zksync provides a go implementation the synchronization primitives that you'll find in the ZooKeeper documentation: locks and barriers. These can be used to coordinate computation across multiple processes.

RWMutex provides a read-write lock. Readers can share concurrent access as long as a writer hasn't claimed the lock. Writers must be exclusive, and will block on any other lock holder.

Lock acquisition is handled on a first-come, first-serve basis, with ordering determined by the ZooKeeper server.

Locks are stored as ephemeral znodes, so if a client disconnects from ZooKeeper unexpectedly, the lock will be released within the session timeout that the client established when they connected.

Barrier provides a shared barrier for multiple clients. When the barrier is set, any call to Barrier.Wait will block any client of the barrier. A call to Barrier.Unset will unblock all clients.

Setting the barrier is an inherently racy process; clients can optimistically call Barrier.Set and handle zk.ErrNodeExists if the barrier is already in place.

Barriers are not ephemeral, since a disconnect from the barrier-setter should not obviously cause all other clients to proceed. This means that any client has permission to remove the barrier.

Double Barriers allow clients to synchronize start and end times of a computation. All participants in the double barrier should agree on the number of participants ahead of time. Then, they each call DoubleBarrier.Enter() and block until all participants have called that function and written their data in ZooKeeper.

When a client finishes its computation, it calls DoubleBarrier.Exit(), which will block until all clients have either called DoubleBarrier.Exit() or disconnected from ZooKeeper.

All clients must agree on the number of participants throughout the entry process. If the group is expecting 5 participants, and then only 4 successfully call Enter, then those 4 will all block forever.

Run tests with GOMAXPROCS=4 godep go test -timeout 10s ./... (or some other value of GOMAXPROCS, of course - the point is, you want parallelism).

Tests require a working ZooKeeper cluster. You can set this in either of two ways:

1. Use Vagrant. vagrant up will launch a VM with a 5-node ZooKeeper ensemble listening on 192.168.100.67, ports 2181 through 2185. This is the assumed default in tests.
2. Set the environment variable ZOOKEEPER_PEERS to a comma-separated list of ZooKeeper hostports, like so:

   $ ZOOKEEPER_PEERS=localhost:2181,localhost:2182 GOMAXPROCS=4 godep go test -timeout 10s ./...
   

The vagrant ZooKeeper cluster uses Toxiproxy to simulate network failures. By default, tests will attempt to connect to 192.168.100.67:8474 to talk to Toxiproxy. You can override this by setting a TOXIPROXY environment variable to your preferred hostport. If unable to connect to Toxiproxy, then tests which rely upon it will be skipped.

Either way, the tests will all be run under a namespace, /zksync-test, and will delete everything under that node after each test run.