share/doc/src/replication.rst - couchdb - Git at Google

 .. Licensed under the Apache License, Version 2.0 (the "License"); you may not
 .. use this file except in compliance with the License. You may obtain a copy of
 .. the License at
 ..
 ..   http://www.apache.org/licenses/LICENSE-2.0
 ..
 .. Unless required by applicable law or agreed to in writing, software
 .. distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 .. WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 .. License for the specific language governing permissions and limitations under
 .. the License.

 .. _replication:

 Replication
 ===========

 One of CouchDB's strengths is the ability to synchronize two copies of the same
 database. This enables users to distribute data across several nodes or
 datacenters, but also to move data more closely to clients.

 Replication involves a source and a destination database, which can be one the
 same or on different CouchDB instances. The aim of the replication is that at
 the end of the process, all active documents on the source database are also in
 the destination database and all documents that were deleted in the source
 databases are also deleted on the destination database (if they even existed).


 Triggering Replication
 ----------------------

 Replication is controlled through documents in the :ref:`replicator`, where
 each document describes one replication process (see
 :ref:`replication-settings`).

 A replication is triggered by storing a replication document in the replicator
 database. Its status can be inspected through the active tasks API (see
 :ref:`active-tasks` and :ref:`replication-status`). A replication can be
 stopped by deleting the document, or by updating it with its `cancel` property
 set to `true`.


 Replication Procedure
 ---------------------

 During replication, CouchDB will compare the source and the destination
 database to determine which documents differ between the source and the
 destination database. It does so by following the :ref:`changes` on the source
 and comparing the documents to the destination. Changes are submitted to the
 destination in batches where they can introduce conflicts. Documents that
 already exist on the destination in the same revision are not transferred. As
 the deletion of documents is represented by a new revision, a document deleted
 on the source will also be deleted on the target.

 A replication task will finish once it reaches the end of the changes feed. If
 its `continuous` property is set to true, it will wait for new changes to
 appear until the task is cancelled. Replication tasks also create checkpoint
 documents on the destination to ensure that a restarted task can continue from
 where it stopped, for example after it has crashed.

 When a replication task is initiated on the sending node, it is called *push*
 replication, if it is initiated by the receiving node, it is called *pull*
 replication.


 Master - Master replication
 ---------------------------

 One replication task will only transfer changes in one direction. To achieve
 master-master replication it is possible to set up two replication tasks in
 different directions. When a change is replication from database A to B by the
 first task, the second will discover that the new change on B already exists in
 A and will wait for further changes.


 Controlling which Documents to Replicate
 ----------------------------------------

 There are two ways for controlling which documents are replicated, and which
 are skipped. *Local* documents are never replicated (see :ref:`api-local`).

 Additionally, :ref:`filterfun` can be used in a replication documents (see
 :ref:`replication-settings`). The replication task will then evaluate
 the filter function for each document in the changes feed. The document will
 only be replicated if the filter returns `true`.


 Migrating Data to Clients
 -------------------------

 Replication can be especially useful for bringing data closer to clients.
 `PouchDB <http://pouchdb.com/>`_ implements the replication algorithm of CouchDB
 in JavaScript, making it possible to make data from a CouchDB database
 available in an offline browser application, and synchronize changes back to
 CouchDB.
	.. Licensed under the Apache License, Version 2.0 (the "License"); you may not
	.. use this file except in compliance with the License. You may obtain a copy of
	.. the License at
	..
	.. http://www.apache.org/licenses/LICENSE-2.0
	..
	.. Unless required by applicable law or agreed to in writing, software
	.. distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
	.. WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
	.. License for the specific language governing permissions and limitations under
	.. the License.

	.. _replication:

	Replication
	===========

	One of CouchDB's strengths is the ability to synchronize two copies of the same
	database. This enables users to distribute data across several nodes or
	datacenters, but also to move data more closely to clients.

	Replication involves a source and a destination database, which can be one the
	same or on different CouchDB instances. The aim of the replication is that at
	the end of the process, all active documents on the source database are also in
	the destination database and all documents that were deleted in the source
	databases are also deleted on the destination database (if they even existed).


	Triggering Replication
	----------------------

	Replication is controlled through documents in the :ref:`replicator`, where
	each document describes one replication process (see
	:ref:`replication-settings`).

	A replication is triggered by storing a replication document in the replicator
	database. Its status can be inspected through the active tasks API (see
	:ref:`active-tasks` and :ref:`replication-status`). A replication can be
	stopped by deleting the document, or by updating it with its `cancel` property
	set to `true`.


	Replication Procedure
	---------------------

	During replication, CouchDB will compare the source and the destination
	database to determine which documents differ between the source and the
	destination database. It does so by following the :ref:`changes` on the source
	and comparing the documents to the destination. Changes are submitted to the
	destination in batches where they can introduce conflicts. Documents that
	already exist on the destination in the same revision are not transferred. As
	the deletion of documents is represented by a new revision, a document deleted
	on the source will also be deleted on the target.

	A replication task will finish once it reaches the end of the changes feed. If
	its `continuous` property is set to true, it will wait for new changes to
	appear until the task is cancelled. Replication tasks also create checkpoint
	documents on the destination to ensure that a restarted task can continue from
	where it stopped, for example after it has crashed.

	When a replication task is initiated on the sending node, it is called push
	replication, if it is initiated by the receiving node, it is called pull
	replication.


	Master - Master replication
	---------------------------

	One replication task will only transfer changes in one direction. To achieve
	master-master replication it is possible to set up two replication tasks in
	different directions. When a change is replication from database A to B by the
	first task, the second will discover that the new change on B already exists in
	A and will wait for further changes.


	Controlling which Documents to Replicate
	----------------------------------------

	There are two ways for controlling which documents are replicated, and which
	are skipped. Local documents are never replicated (see :ref:`api-local`).

	Additionally, :ref:`filterfun` can be used in a replication documents (see
	:ref:`replication-settings`). The replication task will then evaluate
	the filter function for each document in the changes feed. The document will
	only be replicated if the filter returns `true`.


	Migrating Data to Clients
	-------------------------

	Replication can be especially useful for bringing data closer to clients.
	`PouchDB <http://pouchdb.com/>`_ implements the replication algorithm of CouchDB
	in JavaScript, making it possible to make data from a CouchDB database
	available in an offline browser application, and synchronize changes back to
	CouchDB.