Go Channel Axioms

Dec 14, 2015

A while ago I was watching a tech talk by Blake Caldwell on Building Resilient Services with Go. In his presentation he had a slide which listed go channel axioms. I have listed his channel axioms here and provided some short code snippets to hopefully clarify them.

A send to a nil channel blocks forever

var ch chan bool
ch <- true // will always block

We declare a channel ‘ch’ but do not initialize it (with make), so it is a nil channel. We then attempt to send a value down the channel, this causes a blocking operation.

A receive from a nil channel blocks forever

var ch chan bool
v := <-ch // will always block

We declare a channel ‘ch’ but do not initialize it (with make), so it is a nil channel, We then attempt to receive a value from the channel, this causes a blocking operation.

A send to a closed channel panics

ch := make(chan bool)
...
close(ch)
...
ch <- true // panics!

We initialize a channel ‘ch’ then later close the channel, and then later attempt to send a value down the channel. This causes a panic, and we receive a stack trace. In this case, before we send a value down the channel, we may want check if the channel is open:

if v, ok := <-ch; ok {
    // ch is open
}

A receive from a closed channel returns the zero value immediately

ch := make(chan int)
...
close(ch)
...
v := <-ch // v = 0

We initialize a channel ‘ch’ then later close the channel, and then later attempt to receive a value from the channel. This causes the zero value to be returned immediately, this is a non-blocking operation.