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109 lines
5.4 KiB
ReStructuredText
109 lines
5.4 KiB
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Architecture
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============
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.. index:: architecture
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The ownCloud project provides desktop sync clients to synchronize the
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contents of local directories on the desktop machines to the ownCloud.
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The syncing is done with csync_, a bidirectional file synchronizing tool which
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provides both a command line client as well as a library. A special module for
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csync was written to synchronize with ownCloud’s built-in WebDAV server.
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The ownCloud sync client is based on a tool called mirall initially written by
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Duncan Mac Vicar. Later Klaas Freitag joined the project and enhanced it to work
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with ownCloud server. Both mirall and ownCloud Client (oCC) build from the same
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source, currently hosted in the ownCloud source repo on gitorious.
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oCC is written in C++ using the `Qt Framework`_. As a result oCC runs on the
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three important platforms Linux, Windows and MacOS.
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.. _csync: http://www.csync.org
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.. _`Qt Framework`: http://www.qt-project.org
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The Sync Process
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----------------
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First it is important to recall what syncing is. Syncing tries to keep the files
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on both repositories the same. That means if a file is added to one repository
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it is going to be copied to the other repository. If a file is changed on one
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repository, the change is propagated to the other repository. Also, if a file
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is deleted on one side, it is deleted on the other. As a matter of fact, in
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ownCloud syncing we do not have a typical client/server system where the
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server is always master.
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This is the major difference to other systems like a file backup where just
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changes and new files are propagated but files never get deleted.
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Sync Direction and Strategies
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-----------------------------
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.. index:: time stamps, file times, etag, unique id
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Until the release of ownCloud 4.5 and ownCloud Client 1.1, ownCloud employed
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a single file property to decide which file is newer and hence needs to be
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synced to the other repository: the files modification time.
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The *modification timestamp* is part of the files metadata. It is available on
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every relevant filesystem and is the natural indicator for a file change.
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modification timestamps do not require special action to create and have
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a general meaning. One design goal of csync is to not require a special server
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component, that’s why it was chosen as the backend component.
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To compare the modification times of two files from different systems,
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it is needed to operate on the same base. Before version 1.1.0,
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csync requires both sides running on the exact same time, which can
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be achieved through enterprise standard `NTP time synchronisation`_ on all
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machines.
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Since this strategy is rather fragile without NTP, ownCloud 4.5 introduced a
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unique number, which changes whenever the file changes. Although it is a unique
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value, it is not a hash of the file, but a randomly chosen number, which it will
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transmit in the Etag_ field. The client will store this number in a
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per-directory database, located in the application directory (version 1.1) or
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as a hidden file right in the directory to be synced (later versions).
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Since the file number is guaranteed to change if the file changes, it can now be
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used to determine if one of the files has changed.
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.. todo:: describe what happens if both sides change
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If the per-directory database gets removed, oCC's CSync backend will fall back
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to a time-stamp based sync process to rebuild the database. Thus it should be
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made sure that both server and client synchronized to NTP time before
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restarting the client after a database removal. If time deviates, the sync
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process might create faux conflict files, which only differ in their time.
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Those need to be cleaned up manually later on and will not be synced back
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to the server. However, no files will get deleted in this process.
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Just like files, directories also hold a unique id, which changes whenever
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one of the contained files or directories gets modified. Since this is a
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recursive process, it significantly reduces the effort required for a sync
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cycle, because the client will only walk directories with a modified unique id.
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.. note:: oCC 1.1 and newer require file ID capabilities on the ownCloud server,
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hence using them with a server earlier than 4.5.0 is not supported.
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This table outlines the different sync methods attempted depending
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on server/client combination:
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.. index:: compatiblity table
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+--------------------+-------------------+----------------------------+
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| Server Version | Client Version | Sync Methods |
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+====================+===================+============================+
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| 4.0.x or earlier | 1.0.5 or earlier | Time Stamp |
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+--------------------+-------------------+----------------------------+
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| 4.0.x or earlier | 1.1 or later | n/a (incompatible) |
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+--------------------+-------------------+----------------------------+
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| 4.5 or later | 1.0.5 or earlier | Time Stamp |
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+--------------------+-------------------+----------------------------+
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| 4.5 or later | 1.1 or later | File ID, Time Stamp |
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+--------------------+-------------------+----------------------------+
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It is highly recommended to upgrade to ownCloud 4.5 or later with ownCloud
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Client 1.1 or later, since the time stamp-based sync mechanism can
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lead to data loss in certain edge-cases, especially when multiple clients
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are involved and one of them is not in sync with NTP time.
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.. _`NTP time synchronisation`: http://en.wikipedia.org/wiki/Network_Time_Protocol
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.. _Etag: http://en.wikipedia.org/wiki/HTTP_ETag
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