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File synchronization (orsyncing) in computing is the process of ensuring thatcomputer files in two or more locations are updated via certain rules.

Inone-way file synchronization, also calledmirroring, updated files are copied from a source location to one or more target locations, but no files are copied back to the source location. Intwo-way file synchronization, updated files are copied in both directions, usually with the purpose of keeping the two locations identical to each other. In this article, the term synchronization refers exclusively to two-way file synchronization.

File synchronization is commonly used for home backups on external hard drives or updating for transport onUSB flash drives.BitTorrent Sync,Dropbox,SKYSITE,Nextcloud,OneDrive,Google Drive andiCloud are prominent products. Somebackup software also support real-time file sync. The automatic process prevents copying already identical files and thus can be faster and save much time versus a manual copy, and is less error prone.^[1] However this suffers from the limit that the synchronized files must physically fit in the portable storage device. Synchronization software that only keeps a list of files and the changed files eliminates this problem (e.g. the "snapshot" feature inBeyond Compare or the "package" feature inSynchronize It!). It is especially useful for mobile workers, or others that work on multiple computers.

It is possible to synchronize multiple locations by synchronizing them one pair at a time. TheUnison Manual^[2] describes how to do this:

If you need to do this, the most reliable way to set things up is to organize the machines into a "star topology," with one machine designated as the "hub" and the rest as "spokes," and with each spoke machine synchronizing only with the hub. The big advantage of the star topology is that it eliminates the possibility of confusing "spurious conflicts" arising from the fact that a separate archive is maintained byUnison for every pair of hosts that it synchronizes.

Common features of file synchronization systems include:^{[citation needed]}

• Encryption forsecurity, especially when synchronizing across theInternet.
• Compressing any data sent across a network.
• Conflict detection where a file has been modified on both sources, as opposed to where it has only been modified on one. Undetected conflicts can lead to overwriting copies of the file with the most recent version, causing data loss. For conflict detection, the synchronization software needs to keep a database of the synchronized files. Distributed conflict detection can be achieved byversion vectors.
• Open Files Support ensures data integrity when copying data or application files that are in-use or database files that are exclusivelylocked.
• Specific support for using an intermediate storage device, such as a removable flash disc, to synchronize two machines. Most synchronizing programs can be used in this way, but providing specific support for this can reduce the amount of data stored on a device.
• The ability to preview any changes before they are made.
• The ability to view differences in individual files.
• Backup between operating systems and transfer between network computers.^[3]
• Ability to edit or use files on multiple computers or operating systems.

Consumer-grade file synchronization solutions are popular, however for business use, they create a concern of allowing corporate information to sprawl to unmanaged devices and cloud services which are uncontrolled by the organization.^{[citation needed]}

When using cloud services, data privacy risks can be mitigated by using a file synchronization solution that featuresend-to-end encryption instead of simple transport (HTTPS) or at-rest encryption.

• Backup software
• Comparison of file synchronization software
• Comparison of online backup services
• Data comparison
• Data synchronization
• Mirror (computing)
• Remote backup service
• Shared file access
• List of backup software

• Data synchronization
• Storage software
• Utility software types

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