algorithm.md

Algorithm

The primary idea behind pysync is to use rsync to take care of transfering files from one machine to another without worrying about what needs to be updated or deleted.

pysync first calls rsync with the option --dry-run and --delete to see what rsync would do when bringing files from the remote directory to the local one. This gives a list of files which might be brought over and files that it plans on deleting. rsync is also used with the option --out-format='%n<>%M' to display the names of the files and the last modified date of the file.

Incoming Files

This is a list of files that rsync would like to update on your local machine. It compares the last modified date of file in the remote machine and the last modified date of the local machine. If the local and remote files have a date older than the last sync date it means that the file was modified in both machines and there might be some information that might be lost if we use rsync to receive or send files. The action that I decided to take is to rename the local file by appending the name of the machine and the last modified date of the file. Now there won't be a conflict with the files.

Some files will may have been deleted in the remote directory. If the local last modified date is after the last date synced then the file needs to stay.

rsync REMOTE to LOCAL

The previous step generated the files exclude.txt and remove.txt. The exclude file is used when tranfering files from the remote directory to the local directory.

Clean Local Directory

Next we iterate over the list of files and directories specified in remove.txt to delete them from the system. A file/directory will only be on this list if rsync detected that the file does not exist on the remote directory. This is the reason we first do a dry run with the --delete option.

Sync LOCAL to REMOTE

Now we sync the local directory to the remote with the --delete option so that both local and remote have the same contents.

Snapshot

To help keep track of the files that are meant to be in both directories we store a list of them in a file that is managed by the entry. This file is used in subsequent calls to pysync to determine the list of files to exclude and remove.

Naming files

pysync creates a list of files to exclude some files when using rsync. This puts a restriction on the name of the files: Do not use the character set *?[. Otherwise you might run into problems (rsync may interpret as regex). Try to keep the name of the files fairly simple.