Erlang and Concurrency

Here I write a lot about my experiments with Terracotta which is shared durable memory for Java applications which are multi-threaded. But some days before I came across erlang, language which is created at Erricsson for running fault-tolerant telecom applications. Whats so special about it?. If you google or rather "Bing" it you will find plenty of things about it. How cool and scalable it is?. If you read this you will even come to know How Goldman Sachs was using it to gain significant advantage in program trading over other competitors and how others are trying to steal leaked source code.

So erlang is not procedural programming language its functional programming language. Frankly I also need to understand whats so different about it. But I saw this presentation of infoQ site about erlang concurrency and was amazed. Right from starting I always thought about following graph - throughput increases as a function of in-coming request rate till some point but after it stabilizes and then it drops. It drops because of system overload. In a perfectly cpu-intensive lock-contention free application this will happen because of cpu context switching.


But in erlang it stays constant instead your latency (response time) increases. This is according to Little's Law. Little law says relation between throughput and latency is number of users in system. N = RX.




Above is famous graph of benchmark of YAWS (Http server written in erlang) against Apache and you can see how early apache gets saturated and dies. You can read details here but explanation is given here is :

"The problem with Apache is not related to the Apache code per se but is due to the manner in which the underlying operating system (Linux) implements concurrency. We believe that any system implemented using operating system threads and processes would exhibit similar performance. Erlang does not make use of the underlying OS's threads and processes for managing its own process pool and thus does not suffer from these limitations."


So basically all magic is erlang's concurrency model : No Shared State, Only message passing between light-weight processes. Erlang processes are way lighter than Java Threads since they are logical entities and not tied to user-level or kernel-threads. Thus erlang shows "No Shared State" concurrency model scales well. Since now JVM is touted as platform, I am looking forward to see erlang implementation on JVM and see how it does against other concurrent interpreted languages - scala. This is great post why JVM is unfit for such porting. May be Java 8 ( I think closures are not part of Java 7). This is also interesting read about Erlang on Java : Erlang Concurrency model on JVM . Some of work on writing OTP(erlang's sdk for writing applications) for scala http://github.com/jboner/scala-otp/tree/master/.

I have already got Programming Erlang book now looking forward to write first program in OTP.

Tushar