He wasn't talking about rsync the binary but rather about librsync [0], the library that allows you to calculate a diff between two byte arrays without needing

He wasn't talking about rsync the binary but rather about librsync [0], the library that allows you to calculate a diff between two byte arrays without needing the cooperation of the other side -- ie you can calculate an efficient delta to send to remote without needing remote to be online at the moment you calculate, and without needing the complete old version, only a signature of it. A good example use of it is the rdiff tool that allows you to do exactly what I said before. A better real-world example would be duplicity [1] or rdiff-backup [2] that use librsync to de-duplicate backups, without needing access to the whole previous value, only a small signature of it. [0] https://github.com/librsync/librsync https://github.com/librsync/librsync [1] http://www.nongnu.org/duplicity/ http://www.nongnu.org/duplicity/ [2] http://www.nongnu.org/rdiff-backup/ http://www.nongnu.org/rdiff-backup/