Message Passing

Mailbox Drain

Process Messages in Order

A mailbox can be modeled as a queue of messages drained by one receiver in first-in, first-out order.

Program

Play the program to choose the second message and drain the mailbox into one summary.

second
mailbox_drain.rs
Replay: real traced execution (multi-file project)
use std::collections::VecDeque;

fn main() {
    let second = "ack";
    let mut mailbox = VecDeque::new();
    mailbox.push_back("open");
    mailbox.push_back(second);
    let summary = drain(&mut mailbox);
    println!("{summary}");
}

fn drain(mailbox: &mut VecDeque<&str>) -> String {
    let mut seen = Vec::new();
    while let Some(message) = mailbox.pop_front() {
        seen.push(message);
    }
    seen.join(",")
}

use std::collections::VecDeque;

fn main() {
    let second = "retry";
    let mut mailbox = VecDeque::new();
    mailbox.push_back("open");
    mailbox.push_back(second);
    let summary = drain(&mut mailbox);
    println!("{summary}");
}

fn drain(mailbox: &mut VecDeque<&str>) -> String {
    let mut seen = Vec::new();
    while let Some(message) = mailbox.pop_front() {
        seen.push(message);
    }
    seen.join(",")
}

use std::collections::VecDeque;

fn main() {
    let second = "done";
    let mut mailbox = VecDeque::new();
    mailbox.push_back("open");
    mailbox.push_back(second);
    let summary = drain(&mut mailbox);
    println!("{summary}");
}

fn drain(mailbox: &mut VecDeque<&str>) -> String {
    let mut seen = Vec::new();
    while let Some(message) = mailbox.pop_front() {
        seen.push(message);
    }
    seen.join(",")
}

  1. second ← "ack", mailbox ← []

    3fn main() {4    let secon→ "ack"d = "ack"; //@second="ack", "retry", "done"5    let mut mailbo→ []x = VecDeque::new();6    mailbox.push_back("open");7    mailbox.push_back(second);8    let summary = drain(&mut mailbo["open", "ack"]x);9    println!("{summary}");

  2. seen ← []

    12fn drain(mailbox: &mut VecDeque<&str>) -> String {13    let mut see→ []n = Vec::new();14    while let Some(message) = mailbox.pop_front() {

  3. seen.join(",")

    16    }17    seen.join(",")18}

  4. mailbox ← [], summary ← "open,ack"

    7    mailbox.push_back(second);8    let summar→ "open,ack"y = drain(&mut mailbo→ []x);9    println!("{summary}");10}
    outputopen,ack

  1. second ← "retry", mailbox ← []

    3fn main() {4    let secon→ "retry"d = "retry";5    let mut mailbo→ []x = VecDeque::new();6    mailbox.push_back("open");7    mailbox.push_back(second);8    let summary = drain(&mut mailbo["open", "retry"]x);9    println!("{summary}");

  2. seen ← []

    12fn drain(mailbox: &mut VecDeque<&str>) -> String {13    let mut see→ []n = Vec::new();14    while let Some(message) = mailbox.pop_front() {

  3. seen.join(",")

    16    }17    seen.join(",")18}

  4. mailbox ← [], summary ← "open,retry"

    7    mailbox.push_back(second);8    let summar→ "open,retry"y = drain(&mut mailbo→ []x);9    println!("{summary}");10}
    outputopen,retry

  1. second ← "done", mailbox ← []

    3fn main() {4    let secon→ "done"d = "done";5    let mut mailbo→ []x = VecDeque::new();6    mailbox.push_back("open");7    mailbox.push_back(second);8    let summary = drain(&mut mailbo["open", "done"]x);9    println!("{summary}");

  2. seen ← []

    12fn drain(mailbox: &mut VecDeque<&str>) -> String {13    let mut see→ []n = Vec::new();14    while let Some(message) = mailbox.pop_front() {

  3. seen.join(",")

    16    }17    seen.join(",")18}

  4. mailbox ← [], summary ← "open,done"

    7    mailbox.push_back(second);8    let summar→ "open,done"y = drain(&mut mailbo→ []x);9    println!("{summary}");10}
    outputopen,done
mailbox `VecDeque` models a queue that can push at the back and pop from the front.
drain `while let Some(...)` keeps receiving until the mailbox is empty.
ordering `pop_front` preserves the order in which messages were queued.