rsync - How do I synchronize in both directions?

Question

I want to use rsync to synchronize two directories in both directions.

I refer to synchronization in classical sense (not how it is meant in rsync manuals):
I want to update the directories in both directions, depending on which of them is newer.

Can this be done by rsync (preferable in a Linux-way)?
If not, what other solutions exist?

Answer 1

Just run it twice, with "newer" mode (-u or --update flag) plus -t (to copy file modified time), -r (for recursive folders), and -v (for verbose output to see what it is doing):

rsync -rtuv /path/to/dir_a/* /path/to/dir_b
rsync -rtuv /path/to/dir_b/* /path/to/dir_a

This won't handle deletes, but I'm not sure there is a good solution to that problem with only periodic sync'ing.