Erlang allows for massively scalable concurrency, often with millions of lightweight, thread-like components known as actors. Unfortunately, using Erlang requires rewriting all of your legacy code into a rather esoteric language. But there are other options, such as the little known CCR platform that was developed by .NET's robotics department.
Actor based languages such as Erlang are able to achieve high degrees of parallelism by using the Actor model. Under this model the fundamental unit of concurrency is not a thread or fiber, but rather something much smaller. Known as a "process" in Erlang, each unit of concurrency has a base overhead of about 1200 bytes on a 32-bit system. By comparison, a thread on the Windows operating system defaults to 1 MB just for the stack, additional space is also needed for bookkeeping and thread local storage. Because they are so lightweight, an application can spawn literally millions of processes simultaneously.
At any given time most processes will be idle. When a process receives a message, the platform assigns a thread to it so that it can respond to the message. This response may include creating new processes, sending messages to other processes, and/or changing its own state. Once the message is handled, the process either dies or blocks on waiting for the next message.
High levels of concurrency and performance are achieved via the message passing system. Each message is sent asynchronously, allowing for a high degree of independence among the processes. The messages also allow the platform to know which process to awaken. Since a process can be executed on any thread, there is a greatly reduced need for relatively expensive context switching.
The .NET answer to Erlang's model is the CCR or Coordination Currency Runtime. The CCR, originally pioneered for robotics, is already finding acceptance in the wider marketplace. One developer at Siemens Automation was able to integrate CCR into their current backboard code base in only a few days. Blackboard, which routes mail using AI agents and conveyer belts moving at 10 meters per second, has millions of lines of legacy code. Tyco, a security company that works with everything from small stores to the Whitehouse, was also able to integrate CCR within a week. These were not solicited case studies; both Siemens and Tyco did the bulk of the work without the assistance or knowledge of Microsoft.
The core of CCR is a new API level concept known as "ports". Instead of calling methods on classes, developers wanting to perform an action need to post messages on a port. An arbiter attached to the port reads messages and, if they satisfy certain criteria, bundle the message into a task. That task is then placed on a dispatcher queue to be executed by the thread pool at a later time.
Arbiters form the basis of coordination primitives. They can be as simple as a single port receiver or a join/choice across multiple ports. They can even be composed when more complex logic is needed. However constructed they ultimately have one purpose, to wake up a bit of code when data is received.
The real power of CCR comes from combining it with C#'s iterator syntax, "yield return". Yield return is a type of continuation, a way to pause a thread of execution and continue it later without having to suspend a real thread. Normally it is used only for iteration, but with CCR it can be extended for any type of asynchronous operation. The real beauty of it is that it does so without drastically changing your code.
This code sample from the 2008 PDC shows making an asynchronous call using a Port. Rather than blocking or using the asynchronous call pattern with its explicit callbacks, one simple uses a yield return statement to leave the function. Once the data is received and a thread is available, the function continue on the next line as if nothing happened.
fs.BeginRead(buffer, 0, buffer.Length, arPort.Post, null);
// yield until stream posts IAsyncResult on the CCR port
yield return arPort.Receive();
// extract result, it must be in the port
var ar = (IAsyncResult)arPort.Test();
int read = fs.EndRead(ar);
CCR is expected to be included in .NET 4/Visual Studio 2010.
