Since we replaced GitIndex by DirCache JGit didn't fire
IndexChangedEvents anymore. For EGit this still worked with a high
latency since its RepositoryChangeScanner which is scheduled to
run each 10 seconds fires the event in case the index changes.
This scanner is meant to detect index changes induced by a different
process e.g. by calling "git add" from native git.

When the index is changed from within the same process we should fire
the event synchronously. Compare the index checksum on write to index
checksum when index was read earlier to determine if index really
changed. Use IndexChangedListener interface to keep DirCache decoupled
from Repository.

Change-Id: Id4311f7a7859ffe8738863b3d86c83c8b5f513af
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

This is required since RefList.put returns a new RefList.

Change-Id: I717d75d6f6154a6e0dc7cde3b72b0a59c68d955c
Signed-off-by: Kevin Sawicki <kevin@github.com>

Instead of fixing the prefetch queue and recent chunk queue as
different sizes, allow these to share the same limit but be scaled
based on the work being performed.

During walks about 20% of the space will be given to the prefetcher,
and the other 80% will be used by the recent chunks cache. This
should improve cases where there is bad locality between chunks.

During writing of a pack stream, 90-100% of the space should be
made available to the prefetcher, as the prefetch plan is usually
very accurate about the order chunks will be needed in.

Change-Id: I1ca7acb4518e66eb9d4138fb753df38e7254704d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

A linear search is somewhat acceptable for only 4 recent chunks, but
a HashMap based lookup would be better. The table will have 16 slots
by default and given the hashCode() of ChunkKey is derived from the
SHA-1 of the chunk, each chunk will fall into its own bucket within
the table and thus evaluate only 1 entry during lookup instead of 4.

Some users may also want to devote more memory to the recent chunks,
in which case expanding this list to a longer length will help to
reduce chunk faults, but would increase search time. Using a HashMap
will help this code to scale to larger sizes better.

Change-Id: Ia41b7a1cc69ad27b85749e3b74cbf8d0aa338044
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

The RecentChunks cache assumes there is always at least one recent
chunk in the maxSize that it receives from the DhtReaderOptions.
Ensure that is true by requiring the size to be at least 1.

Running with 0 recent chunk cache is very a bad idea, often
during commit walking the parents of a commit will be found
on the same chunk as the commit that was just accessed. In
these cases its a good idea to keep that last chunk around
so the parents can be quickly accessed.

Change-Id: I33b65286e8a4cbf6ef4ced28c547837f173e065d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

The Prefetcher may have loaded a chunk that is a fragment, if the
DhtReader is scanning the Prefetcher's chunks for a particular
object fragment chunks will be missing the index and NPE during
the findOffset() call into the index itself.

Change-Id: Ie2823724c289f745655076c5209acec32361a1ea
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Originally I put the first two digits of the object SHA-1 into the
start of a row key to try and spread the load of objects around a DHT
service. Unfortunately this tends to not work as well as I had hoped.

Servers reading a repository need to contact every node in a DHT
cluster if the cluster tries to evenly distribute the object rows.
This is a lot of connections, especially if the cluster has many
backend storage servers.  If the library has an open connection
limit (possibly due to JVM file descriptor limitations) it may need
to open and close a lot of connections to access a repository,
rather than being able to reuse the same connection to a handful
of backend servers.  This results in a lot of connection thrashing
for some DHT type databases, and is inefficient.

Some DHTs are able to operate even if part of the database space
is currently unavailable.  For example, a DHT service might assign
some section of the key space to a node, and then fail that section
over to another node when the primary is noticed as being offline.
During that failover period that section of the key space is not
available, but other sections hosted by other backends are still
ready for service. Spreading keys all over the cluster makes it
likely that any single backend being temporarily down means the
entire cluster is down, rather than only some.

This is a massive schema change, but it should improve relability
and performance for any DHT system.

Change-Id: I6b65bfb4c14b6f7bd323c2bd0638b49d429245be
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

The first step to deleting a repository from the DHT storage is to
remove the name binding in the RepositoryIndexTable, making the
repository unavailable for lookup.

Change-Id: I469bf92f4bf2f555a15949569b21937c14cb142b
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

There is a data corruption issue with the 'running' list if a
background thread schedules something onto the buffer while the
application thread is also using it.

Change-Id: I5ba78b98b6632965d677a9c8f209f0cf8320cc3d
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

RefData now uses a sequence number as part of the field, ensuring
that updates always increase the sequence number by one whenever
a reference is modified.

Attaching a sequence number to RefData will help with storing
reference log entries during updates. As the sequence number should
be unique within the reference name space, log entries can be keyed
by the sequence number and remain unique.  Making this work over
reference delete-create cycles will require an additional RefTable
API to return the oldest sequence number previously used in the
reference log to seed the recreated reference.

Change-Id: I11cfff2a96ef962e57f29925a3eef41bdbf9f9bb
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

The standard Google distribution of Protocol Buffers in Java is better
maintained than TinyProtobuf, and should be faster for most uses.  It
does use slightly more memory due to many of our key types being
stored as strings in protobuf messages, but this is probably worth the
small hit to memory in exchange for better maintained code that is
easier to reuse in other applications.

Exposing all of our data members to the underlying implementation
makes it easier to develop reporting and data mining tools, or to
expand out a nested structure like RefData into a flat format in a SQL
database table.

Since the C++ `protoc` tool is necessary to convert the protobuf
script into Java code, the generated files are committed as part of
the source repository to make it easier for developers who do not have
this tool installed to still build the overall JGit package and make
use of it.  Reviewers will need to be careful to ensure that any edits
made to a *.proto file come in a commit that also updates the
generated code to match.

CQ: 5135
Change-Id: I53e11e82c186b9cf0d7b368e0276519e6a0b2893
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.com>

Performance testing has indicated the per-process ChunkCache isn't
very effective for the DHT storage implementation.  If a server is
using the DHT storage backend, it is most likely part of a larger
cluster where requests are distributed in a round-robin fashion
between the member servers.

In such a scenario there is insufficient data locality between
requests to get a good hit ratio on the per-process ChunkCache.  A low
hit ratio means the cache is actually hurting performance by eating up
memory that could otherwise be used for transient request data, and
increasing pressure on the GC when it needs to find free space.

Remove all of the ChunkCache code.  Installations that want to cache
(to reduce database usage) should wrap their Database with a
CacheDatabase and use a network based CacheServer.

I left the ChunkCache in the original DHT storage commit because I
wanted to document in the history of the project that its probably
worth *not* having, but leave open a door for someone to revert this
change if they find otherwise at a later date.

Change-Id: I364d0725c46c5a19f7443642a40c89ba4d3fdd29
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Chris Aniszczyk <caniszczyk@gmail.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>