From eb3bc0bcc81bc3c55e293ddbaf93af276c5e19f6 Mon Sep 17 00:00:00 2001
From: Julio Biason <j.biason@engys.com>
Date: Fri, 29 Jul 2022 17:21:09 -0300
Subject: [PATCH] WIP: Async mental model

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 create mode 100644 content/thoughts/a-mental-models-for-async-rust.md

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+title = "A Mental Model for Async Rust"
+date = 2022-07-29
+draft = true
+
+[taxonomies]
+tags = ["rust", "async"]
++++
+
+When I tried to do async Rust, I got a bunch of errors from the borrow checker
+that, to me, it didn't make sense -- and wouldn't be an issue if I was using
+threads.
+
+It took me awhile to figure out a mental model for doing it right.
+
+<!-- more -->
+
+## A problem with naming
+
+I think my initial problem started with naming. The concept of async/await is
+quite recent, but for a long time we've been talking about "greenthreads" and
+"light-weight threads" -- "threads" that are managed by the application and not
+the OS. While there are some differences between greenthreads and async things, 
+the naming stuck with me (and I *think* I saw some posts linking the two).
+
+Still on naming, [Tokio](https://tokio.rs/), the most popular async framework
+in Rust, uses `task::spawn` to spawn a new task, which is pretty close to the
+thread call, `thread::spawn` -- and both return a structure called 
+`JoinHandle` -- so this mixture of "tasks/greenthreads are threads" got pretty
+ingrained to me.
+
+{% note() %}
+Yeah, yeah, other languages avoid this by using their own words, but my contact
+with async was with Rust, so...
+{% end %}
+
+## A problem with structure
+
+So you get this "async is thread" mentality due aproximation. And then you try
+to build something async using the same model.
+
+For example, a producer/consumer in Rust would be something like:
+
+```rust
+use std::sync::mpsc;
+use std::thread;
+
+fn main() {
+    let (tx, rx) = mpsc::channel();
+    let self_tx = tx.clone();
+
+    let consumer = thread::spawn(move || {
+        while let Ok(msg) = rx.recv() {
+            println!("Message: {}", msg);
+
+            if msg > 1000 {
+                // actually, we just need to drop self_tx, otherwise the consumer will keep waiting
+                // for inputs from it, even when tx was already dropped when the producer ended.
+                // the problem with a direct drop is that rustc can't see that it won't be used
+                // anymore.
+                break;
+            } else if msg % 2 == 0 {
+                if self_tx.send(msg * 2).is_err() {
+                    println!("Failed to push new value to consumer");
+                    break;
+                };
+            }
+        }
+    });
+
+    let producer = thread::spawn(move || {
+        for i in 1..12 {
+            if tx.send(i).is_err() {
+                println!("Failed to send {}, ending producer", i);
+                break;
+            }
+        }
+        // tx.send(0);
+    });
+
+    producer.join().unwrap();
+    consumer.join().unwrap();
+}
+```
+
+(Yeah, I did all in a single file. Sue me.)
+
+[async is channel, not spawn]
+