# Package org.apache.lucene.replicator

Files replication framework

See: Description

Interface Summary
ReplicationClient.ReplicationHandler: Handler for revisions obtained by the client.
ReplicationClient.SourceDirectoryFactory: Resolves a session and source into a `Directory` to use for copying the session files to.
Replicator: An interface for replicating files.
Revision: A revision comprises lists of files that come from different sources and need to be replicated together to e.g.

Class Summary
IndexAndTaxonomyReplicationHandler: A `ReplicationClient.ReplicationHandler` for replication of an index and taxonomy pair.
IndexAndTaxonomyRevision: A `Revision` of a single index and taxonomy index files which comprises the list of files from both indexes.
IndexAndTaxonomyRevision.SnapshotDirectoryTaxonomyWriter: A `DirectoryTaxonomyWriter` which sets the underlying `IndexWriter`'s `IndexDeletionPolicy` to `SnapshotDeletionPolicy`.
IndexInputInputStream: An `InputStream` which wraps an `IndexInput`.
IndexReplicationHandler: A `ReplicationClient.ReplicationHandler` for replication of an index.
IndexRevision: A `Revision` of a single index files which comprises the list of files that are part of the current `IndexCommit`.
LocalReplicator: A `Replicator` implementation for use by the side that publishes `Revision`s, as well for clients to `check for updates`.
PerSessionDirectoryFactory: A `ReplicationClient.SourceDirectoryFactory` which returns `FSDirectory` under a dedicated session directory.
ReplicationClient: A client which monitors and obtains new revisions from a `Replicator`.
RevisionFile: Describes a file in a `Revision`.
SessionToken: Token for a replication session, for guaranteeing that source replicated files will be kept safe until the replication completes.

Exception Summary
SessionExpiredException: Exception indicating that a revision update session was expired due to lack of activity.

## Package org.apache.lucene.replicator Description

# Files replication framework

The Replicator allows replicating files between a server and client(s). Producers publish revisions and consumers update to the latest revision available. ReplicationClient is a helper utility for performing the update operation. It can be invoked either manually or periodically by starting an update thread. HttpReplicator can be used to replicate revisions by consumers that reside on a different node than the producer.

The replication framework supports replicating any type of files, with built-in support for a single search index as well as an index and taxonomy pair. For a single index, the application should publish an IndexRevision and set IndexReplicationHandler on the client. For an index and taxonomy pair, the application should publish an IndexAndTaxonomyRevision and set IndexAndTaxonomyReplicationHandler on the client.

When the replication client detects that there is a newer revision available, it copies the files of the revision and then invokes the handler to complete the operation (e.g. copy the files to the index directory, fsync them, reopen an index reader etc.). By default, only files that do not exist in the handler's current revision files are copied, however this can be overridden by extending the client.

An example usage of the Replicator:

     // ++++++++++++++ SERVER SIDE ++++++++++++++ //
     IndexWriter publishWriter; // the writer used for indexing
     Replicator replicator = new LocalReplicator();
     replicator.publish(new IndexRevision(publishWriter));

     // ++++++++++++++ CLIENT SIDE ++++++++++++++ //
     // either LocalReplictor, or HttpReplicator if client and server are on different nodes
     Replicator replicator;

     // callback invoked after handler finished handling the revision and e.g. can reopen the reader.
     Callable<Boolean> callback = null; // can also be null if no callback is needed
     ReplicationHandler handler = new IndexReplicationHandler(indexDir, callback);
     SourceDirectoryFactory factory = new PerSessionDirectoryFactory(workDir);
     ReplicationClient client = new ReplicationClient(replicator, handler, factory);

     // invoke client manually
     client.updateNow();

     // or, periodically
     client.startUpdateThread(100); // check for update every 100 milliseconds