Chapter 8: Concurrency and Channels
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.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.