The Rsync algorithm is a file synchronization and transfer algorithm that efficiently detects and transfers the differences between two files or directories. It was developed by Andrew Tridgell and Paul Mackerras in 1996.
The key idea behind the Rsync algorithm is the concept of delta encoding. Instead of transferring an entire file, Rsync identifies the portions of the file that have changed and transfers only those differences (called deltas). This makes the algorithm particularly efficient when transferring large files or synchronizing files over a network.
The Rsync algorithm operates in two phases: the sender and the receiver.
Sender Phase:
- The sender breaks the source file into fixed-size blocks (typically 2KB or 4KB) and calculates a rolling checksum for each block. The rolling checksum is a hash function that produces a fixed-size checksum for each block based on its content.
- The sender sends the list of checksums along with the corresponding block positions to the receiver.
Receiver Phase:
- The receiver compares the checksums received from the sender with the checksums of the blocks in the destination file.
- If a checksum match is found, it means that the block is already present in the destination file, and the receiver skips it.
- If a checksum mismatch occurs, the receiver identifies the differing blocks and requests the sender to transmit only those blocks.
- The sender sends the requested blocks, and the receiver integrates them into the destination file, reconstructing the updated file.
This process continues until the receiver has synchronized the entire file or directory with the sender.
The Rsync algorithm's efficiency lies in its ability to minimize the amount of data transmitted by transferring only the differences between files. This makes it an excellent choice for efficient file synchronization, remote backups, and network transfers with limited bandwidth.