HBASE-18592 [hbase-thirdparty] Doc on new hbase-thirdparty dependency for the refguide

Signed-off-by: Chia-Ping Tsai <chia7712@gmail.com>

src/main/asciidoc/_chapters/developer.adoc

@@ -2177,6 +2177,60 @@ However any substantive discussion (as with any off-list project-related discuss
 
 Misspellings and/or bad grammar is preferable to the disruption a JIRA comment edit causes: See the discussion at link:http://search-hadoop.com/?q=%5BReopened%5D+%28HBASE-451%29+Remove+HTableDescriptor+from+HRegionInfo&fc_project=HBase[Re:(HBASE-451) Remove HTableDescriptor from HRegionInfo]
 
+[[thirdparty]]
+=== The hbase-thirdparty dependency and shading/relocation
+
+A new project was created for the release of hbase-2.0.0. It was called
+`hbase-thirdparty`. This project exists only to provide the main hbase
+project with relocated -- or shaded -- versions of popular thirdparty
+libraries such as guava, netty, and protobuf. The mainline HBase project
+relies on the relocated versions of these libraries gotten from hbase-thirdparty
+rather than on finding these classes in their usual locations. We do this so
+we can specify whatever the version we wish. If we don't relocate, we must
+harmonize our version to match that which hadoop and/or spark uses.
+
+For developers, this means you need to be careful referring to classes from
+netty, guava, protobuf, gson, etc. (see the hbase-thirdparty pom.xml for what
+it provides). Devs must refer to the hbase-thirdparty provided classes. In
+practice, this is usually not an issue (though it can be a bit of a pain). You
+will have to hunt for the relocated version of your particular class. You'll
+find it by prepending the general relocation prefix of `org.apache.hadoop.hbase.shaded.`.
+For example if you are looking for `com.google.protobuf.Message`, the relocated
+version used by HBase internals can be found at
+`org.apache.hadoop.hbase.shaded.com.google.protobuf.Message`.
+
+For a few thirdparty libs, like protobuf (see the protobuf chapter in this book
+for the why), your IDE may give you both options -- the `com.google.protobuf.*`
+and the `org.apache.hadoop.hbase.shaded.com.google.protobuf.*` -- because both
+classes are on your CLASSPATH. Unless you are doing the particular juggling
+required in Coprocessor Endpoint development (again see above cited protobuf
+chapter), you'll want to use the shaded version, always.
+
+Of note, the relocation of netty is particular. The netty folks have put in
+place facility to aid relocation; it seems like shading netty is a popular project.
+One case of this requires the setting of a peculiar system property on the JVM
+so that classes out in the bundld shared library (.so) can be found in their
+relocated location. Here is the property that needs to be set:
+
+`-Dorg.apache.hadoop.hbase.shaded.io.netty.packagePrefix=org.apache.hadoop.hbase.shaded.`
+
+(Note that the trailing '.' is required). Starting hbase normally or when running
+test suites, the setting of this property is done for you. If you are doing something
+out of the ordinary, starting hbase from your own context, you'll need to provide
+this property on platforms that favor the bundled .so. See release notes on HBASE-18271
+for more. The complaint you see is something like the following:
+`Cause: java.lang.RuntimeException: Failed construction of Master: class org.apache.hadoop.hbase.master.HMasterorg.apache.hadoop.hbase.shaded.io.netty.channel.epoll.`
+
+The `hbase-thirdparty` project has groupid of `org.apache.hbase.thirdparty`.
+As of this writing, it provides three jars; one for netty with an artifactid of
+`hbase-thirdparty-netty`, one for protobuf at `hbase-thirdparty-protobuf` and then
+a jar for all else -- gson, guava -- at `hbase-thirdpaty-miscellaneous`.
+
+The hbase-thirdparty artifacts are a product produced by the Apache HBase
+project under the aegis of the HBase Project Management Committee. Releases
+are done via the usual voting project on the hbase dev mailing list. If issue
+in the hbase-thirdparty, use the hbase JIRA and mailing lists to post notice.
+
 [[hbase.archetypes.development]]
 === Development of HBase-related Maven archetypes