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[[protobuf]]
= Protobuf in HBase
:doctype: book
:numbered:
:toc: left
:icons: font
:experimental:


== Protobuf
HBase uses Google's link:https://developers.google.com/protocol-buffers/[protobufs] wherever
it persists metadata -- in the tail of hfiles or Cells written by
HBase into the system hbase:meta table or when HBase writes znodes
to zookeeper, etc. -- and when it passes objects over the wire making
xref:hbase.rpc[RPCs]. HBase uses protobufs to describe the RPC
Interfaces (Services) we expose to clients, for example the `Admin` and `Client`
Interfaces that the RegionServer fields,
or specifying the arbitrary extensions added by developers via our
xref:cp[Coprocessor Endpoint] mechanism.

In this chapter we go into detail for  developers who are looking to
understand better how it all works. This chapter is of particular
use to those who would amend or extend HBase functionality.

With protobuf, you describe serializations and services in a `.protos` file.
You then feed these descriptors to a protobuf tool, the `protoc` binary,
to generate classes that can marshall and unmarshall the described serializations
and field the specified Services.

See the `README.txt` in the HBase sub-modules for details on how
to run the class generation on a per-module basis;
e.g. see `hbase-protocol/README.txt` for how to generate protobuf classes
in the hbase-protocol module.

In HBase, `.proto` files are either in the `hbase-protocol` module; a module
dedicated to hosting the common proto files and the protoc generated classes
that HBase uses internally serializing metadata. For extensions to hbase
such as REST or Coprocessor Endpoints that need their own descriptors; their
protos are located inside the function's hosting module: e.g. `hbase-rest`
is home to the REST proto files and the `hbase-rsgroup` table grouping
Coprocessor Endpoint has all protos that have to do with table grouping.

Protos are hosted by the module that makes use of them. While
this makes it so generation of protobuf classes is distributed, done
per module, we do it this way so modules encapsulate all to do with
the functionality they bring to hbase.

Extensions whether REST or Coprocessor Endpoints will make use
of core HBase protos found back in the hbase-protocol module. They'll
use these core protos when they want to serialize a Cell or a Put or
refer to a particular node via ServerName, etc., as part of providing the
CPEP Service. Going forward, after the release of hbase-2.0.0, this
practice needs to whither. We'll explain why in the later
xref:shaded.protobuf[hbase-2.0.0] section.

[[shaded.protobuf]]
=== hbase-2.0.0 and the shading of protobufs (HBASE-15638)

As of hbase-2.0.0, our protobuf usage gets a little more involved. HBase
core protobuf references are offset so as to refer to a private,
bundled protobuf. Core stops referring to protobuf
classes at com.google.protobuf.* and instead references protobuf at
the HBase-specific offset
org.apache.hadoop.hbase.shaded.com.google.protobuf.*.  We do this indirection
so hbase core can evolve its protobuf version independent of whatever our
dependencies rely on. For instance, HDFS serializes using protobuf.
HDFS is on our CLASSPATH. Without the above described indirection, our
protobuf versions would have to align. HBase would be stuck
on the HDFS protobuf version until HDFS decided to upgrade. HBase
and HDFS versions would be tied.

We had to move on from protobuf-2.5.0 because we need facilities
added in protobuf-3.1.0; in particular being able to save on
copies and avoiding bringing protobufs onheap for
serialization/deserialization.

In hbase-2.0.0, we introduced a new module, `hbase-protocol-shaded`
inside which we contained all to do with protobuf and its subsequent
relocation/shading. This module is in essence a copy of much of the old
`hbase-protocol` but with an extra shading/relocation step.
Core was moved to depend on this new module.

That said, a complication arises around Coprocessor Endpoints (CPEPs).
CPEPs depend on public HBase APIs that reference protobuf classes at
`com.google.protobuf.*` explicitly. For example, in our Table Interface
we have the below as the means by which you obtain a CPEP Service
to make invocations against:

[source,java]
----
...
  <T extends com.google.protobuf.Service,R> Map<byte[],R> coprocessorService(
   Class<T> service, byte[] startKey, byte[] endKey,
     org.apache.hadoop.hbase.client.coprocessor.Batch.Call<T,R> callable)
  throws com.google.protobuf.ServiceException, Throwable
----

Existing CPEPs will have made reference to core HBase protobufs
specifying ServerNames or carrying Mutations.
So as to continue being able to service CPEPs and their references
to `com.google.protobuf.*` across the upgrade to hbase-2.0.0 and beyond,
HBase needs to be able to deal with both
`com.google.protobuf.*` references and its internal offset
`org.apache.hadoop.hbase.shaded.com.google.protobuf.*` protobufs.

The `hbase-protocol-shaded` module hosts all
protobufs used by HBase core.

But for the vestigial CPEP references to the (non-shaded) content of
`hbase-protocol`, we keep around most of this  module going forward
just so it is available to CPEPs.  Retaining the most of `hbase-protocol`
makes for overlapping, 'duplicated' proto instances where some exist as
non-shaded/non-relocated here in their old module
location but also in the new location, shaded under
`hbase-protocol-shaded`. In other words, there is an instance
of the generated protobuf class
`org.apache.hadoop.hbase.protobuf.generated.ServerName`
in hbase-protocol and another generated instance that is the same in all
regards except its protobuf references are to the internal shaded
version at `org.apache.hadoop.hbase.shaded.protobuf.generated.ServerName`
(note the 'shaded' addition in the middle of the package name).

If you extend a proto in `hbase-protocol-shaded` for  internal use,
consider extending it also in
`hbase-protocol` (and regenerating).

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.
