In Cloud Haskell, the system topology is determined by your choice of Cloud Haskell Backend.
The basic topology that Cloud Haskell currently ships with is determined by the
simplelocalnet backend, which provides for a fully connected grid of nodes with optional
master-slave configuration. This backend allows nodes to discover one another using UDP multicast.
It is a zero-configuration backend designed to get you going with Cloud Haskell quickly without
imposing any particular structure on your application.
Other backends might work in a completely different manner, offering different types of (and relationships between) nodes, or simply by handling discovery differently (e.g., pre-defined node names/addresses, or by using some form of registrar such as DNS-SD/Bonjour).
Here is an example program built against the
simplelocalnet backend, that periodically
searches for a list of peer nodes, and sends a message to a registered (named) process on each.
Clearly the program isn’t very useful, but it illustrates the two key concepts that
simplelocalnet relies on. Firstly, that we
initializeBackend in order to get
connected to an underlying communications infrastructure and secondly, that we can
findPeers at any time to obtain the set of other nodes that have broadcast
Here we simply rehash the master/slave example from the
With the same imports as the example above, we add a no-op slave and a master that
takes a list of its (known) slaves, which it prints out before terminating them all.
And the master node is defined thus:
Many other topologies are in development, including one that runs on Windows Azure,
which is available here. Some third party backends have also been developed,
such as the
distributed-process-p2p backend, which given a known node address,
discovers and maintains knowledge of it’s peers.
Here is an example of node discovery using the