> Does this mean a web-endpoint has to be immutable? If you send it the same parameters multiple times, is it required to respond with the same response every time? If not, does that not mean it is in fact mutable?

This is a concept called purity, and it's only loosely related to immutability. Immutability makes purity easier to implement and reason about, but does not guarantee it. Erlang/Elixer are not pure. For example, `DateTime.now("Etc/UTC")` will return different things at different times.

As a counter-example, Haskell functions are pure, so the `getCurrentTime` function cannot return a value directly as it would be different every time. Instead it uses the return type `IO UTCTime` which act like instructions on how to calculate the time, rather than the time itself.

https://en.wikipedia.org/wiki/Pure_function

When they say all the data in Erlang/Elixir is immutable, it does not mean that there is no state. There's definitely data in the programs that change values, because like you point out, how in the heck can you write a useful system that doesn't have any state anywhere?

State or data that changes values is typically put in one of 3 different places:

1. On the stack. It's pretty typical to have a function like handle_message(app_state, request). The current state of the app is in the call parameters. At the end of this message, handle_call() would call itself again with the new updated state. Somewhere else in the system we keep track of the last value of that state, and if handle_call crashes, we just use the last state.

2. Another place to hold state is in external storage somewhere.

3. The third main place to hold state is via references to other objects, which do #1 or #2 above.

Regarding whether there is a difference between messaging local versus remote objects-- there's an operator for sending messages to another object. It works the same for local and remote. I think it's possible to inspect the actor address and see where it is, but the mechanism works the same.