Part 24: Select

Welcome to tutorial no. 24 in Golang tutorial series.

What is select?

The select statement is used to choose from multiple send/receive channel operations. The select statement blocks until one of the send/receive operation is ready. If multiple operations are ready, one of them is chosen at random. The syntax is similar to switch except that each of the case statement will be a channel operation. Lets dive right into some code for better understanding.

Example

package main

import (  
    "fmt"
    "time"
)

func server1(ch chan string) {  
    time.Sleep(6 * time.Second)
    ch <- "from server1"
}
func server2(ch chan string) {  
    time.Sleep(3 * time.Second)
    ch <- "from server2"

}
func main() {  
    output1 := make(chan string)
    output2 := make(chan string)
    go server1(output1)
    go server2(output2)
    select {
    case s1 := <-output1:
        fmt.Println(s1)
    case s2 := <-output2:
        fmt.Println(s2)
    }
}

Run in playground

In the program above, the server1 function in line no. 8 sleeps for 6 seconds then writes the text from server1 to the channel ch. The server2 function in line no. 12 sleeps for 3 seconds and then writes from server2 to the channel ch.

The main function calls the go Goroutines server1 and server2 in line nos. 20 and 21 respectively.

In line no. 22, the control reaches the select statement. The select statement blocks until one of its cases is ready. In our program above, the server1 Goroutine writes to the output1 channel after 6 seconds whereas the server2 writes to the output2 channel after 3 seconds. So the select statement will block for 3 seconds and will wait for server2 Goroutine to write to the output2 channel. After 3 seconds, the program prints,

from server2  

and then will terminate.



Practical use of select

The reason behind naming the functions in the above program as server1 and server2 is to illustrate the practical use of select.

Lets assume we have a mission critical application and we need to return the output to the user as quickly as possible. The database for this application is replicated and stored in different servers across the world. Assume that the functions server1 and server2 are in fact communicating with 2 such servers. The response time of each server is dependant on the load on each and the network delay. We send the request to both the servers and then wait on the corresponding channels for the response using the select statement. The server which responds first is chosen by the select and the other response is ignored. This way we can send the same request to multiple servers and return the quickest response to the user :).

Default case

The default case in a select statement is executed when none of the other case is ready. This is generally used to prevent the select statement from blocking.

package main

import (  
    "fmt"
    "time"
)

func process(ch chan string) {  
    time.Sleep(10500 * time.Millisecond)
    ch <- "process successful"
}

func main() {  
    ch := make(chan string)
    go process(ch)
    for {
        time.Sleep(1000 * time.Millisecond)
        select {
        case v := <-ch:
            fmt.Println("received value: ", v)
            return
        default:
            fmt.Println("no value received")
        }
    }

}

Run in playground

In the program above, the process function in line no. 8 sleeps for 10500 milliseconds (10.5 seconds) and then writes process successful to the ch channel. This function is called concurrently in line no. 15 of the program.

After calling the process Goroutine concurrently, an infinite for loop is started in the main Goroutine. The infinite loop sleeps for 1000 milliseconds (1 second) during the start of each iteration and them performs a select operation. During the first 10500 milliseconds, the first case of the select statement namely case v := <-ch: will not be ready since the process Goroutine will write to the ch channel only after 10500 milliseconds. Hence thedefault case will be executed during this time and the program will print no value received 10 times.

After 10.5 seconds, the process Goroutine writes process successful to ch in line no. 10. Now the first case of the select statement will be executed and the program will print received value: process successful and then it will terminate. This program will output,

no value received  
no value received  
no value received  
no value received  
no value received  
no value received  
no value received  
no value received  
no value received  
no value received  
received value:  process successful  



Deadlock and default case

package main

func main() {  
    ch := make(chan string)
    select {
    case <-ch:
    }
}

Run in playground

In the program above, we have created a channel ch in line no. 4. We try to read from this channel inside the select in line no. 6. The select statement will block forever since no other Goroutine is writing to this channel and hence will result in deadlock. This program will panic at runtime with the following message,

fatal error: all goroutines are asleep - deadlock!

goroutine 1 [chan receive]:  
main.main()  
    /tmp/sandbox416567824/main.go:6 +0x80

If a default case is present, this deadlock will not happen since the default case will be executed when no other case is ready. The program above is rewritten with a default case below.

package main

import "fmt"

func main() {  
    ch := make(chan string)
    select {
    case <-ch:
    default:
        fmt.Println("default case executed")
    }
}

Run in playground

The above program will print,

default case executed  

Similarly the default case will be executed even if the select has only nil channels.

package main

import "fmt"

func main() {  
    var ch chan string
    select {
    case v := <-ch:
        fmt.Println("received value", v)
    default:
        fmt.Println("default case executed")

    }
}

Run in playground

In the program above ch is nil and we are trying to read from ch in the select in line no. 8. If the default case was not present , the select would have blocked forever and caused a deadlock. Since we have a default case inside the select, it will be executed and the program will print,

default case executed  

Random selection

When multiple cases in a select statement are ready, one of them will be executed at random.

package main

import (  
    "fmt"
    "time"
)

func server1(ch chan string) {  
    ch <- "from server1"
}
func server2(ch chan string) {  
    ch <- "from server2"

}
func main() {  
    output1 := make(chan string)
    output2 := make(chan string)
    go server1(output1)
    go server2(output2)
    time.Sleep(1 * time.Second)
    select {
    case s1 := <-output1:
        fmt.Println(s1)
    case s2 := <-output2:
        fmt.Println(s2)
    }
}

Run in playground

In the program above, the server1 and server2 go routines are called in line no. 18 and 19 respectively. Then the main program sleeps for 1 second in line no. 20. When the control reaches the select statement in line no. 21, server1 would have written from server1 to the output1 channel and server2 would have written from server2 to the output2 channel and hence both the cases of the select statement are ready to be executed. If you run this program multiple times, the output will vary between from server1 or from server2 depending on which case is chosen in random.

Please run this program in your local system to get this randomness. If this program is run in the playground it will print the same output since the playground is deterministic.

Gotcha - Empty select

package main

func main() {  
    select {}
}

Run in playground

What do you think will be the output of the program above?

We know that the select statement will block until one of its cases is executed. In this case the select statement doesn't have any cases and hence it will block forever resulting in a deadlock. This program will panic with the following output,

fatal error: all goroutines are asleep - deadlock!

goroutine 1 [select (no cases)]:  
main.main()  
    /tmp/sandbox299546399/main.go:4 +0x20

This brings us to an end of this tutorial. Have a good day.

Next tutorial - Mutex