Explain the execution flow of a gen_server call?

gen_server:call/2 looks like a normal function call, but underneath it's a request/reply protocol built out of plain message passing plus a monitor for safety.

sequenceDiagram
    participant Caller
    participant GenServer as gen_server process
    Caller->>GenServer: {'$gen_call', {Caller,Ref}, Request}
    Note over GenServer: handle_call/3 runs
    GenServer-->>Caller: {Ref, Reply}
    Note over Caller: call/2 unwraps Reply and returns it

Step by step: the caller sends a tagged message containing its own PID and a fresh unique reference, then sets up a monitor on the server and blocks in a receive waiting for a reply tagged with that same reference (with a default 5-second timeout). The server's handle_call/3 callback runs, returning {reply, Reply, NewState}, and the framework sends Reply back tagged with the original reference. The monitor exists so that if the server dies mid-call, the caller gets a 'DOWN' message instead of hanging forever.

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