Sync allows to synchronize multiple processes, so that they could perform their jobs at the same time.

The main purpose for Sync is to synchronize multiple streams within a pipeline. The flow of usage goes as follows:

• A Sync process is started.
• Processes register themselves (or are registered) in the Sync, using register/2. Registered processes are not being synchronized till the Sync becomes active (see the next step). Each registered process is monitored and automatically unregistered upon exit. Sync can be setup to exit when all the registered processes exit by passing the empty_exit? option to start_link/2.
• When all processes that need to be registered are registered, the Sync can be activated with activate/1 function. This disables registration and enables synchronization.
• Once a process needs to sync, it invokes sync/2, which results in blocking until all the registered processes invoke sync/2. This works only when the Sync is active - otherwise calling sync/2 returns immediately.
• Once all the ready processes invoke sync/2, the calls return, and they become registered again.
• When synchronization needs to be turned off, the Sync should be deactivated with deactivate/2. This disables synchronization and enables registration again. All the calls to sync/2 return immediately.

If a process designed to work with Sync should not be synced, no_sync/0 should be used. Then all calls to sync/2 return immediately.