Did you remember Chapter 1? We did a concurrent Hello World!
Here’s a quick reminder.
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channel =Channel(String).new
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10.times do
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spawn {
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channel.send "Hello?"
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}
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puts channel.receive
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end
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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.
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channel =Channel(String).new
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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.
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10.times do
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spawn {
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channel.send "Hello?"
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}
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puts channel.receive
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end
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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:
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channel =Channel(String).new
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channel.send "Hello?"# This blocks the program execution
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puts channel.receive
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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.
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spawn {
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channel.send "Hello?"
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}
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puts channel.receive
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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.