Recipe: Double Barrier

[eric]

From http://zookeeper.apache.org/doc/trunk/recipes.html

Double barriers enable clients to synchronize the beginning and the end of a computation. When enough processes have joined the barrier, processes start their computation and leave the barrier once they have finished. This recipe shows how to use a ZooKeeper node as a barrier.

The pseudo code in this recipe represents the barrier node as b. Every client process p registers with the barrier node on entry and unregisters when it is ready to leave. A node registers with the barrier node via the Enter procedure below, it waits until x client process register before proceeding with the computation. (The x here is up to you to determine for your system.)

Enter

1. Create a name n = b+"/"+p
2. Set watch: exists(b + "/ready", true)
3. Create child: create( n, EPHEMERAL)
4. L = getChildren(b, false)
5. if fewer children in L than x, wait for watch event
6. else create(b + "/ready", REGULAR)

Leave

1. L = getChildren(b, false)
2. if no children, exit
3. if p is only process node in L, delete(n) and exit
4. if p is the lowest process node in L, wait on highest process node in P
5. else delete(n) if still exists and wait on lowest process node in L
6. goto 1

Like #42, it seems like this should go in the same gem as the locker.