Channels

All our spawned threads have been fairly short-lived so far.
Get some input, run a computation, return the result, shut down.

For our ticket management system, we want to do something different: a client-server architecture.

We will have one long-running server thread, responsible for managing our state, the stored tickets.

We will then have multiple client threads.
Each client will be able to send commands and queries to the stateful thread, in order to change its state (e.g. add a new ticket) or retrieve information (e.g. get the status of a ticket).
Client threads will run concurrently.

Communication

So far we've only had very limited parent-child communication:

  • The spawned thread borrowed/consumed data from the parent context
  • The spawned thread returned data to the parent when joined

This isn't enough for a client-server design.
Clients need to be able to send and receive data from the server thread after it has been launched.

We can solve the issue using channels.

Channels

Rust's standard library provides multi-producer, single-consumer (mpsc) channels in its std::sync::mpsc module.
There are two channel flavours: bounded and unbounded. We'll stick to the unbounded version for now, but we'll discuss the pros and cons later on.

Channel creation looks like this:

use std::sync::mpsc::channel;

let (sender, receiver) = channel();

You get a sender and a receiver.
You call send on the sender to push data into the channel.
You call recv on the receiver to pull data from the channel.

Multiple senders

Sender is clonable: we can create multiple senders (e.g. one for each client thread) and they will all push data into the same channel.

Receiver, instead, is not clonable: there can only be a single receiver for a given channel.

That's what mpsc (multi-producer single-consumer) stands for!

Message type

Both Sender and Receiver are generic over a type parameter T.
That's the type of the messages that can travel on our channel.

It could be a u64, a struct, an enum, etc.

Errors

Both send and recv can fail.
send returns an error if the receiver has been dropped.
recv returns an error if all senders have been dropped and the channel is empty.

In other words, send and recv error when the channel is effectively closed.

Exercise

The exercise for this section is located in 07_threads/05_channels