Do Go Channels Guarantee Order When Writes Are Blocked?

Do Go Channels Preserve Order When Blocked?

In Go, goroutines concurrently execute tasks, often communicating through channels. When multiple goroutines attempt to write to a non-blocking channel simultaneously, the order in which their values are sent is crucial. This question explores whether Go channels maintain order in such scenarios.

Unpredictable Order with Blocked Writes

The provided code snippet demonstrates a function, broadcast, that sends messages to a slice of channels:

<code class="go">func broadcast(c <-chan string, chans []chan<- string) {
    for msg := range c {
        go func() { ch <- msg }()
    }
}</code>

In this implementation, goroutines are used to asynchronously send messages to the channels to avoid blocking the caller. However, the question raises concerns about the order of messages received by each channel, especially when multiple writers are involved.

The Go channel specification states that when the channel has a capacity greater than zero, it behaves asynchronously. In such cases, writes succeed without blocking unless the channel is full. Messages are also received in the order they are sent.

Nevertheless, the specification remains silent on the order of writes when multiple goroutines experience blocking. This silence leads to the question: Are there any guarantees about the order of sends after a channel becomes unblocked?

Lack of Guarantees

The answer to this question is frustrating: no, there are no guarantees. Even when the channel has available capacity, the order in which multiple goroutines write to it cannot be assured.

Imagine a scenario where two goroutines are scheduled to send messages to the channel almost simultaneously. The goroutine that initiated first may not necessarily execute first, leading to unpredictable message order.

Therefore, it is crucial to understand that Go channels do not preserve order when goroutines experience blocking on writes. If the order of messages is critical, alternative mechanisms, such as queues or message brokers, should be considered.

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