Multiple channels

Learn multiple cases inside select.

Multiple channels

select shows its power when waiting on many channels. case v := <-ch1: and case v := <-ch2: wait on two channels at once; whichever sends first runs.

This lets a goroutine gather data from many sources. For example a worker waits on both a jobs channel and a cancel channel; it reacts to whichever signals first.

Multi-channel pattern

A common pattern with multiple channels: one carries work, the other carries cancellation. select waits on both; if cancel arrives the goroutine exits, if work arrives it processes.

Combined with the context package this makes a strong cancellation mechanism. Workers are not killed; they check and exit themselves.

Exercise Wait on multiple channels

Inside main, add a select { case ... case ... } block and print multi ready with fmt.Println.

Show example solution

package main

import "fmt"

func main() {
    ch1 := make(chan string)
    ch2 := make(chan string)
    go func() { ch1 <- "çoklu hazır" }()
    select {
    case m := <-ch1:
        fmt.Println(m)
    case m := <-ch2:
        fmt.Println(m)
    }
}

Test yourself

If several cases are ready in select, what happens?
- The first case runs
- All run in order
- It errors
- A random one runs
Show answer

A random one runs — a random one runs. Not the first; random pick is fair.

Multiple cases

select {
case v := <-ch1:
    fmt.Println(v)
____:
    fmt.Println("ch2")
}

Show answer

select {
case v := <-ch1:
    fmt.Println(v)
case v := <-ch2:
    fmt.Println("ch2")
}

Match multi-channel pattern

ch1

Work
ch2

Cancel
select

Wait
random

Fair pick

Next lesson: default and non-blocking

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