Change-Id: I35292f9f6fb5ebc591308fdd2d069203413e189d
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: Iba44e71b6441a0e39122ca8666b51989e605f25f
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: I5e6994844405f7839ad3b3439f98bcadb59d329b
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: I629990189083bab4737938ad712080fba7917582
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: I67ee2912ef54462cf860dc4ec0a6334e9c619384
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: Ic8f49336ba96c8dcf4bab2f74c0f1efc1ab55131
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: I4839877e1a6fa7782f37423213af8d579727a494
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: I39f4a23cf284505395d511dfedf02b7f5608df95
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: I4dec8eba7e35858aef65fcc10f91fad3fe5b52b9
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: I574a05200471c431b3a02ac6ff208dc6aa90f539
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

Change-Id: I6018ce0cd3b7c8137e137848fe1f04551b257538
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

jgit.storage.dht is a storage provider implementation for JGit that
permits storing the Git repository in a distributed hashtable, NoSQL
system, or other database.  The actual underlying storage system is
undefined, and can be plugged in by implementing 7 small interfaces:

  *  Database
  *  RepositoryIndexTable
  *  RepositoryTable
  *  RefTable
  *  ChunkTable
  *  ObjectIndexTable
  *  WriteBuffer

The storage provider interface tries to assume very little about the
underlying storage system, and requires only three key features:

  *  key -> value lookup (a hashtable is suitable)
  *  atomic updates on single rows
  *  asynchronous operations (Java's ExecutorService is easy to use)

Most NoSQL database products offer all 3 of these features in their
clients, and so does any decent network based cache system like the
open source memcache product.  Relying only on key equality for data
retrevial makes it simple for the storage engine to distribute across
multiple machines.  Traditional SQL systems could also be used with a
JDBC based spi implementation.

Before submitting this change I have implemented six storage systems
for the spi layer:

  * Apache HBase[1]
  * Apache Cassandra[2]
  * Google Bigtable[3]
  * an in-memory implementation for unit testing
  * a JDBC implementation for SQL
  * a generic cache provider that can ride on top of memcache

All six systems came in with an spi layer around 1000 lines of code to
implement the above 7 interfaces.  This is a huge reduction in size
compared to prior attempts to implement a new JGit storage layer.  As
this package shows, a complete JGit storage implementation is more
than 17,000 lines of fairly complex code.

A simple cache is provided in storage.dht.spi.cache.  Implementers can
use CacheDatabase to wrap any other type of Database and perform fast
reads against a network based cache service, such as the open source
memcached[4].  An implementation of CacheService must be provided to
glue this spi onto the network cache.

[1] https://github.com/spearce/jgit_hbase
[2] https://github.com/spearce/jgit_cassandra
[3] http://labs.google.com/papers/bigtable.html
[4] http://memcached.org/

Change-Id: I0aa4072781f5ccc019ca421c036adff2c40c4295
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>