Dispatchers

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Sometimes you want route decision making to be done by the sender, and not the receiver. That's where dispatchers come in. Here's an example:

    var pid1 = spawn("proc1", ... );
    var pid2 = spawn("proc2", ... );
    var pid3 = spawn("proc3", ... );

    ProcessId pid = Dispatch.broadcast(pid1, pid2, pid3);
    tell(pid, msg);

In that example 3 processes are grouped into one ProcessId. You can then tell, ask, subscribe, etc. because it's just a regular ProcessId. The Process system itself can spot that there are multiple processes referenced and it deals with it at the time of dispatch, without a router or proxy Process. So 3 messages will be sent to 3 processes, and its the sender that is doing it.

In the above example pid looks like this:

    /disp/broadcast/[/node-name/user/proc1,/node-name/user/proc2,/node-name/user/proc3]

The disp part tells the system to use a named dispatcher, the broadcast part is the name of the dispatcher (you can register your own dispatchers via Dispatch.register(...)). There are several built in dispatchers:

• Dispatch.broadcast(...)
• Dispatch.random(...)
• Dispatch.roundRobin(...)
• Dispatch.random(...)
• Dispatch.first(...)
• Dispatch.second(...)
• Dispatch.third(...)
• Dispatch.last(...)

The name of the dispatcher decides the bespoke behaviour to run on the rest of the ProcessId: [/node-name/user/pid1,/node-name/user/pid2,/node-name/user/pid3]

You can also register dispatchers as registered processes. This can give router like behaviour even though the dispatcher is doing the routing work.

    ProcessId pid = Dispatch.random(pid1, pid2, pid3);
    Process.register("load-balance", pid);

    // Will send a "Hello" to a random Process in the dispatcher list
    tell("@load-balance", "Hello");  

The name "@load-balance" can be used from anywhere in the cluster, it only needs to be registered once. So as you can see, dispatchers combined with registered processes are a very powerful feature for routing messages without the need for gatekeeper routers.

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