zsync is a file transfer program. It allows you to download a file from a remote server, where you have a copy of an older version of the file on your computer already. zsync downloads only the new parts of the file. It uses the same algorithm as rsync. However, where rsync is designed for synchronising data from one computer to another within an organisation, zsync is designed for file distribution, with one file on a server to be distributed to thousands of downloaders. zsync requires no special server software — just a web server to host the files — and imposes no extra load on the server, making it ideal for large scale file distribution.

zsync is open source, distributed under version 2 of the Artistic License. Feedback, bugs reports and patches are welcome.

Advantages of zsync

zsync fills a gap in the technology available for large-scale file distribution. Three key points explain why zsync provides a genuinely new technique for file distribution:

The special handling of compressed files is, as far as I know, entirely new and unique to zsync. The combination of client-side rsync and HTTP is also unique to zsync, to the best of my knowledge.

I have put up technical papers. There is a current paper, which reflects zsync's latest development, and which I update with new figures as I run new tests and comparisons; there are also older snapshots of the technical paper, included for reference and citation.

Practical Engineering

zsync is only useful if people offer zsync downloads. I have made a deliberate effort to make zsync simple and reliable in use, and easy to deploy.

Because zsync is a file distribution method, it is useful only when people offer .zsyncs for downloads. It's a little hard to demonstrate the program and build up a user base because of this. So I am suggesting a number of uses for zsync on the client page; these include some zsync downloads that I am offering downloads myself to give some immediate use to the program.

If you are a server operator looking to use zsync for file distribution, see the server page, which gives some guidance on how to create and offer .zsync files.

Current Status

I am fairly happy with how zsync works so far, but I am still open to new ideas and optimisations. I have made sure that the format is fairly extensible in a compatible way; I expect that users of the current version of zsync will not experience compatibility problems with later versions for the foreseeable future.

I am also looking to freeze the interface between the zsync library and the current command-line client; I will at some point split zsync into a dynamic library, which implements the format and allows it to be embedded into other programs, and the client program. This to some extent depends on whether anyone is interested in embedding zsync into other programs; if I don't hear of anyone intending to do so, this might wait until I have a use for it myself.

I am interested in hearing feedback about zsync. Bugs, crashes, and feature suggestions are welcome. Perhaps most of all I am interested in hearing about uses of zsync: I would like to hear about any serious use of zsync, because knowing how people are using it will affect which features I work on. If there are features that you need to enable a particular use, then I would like to hear about that too, particularly anyone that is interested in using zsync as a library as part of a larger program; please note that support for multiple files has already been asked about many times, but due to lack of time I am not likely to work on this in the near future.