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<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} 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>

*/
package org.apache.hadoop.hbase.coprocessor;
