What if only one thread is allowed to touch the data?

Level: 201 · anyone who has met an actor, or Go's proverb about sharing memory

Stub — a question and its expected answers, not yet a lesson. No program behind this page has been written or run, so nothing on it has been through the check that backs every finished page. The table is what the finished lesson is expected to find, and every cell is unverified until a program prints it. (Not machine-checked here.)

One line: Instead of locking a total that many threads add to, give the total to one thread and have the others send it their numbers — no lock, no race, and every update applied in the order received — which is the actor model in one sentence, the third fix in Keeping every update, and the design Erlang built a language around.

The question

The counter from chapter 02, once more. This time no thread but the owner ever sees it: ten senders, one owner goroutine with a channel, and a final query message that asks for the total. The page runs it, then asks the questions an actor raises — what happens to a message sent to an owner that has stopped, how the owner is asked a question and answered, and what the mailbox costs against a lock.

What each language is expected to say

	Expected answer, to be verified by the program
Rust	an owner thread on an mpsc::Receiver of an enum Message { Add(u64), Total(Sender<u64>) }
Go	an owner goroutine on a channel of a struct with a reply channel inside; "do not communicate by sharing memory"
C	an owner thread on a pipe, with the reply through a second pipe
C++	an owner std::jthread on a hand-built queue of std::variant messages
Java	an owner thread on a BlockingQueue, replying through a CompletableFuture in the message
Python	an owner thread on a Queue, replying through a Future, or an asyncio task

What the programs have to show

• ten senders, one owner: the total is right without a lock, and the order of application is the order of receipt
• the ask-and-reply message, and the sender that waits for the reply
• a message to an owner that has exited: the error, the panic, or the silent loss, per language

See also

• Before this: How do N workers share one queue of jobs?
• After this: How does one event reach every subscriber?
• Keeping every update
• Who owns a value after it has been sent?
• The Rust library's Who owns the state ↗
• The Go library's A goroutine has no handle ↗
• Concepts: Actor model · Communicating sequential processes · Message passing · Thread confinement · Supervision · OTP behaviours