tests</title>
      <thead>
        <tr>
          <th>Test</th>
          <th>Number of producers</th>
          <th>Number of consumers</th>
        </tr>
      </thead>
      <tbody>
        <tr>
          <td>1</td>
          <td>1</td>
          <td>1</td>
        </tr>
        <tr>
          <td>2</td>
          <td>2</td>
          <td>2</td>
        </tr>
        <tr>
          <td>3</td>
          <td>4</td>
          <td>4</td>
        </tr>
        <tr>
          <td>4</td>
          <td>8</td>
          <td>8</td>
        </tr>
        <tr>
          <td>5</td>
          <td>16</td>
          <td>16</td>
        </tr>
        <tr>
          <td>6</td>
          <td>32</td>
          <td>32</td>
        </tr>
        <tr>
          <td>7</td>
          <td>64</td>
          <td>64</td>
        </tr>
      </tbody>
    </table>
    <para>The test was run against the following Qpid Broker configurations</para>
    <itemizedlist>
      <listitem>
        <para>Non HA Broker</para>
      </listitem>
      <listitem>
        <para>HA 2 Nodes Cluster with durability <emphasis>SYNC,SYNC,ALL</emphasis></para>
      </listitem>
      <listitem>
        <para>HA 2 Nodes Cluster with durability <emphasis>WRITE_NO_SYNC,WRITE_NO_SYNC,ALL</emphasis></para>
      </listitem>
      <listitem>
        <para>HA 2 Nodes Cluster with durability <emphasis>WRITE_NO_SYNC,WRITE_NO_SYNC,ALL</emphasis> and <emphasis>coalescing-sync</emphasis> Qpid mode</para>
      </listitem>
      <listitem>
        <para>HA 2 Nodes Cluster with durability <emphasis>WRITE_NO_SYNC,NO_SYNC,ALL</emphasis> and <emphasis>coalescing-sync</emphasis> Qpid mode</para>
      </listitem>
      <listitem>
        <para>HA 2 Nodes Cluster with durability <emphasis>NO_SYNC,NO_SYNC,ALL</emphasis> and <emphasis>coalescing-sync</emphasis> Qpid option</para>
      </listitem>
    </itemizedlist>
    <para>The evironment used in testing consisted of 2 servers with 4 CPU cores (2x Intel(r) Xeon(R) CPU 5150@2.66GHz), 4GB of RAM
        and running under OS Red Hat Enterprise Linux AS release 4 (Nahant Update 4). Network bandwidth was 1Gbit.
    </para>
    <para>We ran Master node on the first server and Replica and clients(both consumers and producers) on the second server.</para>
    <para>In non-HA case Qpid Broker was run on a first server and clients were run on a second server.</para>
    <para>The table below contains the test results we measured on this environment for different Broker configurations.</para>
    <para>Each result is represented by throughput value in KB/second and difference in % between HA configuration and non HA case for the same number of clients.</para>
    <table border="1">
      <title>Performance Comparison</title>
      <thead>
        <tr>
          <td>Test/Broker</td>
          <td>No HA</td>
          <td>SYNC, SYNC, ALL</td>
          <td>WRITE_NO_SYNC, WRITE_NO_SYNC, ALL</td>
          <td>WRITE_NO_SYNC, WRITE_NO_SYNC, ALL - coalescing-sync</td>
          <td>WRITE_NO_SYNC, NO_SYNC,ALL - coalescing-sync</td>
          <td>NO_SYNC, NO_SYNC, ALL - coalescing-sync</td>
        </tr>
      </thead>
      <tbody>
        <tr>
          <td>1 (1/1)</td>
          <td>0.0%</td>
          <td>-61.4%</td>
          <td>117.0%</td>
          <td>-16.02%</td>
          <td>-9.58%</td>
          <td>-25.47%</td>
        </tr>
        <tr>
          <td>2 (2/2)</td>
          <td>0.0%</td>
          <td>-75.43%</td>
          <td>67.87%</td>
          <td>-66.6%</td>
          <td>-69.02%</td>
          <td>-30.43%</td>
        </tr>
        <tr>
          <td>3 (4/4)</td>
          <td>0.0%</td>
          <td>-84.89%</td>
          <td>24.19%</td>
          <td>-71.02%</td>
          <td>-69.37%</td>
          <td>-43.67%</td>
        </tr>
        <tr>
          <td>4 (8/8)</td>
          <td>0.0%</td>
          <td>-91.17%</td>
          <td>-22.97%</td>
          <td>-82.32%</td>
          <td>-83.42%</td>
          <td>-55.5%</td>
        </tr>
        <tr>
          <td>5 (16/16)</td>
          <td>0.0%</td>
          <td>-91.16%</td>
          <td>-21.42%</td>
          <td>-86.6%</td>
          <td>-86.37%</td>
          <td>-46.99%</td>
        </tr>
        <tr>
          <td>6 (32/32)</td>
          <td>0.0%</td>
          <td>-94.83%</td>
          <td>-51.51%</td>
          <td>-92.15%</td>
          <td>-92.02%</td>
          <td>-57.59%</td>
        </tr>
        <tr>
          <td>7 (64/64)</td>
          <td>0.0%</td>
          <td>-94.2%</td>
          <td>-41.84%</td>
          <td>-89.55%</td>
          <td>-89.55%</td>
          <td>-50.54%</td>
        </tr>
      </tbody>
    </table>
    <para>The figure below depicts the graphs for the performance test results</para>
    <figure>
      <title>Test results</title>
      <graphic fileref="images/HA-perftests-results.png"/>
    </figure>
    <para>On using durability <emphasis>SYNC,SYNC,ALL</emphasis> (without coalescing-sync) the performance drops significantly (by 62-95%) in comparison with non HA broker.</para>
    <para>Whilst, on using durability <emphasis>WRITE_NO_SYNC,WRITE_NO_SYNC,ALL</emphasis> (without coalescing-sync) the performance drops by only half, but with loss of durability guarantee, so is not recommended.</para>
    <para>In order to have better performance with HA, Qpid Broker comes up with the special mode called <link linkend="Java-Broker-High-Availability-DurabilityGuarantee_CoalescingSync">coalescing-sync</link>,
    With this mode enabled, Qpid broker batches the concurrent transaction commits and syncs transaction data into Master disk in one go.
    As result, the HA performance only drops by 25-60% for durability <emphasis>NO_SYNC,NO_SYNC,ALL</emphasis> and by 10-90% for <emphasis>WRITE_NO_SYNC,WRITE_NO_SYNC,ALL</emphasis>.</para>
  </section>

</chapter>
