| Good question. Here are my bullet points: * Fault tolerance comes first. Built in from ground up. * High concurrency. Yap that is it. Ok, it has many other features but they amazingly fall from those one. Here are the secondary features and how they relate to the primary ones. * Fault isolation. If a system is to be fault tolerant, faults must be isolated. If one process crashes it shouldn't bring down the whole system. * Easily parallelizable. Because of isolated processes and the desire for high concurrency. It was easy to build in a sane scheduling algorithm that can spread the load across multiple CPU cores. * Functional. Functional programming discourages handling large states. This a large Java class with 50 instance variable that could be modified by a 20 different methods. Functional programming encourages passing the state along explicitly. * Built in distribution. It is hard to make a fault tolerant system (ok impossible in practice, to be more precise) without redundant hardware. Servers will fail but your service must not. You must have more than one server read to take over. Distributing your application across multiple physical machines is built it. You send a message to a local Erlang process like this: Pid ! Msg. Here is how you send a message to a process running in another data center. Maybe half way across the world. Pid ! Msg. That is pretty nice. * The system is responsive. A non-response system can be considered a failed system in some domain. Think about a mail server. If the user click on a message and it take 5s to return a response and open it, maybe the person would consider the system as broken. This also comes out of concurrency and fault isolation. As the load increases instead of throwing errors everywhere the system gracefully absorbs the load while still staying responsive. |