This has been bugging me for a long time. There are some things we explain because we know other people don't understand them and there are some things we don't explain because we assume that everybody else knows them.
So this is about web-servers written in Erlang and Elixir. The idea to write this came after watching the Making the Web Functional presentation by Chris McCord and Evan Czaplicki at the 2016 Erlang Factory in San Francisco
Imagine an Erlang or Elixir HTTP server managing a couple of million user sessions. Time and again I've heard this said:
But this statement is incorrect and stems from a fundamental misconception.
We do not have ONE web-server handling 2 millions sessions. We have 2 million webservers handling one session each.
The reason people think we have one webserver handling a couple of million users is because this is the way it works in a sequential web server. A server like Apache is actually a single webserver that handles a few million connections.
In Erlang we create very lightweight processes, one per connection and within that process spin up a web server. So we might end up with a few million web-servers with one user each.
If we can accept say 20K requests/second - this is equivalent to saying we can create 20K webservers/second.
On the surface things look very similar. But there is a fundamental difference between having one webserver handling two million connections, and two million web servers handling one connection each.
If there is a software error and the server software crashes we lose either two million connections or one depending upon the model.
In Erlang if the web server software itself is incorrect we'll lose a single connection, which is OK. Since the software is incorrect and crashes we don't know what to do so crashing is a good alternative. What is important is that one session crashing does not effect all the other sessions.
This requirement, goes way back to when we designed Erlang in the mid 1980's. In Telecoms systems, losing one connection due to a software error was acceptable, losing them all due to a bug was big time bad news.
So what happens if you want to make a chat or presence server? Well we've already got one process per connection, so let's also make one process per user and send messages between the connections and user processes when connections come and go - this is so simple that the code practically writes itself.
So why did WhatsApp use Erlang and why does the Phoenix Framework outperform Ruby on Rails? - precisely because we have millions of tiny webservers - and when we have lots of small things it's easy to spread them over lots of processors and make the system fault-tolerant and scalable.
Packing Erlang or Elixir processes onto cores is easy because they are small and are like packing physical objects. If we want to pack sand in barrels it's easy. The grains of sand are so small that it's easy to completely fill the barrels. Packing huge boulders is difficult, they don't pack well and much space is wasted.
Because we have one web server per user we can easily make the system fault tolerant or scalable
To make a fault tolerent system we use two or more processes per user; One is the master process, the others are replicas on different machines. They must be on different machines since the entire machine where the master runs might crash. We can make it scalable by just buying more machines and spreading the processes out over the machines.