Zsync

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The correct title of this article is zsync. The initial letter is shown capitalized due to technical restrictions.

zsync offers an alternative to the file transfer software rsync.

Although both programs use the same algorithm, zsync does not require any special access or daemon on the server (SSH or rsyncd) to enable clients to download a file. The bulk of the work takes place on the client side: on the server side, zsync needs only use a small control .zsync file accompanying the file intended for zsync-enabled download. The zsync client requests only the updated blocks of the file, making it useful for bandwidth-conservation purposes.

Note that zsync does not meet precisely the same technical requirements as rsync: rsync achieves its task of mirroring a file on the client with only one "round-trip" transmission to the server and back, while zsync must first request the control file from the server, a round-trip transmission in itself, and then uses the control file to minimize the file content information it requests from the server on a second round-trip transmission.

More in-depth information on zsync appears in the technical paper written by its programmer, Colin Phipps. zsync operates under the Artistic License, while rsync uses the GNU General Public License version 2. The Open Source Initiative certifies both licenses.