> But I also saw how hard it is to understand a large system that built using actors.
Indeed, it can be just as much of a spaghetti mess as any other code, but it becomes easier if actors are the preferred abstraction for a platform already, for instance as it is for Erlang/Elixir on the BEAM VM.
The platform comes with a few benefits such as:
1) Immutable data: inside each actor the state is explicitly evolved from one message to the next. It's passed as an explicit argument to functions. Erlang is even better as the variable binding itself is immutable.
2) Isolated heaps: actors all have isolated heaps. You can have millions of them per OS process and they can't reach in and modify each other's memory. They have to send and receive a message.
3) Supervision trees: actors that work together can be grouped into a tree hierarchy so that if one starts, it start the others and they have "links" between them. If some crash, others crash with them. After the crash they can be restarted safely. It can be done safely because they have isolated heaps. Restarting a bunch of OS threads in a regular C/Java/etc program cannot be done safely, usually. These supervision hierarchies is how the system can be organized. A top level actor might serve as the API endpoint for its children so message go through it.
4) Tracing/live debugging: every message that is sent or function call can be traced dynamically by connecting to a live system. That can be helpful of making sense of the mess when debugging.
There are many "actor" systems out there. It's not a big deal to write a function to send a message to a lockless "mailbox" to be received by a thread in pretty much any modern language/platform. Doing that seems like it gets you 90% there to "actors", but without those 4 points above it only gets there 10% of the way. You can build a quick demo, but it would become a nightmare in a production system.