Did you remember Chapter 1? We did a concurrent Hello World!

Here’s a quick reminder.

channel = Channel(String).new
10.times do
  spawn {
    channel.send "Hello?"
  }
  puts channel.receive
end

In Crystal we use the keyword spawn to make something work in the background without blocking the main execution.

To achieve this spawn creates a lightweight thread called Fiber. Fibers are very cheap to create and you can easily create tens of thousands of Fibers on a single core.

Okay, that’s really cool! We can use spawn to make stuff work in the background but how do we get something back from a Fiber.

Now that’s where Channels come to play.

Channel

As the name stands a Channel is a channel between a sender and the receiver. Therefore a Channel lets each other communicate with send and receive methods.

Let’s take a line by line look at our previous example.

channel = Channel(String).new

We create a Channel with Channel(String).new. Note that we are creating a Channel which will send and receive messages with type of String.

10.times do
  spawn {
    channel.send "Hello?"
  }
  puts channel.receive
end

Leaving the loop aside, we are sending a message to our channel inside spawn. You might ask ‘Why are we sending message in the background?’ Well, send is a blocking operation and if we do that in the main program we gonna block the program forever.

Consider this:

channel = Channel(String).new
channel.send "Hello?" # This blocks the program execution
puts channel.receive

What’s the output of this program? Actually this program won’t ever finish because it gets blocked by channel.send "Hello?". Now that we know why we use spawn to send a message let’s continue.

spawn {
  channel.send "Hello?"
}
puts channel.receive

We just sent a message through our channel in the background with spawn. Then we receive it back with channel.receive. In this example the message is Hello? so this program prints Hello?and then finishes.