Agree, sorry. Sure, here are some randomly ordered reasons:
* Immutable data. This ties into "functional" section below but it is worth mentioning separately. The benefit here is it is a better fit for concurrency and correctness.
* Lightweight processes. Can have millions of them. But they don't share the heap so act like OS processes in that respect. That means 2 things:
1) It changes the way you think and solve concurrent problems. It reduces impedance between real life problems and how they are solved. Spawning a new process for each long lived connection is obvious and easy.
2) Fault tolerance. Having one request not crash the whole server is a really a massive improvement. In C++ or Java you don't know if something got messed up because of a crash. Maybe some shared data structure or lock got left in an inconsistent state. There is just no guarantee.
* Hot code loading. This is not just a gimmick but it works. Most places don't use it to update regularly and I don't either. But used it a few times in production to add extra logging, fix a customer specific issue without shutting down the cluster before the fix was released. All the while there were 100k/sec requests pounding it. That is plain old money in the pocket. No downtime, no headaches.
* Tracing. BEAM has very good tracing support. When debugging I am quite fond of just logging into a node and using tracing instead of print or log statements.
* Functional. This is more about Erlang the language, and it kind of goes with immutable data. But because of that it means state is more visible and explicit. Looking at a piece of code on a page, I get more context of what is happening because State is passed, updated and returned. As opposed to being some "this" reference which at that point in the code, I don't have any idea how it was modified or what it has in it.
* Garbage collection. BEAM has excellent garbage collection. It is pause-less, in the sense that GC of one process won't block the world. Java has lots of tweaks for that but it is not a default state of affairs.
Welp, this is just a quick list off the top of my head.
At the end of the day, one can probably go through the above list and say I can do all that in Assembly or C. Which is true. BEAM is written in C in large part. To get fault tolerance, can just spawn OS processes in any language. To get immutability, can just go to Github and find an immutable collections library for Python or Java. Java can also do some code loading. But the difference is, it all comes in a single package ready to use. That is huge and I see it as a great benefit.
