HBASE-23800 Add documentation about the CECPs changes (#1487)

Signed-off-by: stack <stack@apache.org>

src/main/asciidoc/_chapters/cp.adoc

@@ -178,11 +178,28 @@ average or summation for an entire table which spans hundreds of regions.
 
 In contrast to observer coprocessors, where your code is run transparently, endpoint
 coprocessors must be explicitly invoked using the
-link:https://hbase.apache.org/devapidocs/org/apache/hadoop/hbase/client/Table.html#coprocessorService-java.lang.Class-byte:A-byte:A-org.apache.hadoop.hbase.client.coprocessor.Batch.Call-[CoprocessorService()]
+link:https://hbase.apache.org/devapidocs/org/apache/hadoop/hbase/client/AsyncTable.html#coprocessorService-java.util.function.Function-org.apache.hadoop.hbase.client.ServiceCaller-byte:A-[CoprocessorService()]
 method available in
-link:https://hbase.apache.org/devapidocs/org/apache/hadoop/hbase/client/Table.html[Table]
-or
-link:https://hbase.apache.org/devapidocs/org/apache/hadoop/hbase/client/HTable.html[HTable].
+link:https://hbase.apache.org/devapidocs/org/apache/hadoop/hbase/client/AsyncTable.html[AsyncTable].
+
+[WARNING]
+.On using coprocessorService method with sync client
+====
+The coprocessorService method in link:https://hbase.apache.org/devapidocs/org/apache/hadoop/hbase/client/Table.html[Table]
+has been deprecated.
+
+In link:https://issues.apache.org/jira/browse/HBASE-21512[HBASE-21512]
+we reimplement the sync client based on the async client. The coprocessorService
+method defined in `Table` interface directly references a method from protobuf's
+`BlockingInterface`, which means we need to use a separate thread pool to execute
+the method so we avoid blocking the async client(We want to avoid blocking calls in
+our async implementation).
+
+Since coprocessor is an advanced feature, we believe it is OK for coprocessor users to
+instead switch over to use `AsyncTable`. There is a lightweight
+link:https://hbase.apache.org/devapidocs/org/apache/hadoop/hbase/client/Connection.html#toAsyncConnection--[toAsyncConnection]
+method to get an `AsyncConnection` from `Connection` if needed.
+====
 
 Starting with HBase 0.96, endpoint coprocessors are implemented using Google Protocol
 Buffers (protobuf). For more details on protobuf, see Google's
@@ -193,6 +210,12 @@ link:https://issues.apache.org/jira/browse/HBASE-5448[HBASE-5448]). To upgrade y
 HBase cluster from 0.94 or earlier to 0.96 or later, you need to reimplement your
 coprocessor.
 
+In HBase 2.x, we made use of a shaded version of protobuf 3.x, but kept the
+protobuf for coprocessors on 2.5.0. In HBase 3.0.0, we removed all dependencies on
+non-shaded protobuf so you need to reimplement your coprocessor to make use of the
+shaded protobuf version provided in hbase-thirdparty. Please see
+the <<protobuf,protobuf>> section for more details.
+
 Coprocessor Endpoints should make no use of HBase internals and
 only avail of public APIs; ideally a CPEP should depend on Interfaces
 and data structures only. This is not always possible but beware

src/main/asciidoc/_chapters/protobuf.adoc

@@ -148,3 +148,75 @@ consider extending it also in
 Going forward, we will provide a new module of common types for use
 by CPEPs that will have the same guarantees against change as does our
 public API. TODO.
+
+=== protobuf changes for hbase-3.0.0 (HBASE-23797)
+Since hadoop(start from 3.3.x) also shades protobuf and bumps the version to
+3.x, there is no reason for us to stay on protobuf 2.5.0 any more.
+
+In HBase 3.0.0, the hbase-protocol module has been purged, the CPEP
+implementation should use the protos in hbase-protocol-shaded module, and also
+make use of the shaded protobuf in hbase-thirdparty. In general, we will keep
+the protobuf version compatible for a whole major release, unless there are
+critical problems, for example, a critical CVE on protobuf.
+
+Add this dependency to your pom:
+[source,xml]
+----
+<dependency>
+  <groupId>org.apache.hbase.thirdparty</groupId>
+  <artifactId>hbase-shaded-protobuf</artifactId>
+  <!-- use the version that your target hbase cluster uses -->
+  <version>${hbase-thirdparty.version}</version>
+  <scope>provided</scope>
+</dependency>
+----
+
+And typically you also need to add this plugin to your pom to make your
+generated protobuf code also use the shaded and relocated protobuf version
+in hbase-thirdparty.
+[source,xml]
+----
+<plugin>
+  <groupId>com.google.code.maven-replacer-plugin</groupId>
+  <artifactId>replacer</artifactId>
+  <version>1.5.3</version>
+  <executions>
+    <execution>
+      <phase>process-sources</phase>
+      <goals>
+        <goal>replace</goal>
+      </goals>
+    </execution>
+  </executions>
+  <configuration>
+    <basedir>${basedir}/target/generated-sources/</basedir>
+      <includes>
+        <include>**/*.java</include>
+      </includes>
+      <!-- Ignore errors when missing files, because it means this build
+           was run with -Dprotoc.skip and there is no -Dreplacer.skip -->
+      <ignoreErrors>true</ignoreErrors>
+      <replacements>
+        <replacement>
+          <token>([^\.])com.google.protobuf</token>
+          <value>$1org.apache.hbase.thirdparty.com.google.protobuf</value>
+        </replacement>
+        <replacement>
+          <token>(public)(\W+static)?(\W+final)?(\W+class)</token>
+          <value>@javax.annotation.Generated("proto") $1$2$3$4</value>
+        </replacement>
+        <!-- replacer doesn't support anchoring or negative lookbehind -->
+        <replacement>
+          <token>(@javax.annotation.Generated\("proto"\) ){2}</token>
+          <value>$1</value>
+        </replacement>
+      </replacements>
+  </configuration>
+</plugin>
+----
+
+In hbase-examples module, we have some examples under the
+`org.apache.hadoop.hbase.coprocessor.example` package. You can see
+`BulkDeleteEndpoint` and `BulkDelete.proto` for more details, and you can also
+check the `pom.xml` of hbase-examples module to see how to make use of the above
+plugin.