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From Apache Wiki <>
Subject [Couchdb Wiki] Update of "Document revisions" by MartinCzura
Date Sun, 02 Nov 2008 01:09:42 GMT
Dear Wiki user,

You have subscribed to a wiki page or wiki category on "Couchdb Wiki" for change notification.

The following page has been changed by MartinCzura:

The comment on the change is:
page creation

New page:
== Overview ==

CouchDB does not overwrite updated documents in place, instead it creates a new document at
the end of the database file, with the same `_id` but a new `_rev` identifier. This type of
storage system is space-wasteful so regular ["Compaction"] is needed to reclaim disk space.
Note that the older revisions are not available to ["Views"].

Document revisions are used for optimistic concurrency control. If you try to update a document
using an old revision the update will be in conflict. These conflicts should be resolved by
your client, usually by requesting the newest version of the document, modifying and trying
the update again.

@@ How does this relate to replication conflicts?

=== Revision History ===

'''You cannot rely on document revisions for any other purpose than concurrency control.'''

Due to compaction, revisions may disappear at any time. You cannot use them for a client revision

If you wish to implement revisions in your client system a number of patterns have been suggested:

 * Using attachments to store old revisions.
 * Using multiple documents to store old revisions.

@@ Please add to this list and flesh out the solutions as you see fit.

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