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 <h1 title="Package org.apache.hadoop.hbase.coprocessor" class="title">Package org.apache.hadoop.hbase.coprocessor</h1>
 </div>
 <hr>
 <div class="package-signature">package <span class="element-name">org.apache.hadoop.hbase.coprocessor</span></div>
 <section class="package-description" id="package-description">
 <div class="block"><h2>Table of Contents</h2>
 <ul>
 <li><a href="#overview">Overview</a></li>
 <li><a href="#coprocessor">Coprocessor</a></li>
 <li><a href="#regionobserver">RegionObserver</a></li>
 <li><a href="#commandtarget">Endpoint</a></li>
 <li><a href="#load">Coprocessor loading</a></li>
 </ul>

 <h2><a name="overview">Overview</a></h2>
 Coprocessors are code that runs in-process on each region server. Regions
 contain references to the coprocessor implementation classes associated
 with them. Coprocessor classes can be loaded either from local
 jars on the region server's classpath or via the HDFS classloader.
 <p>
 Multiple types of coprocessors are provided to provide sufficient flexibility
 for potential use cases. Right now there are:
 <p>
 <ul>
 <li>Coprocessor: provides region lifecycle management hooks, e.g., region
 open/close/split/flush/compact operations.</li>
 <li>RegionObserver: provides hook for monitor table operations from
 client side, such as table get/put/scan/delete, etc.</li>
 <li>Endpoint: provides on demand triggers for any arbitrary function
 executed at a region. One use case is column aggregation at region
 server. </li>
 </ul>

 <h2><a name="coprocessor">Coprocessor</a></h2>
 A coprocessor is required to
 implement <code>Coprocessor</code> interface so that coprocessor framework
 can manage it internally.
 <p>
 Another design goal of this interface is to provide simple features for
 making coprocessors useful, while exposing no more internal state or
 control actions of the region server than necessary and not exposing them
 directly.
 <p>
 Over the lifecycle of a region, the methods of this interface are invoked
 when the corresponding events happen. The master transitions regions
 through the following states:
 <p>
 &nbsp;&nbsp;&nbsp;
 unassigned -&gt; pendingOpen -&gt; open -&gt; pendingClose -7gt; closed.
 <p>
 Coprocessors have opportunity to intercept and handle events in
 pendingOpen, open, and pendingClose states.
 <p>

 <h3>PendingOpen</h3>
 <p>
 The region server is opening a region to bring it online. Coprocessors
 can piggyback or fail this process.
 <p>
 <ul>
   <li>preOpen, postOpen: Called before and after the region is reported as
  online to the master.</li>
 </ul>
 <p>
 <h3>Open</h3>
 The region is open on the region server and is processing both client
 requests (get, put, scan, etc.) and administrative actions (flush, compact,
 split, etc.). Coprocessors can piggyback administrative actions via:
 <p>
 <ul>
   <li>preFlush, postFlush: Called before and after the memstore is flushed
   into a new store file.</li>
   <li>preCompact, postCompact: Called before and after compaction.</li>
   <li>preSplit, postSplit: Called after the region is split.</li>
 </ul>
 <p>
 <h3>PendingClose</h3>
 The region server is closing the region. This can happen as part of normal
 operations or may happen when the region server is aborting due to fatal
 conditions such as OOME, health check failure, or fatal filesystem
 problems. Coprocessors can piggyback this event. If the server is aborting
 an indication to this effect will be passed as an argument.
 <p>
 <ul>
   <li>preClose and postClose: Called before and after the region is
   reported as closed to the master.</li>
 </ul>
 <p>

 <h2><a name="regionobserver">RegionObserver</a></h2>
 If the coprocessor implements the <code>RegionObserver</code> interface it can
 observe and mediate client actions on the region:
 <p>
 <ul>
   <li>preGet, postGet: Called before and after a client makes a Get
   request.</li>
   <li>preExists, postExists: Called before and after the client tests
   for existence using a Get.</li>
   <li>prePut and postPut: Called before and after the client stores a value.
   </li>
   <li>preDelete and postDelete: Called before and after the client
   deletes a value.</li>
   <li>preScannerOpen postScannerOpen: Called before and after the client
   opens a new scanner.</li>
   <li>preScannerNext, postScannerNext: Called before and after the client
   asks for the next row on a scanner.</li>
   <li>preScannerClose, postScannerClose: Called before and after the client
   closes a scanner.</li>
   <li>preCheckAndPut, postCheckAndPut: Called before and after the client
   calls checkAndPut().</li>
   <li>preCheckAndDelete, postCheckAndDelete: Called before and after the client
   calls checkAndDelete().</li>
 </ul>
 <p>
 Here's an example of what a simple RegionObserver might look like. This
 example shows how to implement access control for HBase. This
 coprocessor checks user information for a given client request, e.g.,
 Get/Put/Delete/Scan by injecting code at certain
 <code>RegionObserver</code>
 preXXX hooks. If the user is not allowed to access the resource, a
 CoprocessorException will be thrown. And the client request will be
 denied by receiving this exception.
 <div style="background-color: #cccccc; padding: 2px">
 <blockquote><pre>
 package org.apache.hadoop.hbase.coprocessor;

 import org.apache.hadoop.hbase.client.Get;

 // Sample access-control coprocessor. It utilizes RegionObserver
 // and intercept preXXX() method to check user privilege for the given table
 // and column family.
 public class AccessControlCoprocessor extends BaseRegionObserverCoprocessor {
   // @Override
   public Get preGet(CoprocessorEnvironment e, Get get)
       throws CoprocessorException {

     // check permissions..
     if (access_not_allowed)  {
       throw new AccessDeniedException(&quot;User is not allowed to access.&quot;);
     }
     return get;
   }

   // override prePut(), preDelete(), etc.
 }
 </pre></blockquote>
 </div>

 <h2><a name="commandtarget">Endpoint</a></h2>
 <code>Coprocessor</code> and <code>RegionObserver</code> provide certain hooks
 for injecting user code running at each region. The user code will be triggered
 by existing <code>HTable</code> and <code>HBaseAdmin</code> operations at
 the certain hook points.
 <p>
 Coprocessor Endpoints allow you to define your own dynamic RPC protocol to communicate
 between clients and region servers, i.e., you can create a new method, specifying custom
 request parameters and return types.  RPC methods exposed by coprocessor Endpoints can be
 triggered by calling client side dynamic RPC functions -- <code>HTable.coprocessorService(...)
 </code>.
 <p>
 To implement an Endpoint, you need to:
 <ul>
  <li>Define a protocol buffer Service and supporting Message types for the RPC methods.
  See the
  <a href="https://developers.google.com/protocol-buffers/docs/proto#services">protocol buffer guide</a>
  for more details on defining services.</li>
  <li>Generate the Service and Message code using the protoc compiler</li>
  <li>Implement the generated Service interface and override get*Service() method in
  relevant Coprocessor to return a reference to the Endpoint's protocol buffer Service instance.
 </ul>
 <p>
 For a more detailed discussion of how to implement a coprocessor Endpoint, along with some sample
 code, see the <code>org.apache.hadoop.hbase.client.coprocessor</code> package documentation.
 </p>

 <h2><a name="load">Coprocessor loading</a></h2>
 A customized coprocessor can be loaded by two different ways, by configuration,
 or by <code>TableDescriptor</code> for a newly created table.
 <p>
 (Currently we don't really have an on demand coprocessor loading mechanism for
 opened regions.)
 <h3>Load from configuration</h3>
 Whenever a region is opened, it will read coprocessor class names from
 <code>hbase.coprocessor.region.classes</code> from <code>Configuration</code>.
 Coprocessor framework will automatically load the configured classes as
 default coprocessors. The classes must be included in the classpath already.

 <p>
 <div style="background-color: #cccccc; padding: 2px">
 <blockquote><pre>
   &lt;property&gt;
     &lt;name&gt;hbase.coprocessor.region.classes&lt;/name&gt;
     &lt;value&gt;org.apache.hadoop.hbase.coprocessor.AccessControlCoprocessor, org.apache.hadoop.hbase.coprocessor.ColumnAggregationProtocol&lt;/value&gt;
     &lt;description&gt;A comma-separated list of Coprocessors that are loaded by
     default. For any override coprocessor method from RegionObservor or
     Coprocessor, these classes' implementation will be called
     in order. After implement your own
     Coprocessor, just put it in HBase's classpath and add the fully
     qualified class name here.
     &lt;/description&gt;
   &lt;/property&gt;
 </pre></blockquote>
 </div>
 <p>
 The first defined coprocessor will be assigned
 <code>Coprocessor.Priority.SYSTEM</code> as priority. And each following
 coprocessor's priority will be incremented by one. Coprocessors are executed
 in order according to the natural ordering of the int.

 <h3>Load from table attribute</h3>
 Coprocessor classes can also be configured at table attribute. The
 attribute key must start with "Coprocessor" and values of the form is
 &lt;path&gt;:&lt;class&gt;:&lt;priority&gt;, so that the framework can
 recognize and load it.
 <p>
 <div style="background-color: #cccccc; padding: 2px">
 <blockquote><pre>
 &#39;COPROCESSOR$1&#39; =&gt; &#39;hdfs://localhost:8020/hbase/coprocessors/test.jar:Test:1000&#39;
 &#39;COPROCESSOR$2&#39; =&gt; &#39;/hbase/coprocessors/test2.jar:AnotherTest:1001&#39;
 </pre></blockquote>
 </div>
 <p>
 &lt;path&gt; must point to a jar, can be on any filesystem supported by the
 Hadoop <code>FileSystem</code> object.
 <p>
 &lt;class&gt; is the coprocessor implementation class. A jar can contain
 more than one coprocessor implementation, but only one can be specified
 at a time in each table attribute.
 <p>
 &lt;priority&gt; is an integer. Coprocessors are executed in order according
 to the natural ordering of the int. Coprocessors can optionally abort
 actions. So typically one would want to put authoritative CPs (security
 policy implementations, perhaps) ahead of observers.
 <p>
 <div style="background-color: #cccccc; padding: 2px">
 <blockquote><pre>
   Path path = new Path(fs.getUri() + Path.SEPARATOR +
     "TestClassloading.jar");

   // create a table that references the jar
   TableDescriptor htd = TableDescriptorBuilder
                         .newBuilder(TableName.valueOf(getClass().getTableName()))
                         .setColumnFamily(ColumnFamilyDescriptorBuilder.of("test"))
                         .setValue(Bytes.toBytes("Coprocessor$1", path.toString()+
                           ":" + classFullName +
                           ":" + Coprocessor.Priority.USER))
                         .build();
   Admin admin = connection.getAdmin();
   admin.createTable(htd);
 </pre></blockquote>
 <h3>Chain of RegionObservers</h3>
 As described above, multiple coprocessors can be loaded at one region at the
 same time. In case of RegionObserver, you can have more than one
 RegionObservers register to one same hook point, i.e, preGet(), etc.
 When a region reach the
 hook point, the framework will invoke each registered RegionObserver by the
 order of assigned priority.
 </div></div>
 </section>
 <section class="summary">
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 <li>
 <div id="related-package-summary">
 <div class="caption"><span>Related Packages</span></div>
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 <div class="col-first odd-row-color"><a href="example/package-summary.html">org.apache.hadoop.hbase.coprocessor.example</a></div>
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 <div class="table-header col-first">Class</div>
 <div class="table-header col-last">Description</div>
 <div class="col-first even-row-color class-summary class-summary-tab2"><a href="BaseEnvironment.html" title="class in org.apache.hadoop.hbase.coprocessor">BaseEnvironment</a>&lt;C extends <a href="../Coprocessor.html" title="interface in org.apache.hadoop.hbase">Coprocessor</a>&gt;</div>
 <div class="col-last even-row-color class-summary class-summary-tab2">
 <div class="block">Encapsulation of the environment of each coprocessor</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="BulkLoadObserver.html" title="interface in org.apache.hadoop.hbase.coprocessor">BulkLoadObserver</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">
 <div class="block">Coprocessors implement this interface to observe and mediate bulk load operations.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab2"><a href="ColumnInterpreter.html" title="class in org.apache.hadoop.hbase.coprocessor">ColumnInterpreter</a>&lt;T,<wbr>S,<wbr>P extends org.apache.hbase.thirdparty.com.google.protobuf.Message,<wbr>Q extends org.apache.hbase.thirdparty.com.google.protobuf.Message,<wbr>R extends org.apache.hbase.thirdparty.com.google.protobuf.Message&gt;</div>
 <div class="col-last even-row-color class-summary class-summary-tab2">
 <div class="block">Defines how value for specific column is interpreted and provides utility methods like compare,
  add, multiply etc for them.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab5"><a href="CoprocessorException.html" title="class in org.apache.hadoop.hbase.coprocessor">CoprocessorException</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab5">
 <div class="block">Thrown if a coprocessor encounters any exception.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab2"><a href="CoprocessorHost.html" title="class in org.apache.hadoop.hbase.coprocessor">CoprocessorHost</a>&lt;C extends <a href="../Coprocessor.html" title="interface in org.apache.hadoop.hbase">Coprocessor</a>,<wbr>E extends <a href="../CoprocessorEnvironment.html" title="interface in org.apache.hadoop.hbase">CoprocessorEnvironment</a>&lt;C&gt;&gt;</div>
 <div class="col-last even-row-color class-summary class-summary-tab2">
 <div class="block">Provides the common setup framework and runtime services for coprocessor invocation from HBase
  services.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab2"><a href="CoprocessorHost.EnvironmentPriorityComparator.html" title="class in org.apache.hadoop.hbase.coprocessor">CoprocessorHost.EnvironmentPriorityComparator</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab2">
 <div class="block">Environment priority comparator.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab1"><a href="CoprocessorHost.ObserverGetter.html" title="interface in org.apache.hadoop.hbase.coprocessor">CoprocessorHost.ObserverGetter</a>&lt;C,<wbr>O&gt;</div>
 <div class="col-last even-row-color class-summary class-summary-tab1">
 <div class="block">Implementations defined function to get an observer of type <code>O</code> from a coprocessor of
  type <code>C</code>.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="CoprocessorService.html" title="interface in org.apache.hadoop.hbase.coprocessor">CoprocessorService</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">Deprecated.
 <div class="deprecation-comment">Since 2.0.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab7"><a href="CoreCoprocessor.html" title="annotation interface in org.apache.hadoop.hbase.coprocessor">CoreCoprocessor</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab7">
 <div class="block">Marker annotation that denotes Coprocessors that are core to HBase.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="EndpointObserver.html" title="interface in org.apache.hadoop.hbase.coprocessor">EndpointObserver</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">
 <div class="block">Coprocessors implement this interface to observe and mediate endpoint invocations on a region.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab1"><a href="HasMasterServices.html" title="interface in org.apache.hadoop.hbase.coprocessor">HasMasterServices</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab1">Deprecated.
 <div class="deprecation-comment">Since 2.0.0 to be removed in 4.0.0.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="HasRegionServerServices.html" title="interface in org.apache.hadoop.hbase.coprocessor">HasRegionServerServices</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">Deprecated.
 <div class="deprecation-comment">Since 2.0.0 to be removed in 4.0.0.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab1"><a href="MasterCoprocessor.html" title="interface in org.apache.hadoop.hbase.coprocessor">MasterCoprocessor</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab1">&nbsp;</div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="MasterCoprocessorEnvironment.html" title="interface in org.apache.hadoop.hbase.coprocessor">MasterCoprocessorEnvironment</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">&nbsp;</div>
 <div class="col-first even-row-color class-summary class-summary-tab1"><a href="MasterObserver.html" title="interface in org.apache.hadoop.hbase.coprocessor">MasterObserver</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab1">
 <div class="block">Defines coprocessor hooks for interacting with operations on the
  <a href="../master/HMaster.html" title="class in org.apache.hadoop.hbase.master"><code>HMaster</code></a> process.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab2"><a href="MetaTableMetrics.html" title="class in org.apache.hadoop.hbase.coprocessor">MetaTableMetrics</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab2">
 <div class="block">A coprocessor that collects metrics from meta table.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab3"><a href="MetaTableMetrics.MetaTableOps.html" title="enum class in org.apache.hadoop.hbase.coprocessor">MetaTableMetrics.MetaTableOps</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab3">&nbsp;</div>
 <div class="col-first odd-row-color class-summary class-summary-tab2"><a href="MetricsCoprocessor.html" title="class in org.apache.hadoop.hbase.coprocessor">MetricsCoprocessor</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab2">
 <div class="block">Utility class for tracking metrics for various types of coprocessors.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab2"><a href="MultiRowMutationEndpoint.html" title="class in org.apache.hadoop.hbase.coprocessor">MultiRowMutationEndpoint</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab2">
 <div class="block">This class implements atomic multi row transactions using
  <a href="../regionserver/HRegion.html#mutateRowsWithLocks(java.util.Collection,java.util.Collection,long,long)"><code>HRegion.mutateRowsWithLocks(Collection, Collection, long, long)</code></a> and Coprocessor
  endpoints.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="ObserverContext.html" title="interface in org.apache.hadoop.hbase.coprocessor">ObserverContext</a>&lt;E extends <a href="../CoprocessorEnvironment.html" title="interface in org.apache.hadoop.hbase">CoprocessorEnvironment</a>&gt;</div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">
 <div class="block">Carries the execution state for a given invocation of an Observer coprocessor
  (<a href="RegionObserver.html" title="interface in org.apache.hadoop.hbase.coprocessor"><code>RegionObserver</code></a>, <a href="MasterObserver.html" title="interface in org.apache.hadoop.hbase.coprocessor"><code>MasterObserver</code></a>, or <a href="WALObserver.html" title="interface in org.apache.hadoop.hbase.coprocessor"><code>WALObserver</code></a>) method.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab2"><a href="ObserverContextImpl.html" title="class in org.apache.hadoop.hbase.coprocessor">ObserverContextImpl</a>&lt;E extends <a href="../CoprocessorEnvironment.html" title="interface in org.apache.hadoop.hbase">CoprocessorEnvironment</a>&gt;</div>
 <div class="col-last even-row-color class-summary class-summary-tab2">
 <div class="block">This is the only implementation of <a href="ObserverContext.html" title="interface in org.apache.hadoop.hbase.coprocessor"><code>ObserverContext</code></a>, which serves as the interface for
  third-party Coprocessor developers.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab2"><a href="ReadOnlyConfiguration.html" title="class in org.apache.hadoop.hbase.coprocessor">ReadOnlyConfiguration</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab2">
 <div class="block">Wraps a Configuration to make it read-only.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab1"><a href="RegionCoprocessor.html" title="interface in org.apache.hadoop.hbase.coprocessor">RegionCoprocessor</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab1">&nbsp;</div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="RegionCoprocessorEnvironment.html" title="interface in org.apache.hadoop.hbase.coprocessor">RegionCoprocessorEnvironment</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">&nbsp;</div>
 <div class="col-first even-row-color class-summary class-summary-tab1"><a href="RegionObserver.html" title="interface in org.apache.hadoop.hbase.coprocessor">RegionObserver</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab1">
 <div class="block">Coprocessors implement this interface to observe and mediate client actions on the region.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab3"><a href="RegionObserver.MutationType.html" title="enum class in org.apache.hadoop.hbase.coprocessor">RegionObserver.MutationType</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab3">
 <div class="block">Mutation type for postMutationBeforeWAL hook</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab1"><a href="RegionServerCoprocessor.html" title="interface in org.apache.hadoop.hbase.coprocessor">RegionServerCoprocessor</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab1">&nbsp;</div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="RegionServerCoprocessorEnvironment.html" title="interface in org.apache.hadoop.hbase.coprocessor">RegionServerCoprocessorEnvironment</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">&nbsp;</div>
 <div class="col-first even-row-color class-summary class-summary-tab1"><a href="RegionServerObserver.html" title="interface in org.apache.hadoop.hbase.coprocessor">RegionServerObserver</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab1">
 <div class="block">Defines coprocessor hooks for interacting with operations on the
  <a href="../regionserver/HRegionServer.html" title="class in org.apache.hadoop.hbase.regionserver"><code>HRegionServer</code></a> process.</div>
 </div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="WALCoprocessor.html" title="interface in org.apache.hadoop.hbase.coprocessor">WALCoprocessor</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">
 <div class="block">WALCoprocessor don't support loading services using <a href="../Coprocessor.html#getServices()"><code>Coprocessor.getServices()</code></a>.</div>
 </div>
 <div class="col-first even-row-color class-summary class-summary-tab1"><a href="WALCoprocessorEnvironment.html" title="interface in org.apache.hadoop.hbase.coprocessor">WALCoprocessorEnvironment</a></div>
 <div class="col-last even-row-color class-summary class-summary-tab1">&nbsp;</div>
 <div class="col-first odd-row-color class-summary class-summary-tab1"><a href="WALObserver.html" title="interface in org.apache.hadoop.hbase.coprocessor">WALObserver</a></div>
 <div class="col-last odd-row-color class-summary class-summary-tab1">
 <div class="block">It's provided to have a way for coprocessors to observe, rewrite, or skip WALEdits as they are
  being written to the WAL.</div>
 </div>
 </div>
 </div>
 </div>
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