Suppose you are running a cluster in Node.JS and you wish to unit-test it. For instance, you'd like to make sure that if a worker dies the cluster takes some action, such as forking another worker and possibly some related job. Or that, under certain conditions, additional workers are spawned.
I suppose that in order to do this one must launch the cluster and have somehow access to its internal state; then (for instance) force workers to get stuck, and check the state after a delay. If so, how to export the state?
You'll have to architect your master to return a reference to its cluster object. In your tests, you can kill one of its workers with cluster.workers[2].kill(). The worker object also has a reference to the child's process object, which you can use to simulate various conditions. You may have to use a setTimeout to ensure the master has the time to do its thing.
The above methods however still creates forks, which may be undesirable in a testing scenario. Your other option is to use a mocking library (SinonJS et al) to mock out cluster's fork method, and then spy the number of calls it gets. You can simulate worker death by using cluster.emit('exit') on the master cluster object.
Note: I'm not sure if this is an issue only with me, but cluster.emit always seems to emit twice for me, for some reason.