zebu works on ZFS filesystems (rather than the entire zpool). Some
filesystems can be configured to be an rsync vault; these filesystems contain
backup copies of data from remote systems, and will trigger an rsync before
they are backed up. All filesystems are represented by a class ZFSfs; vaults
are of class ZEBUVault (which inherits from ZFSfs).
A singleton ZEBUConfig class parses the config files, and manufactures the
array of ZFSfs classes in play. Command-line arguments override values
brought in via ZEBUConfig.
After setup, zebu passes through three phases: snapshot (where filesystems
get new snapshots), cleanup (where old snapshots are deleted), and transmit
(where the latest snapshots are sent). Any phase can be skipped, either
globally or for a single ZFSfs. The last transmitted snapshot is never
deleted during the cleanup phase (even if it's reached its expire time).