Does Channel Blocking Guarantee Order Preservation in Go?

Channel Order Preservation with Blocking

When dealing with a slice of channels that receive the same message, it's crucial to understand if channels preserve order when blocked. The spec states that in an asynchronous channel (with a capacity greater than zero), elements are received in the order they are sent if the buffer is not full.

However, what happens when a channel becomes blocked due to multiple goroutines attempting to write to it? The question arises: are there any guarantees about the order of sends after the channel becomes unblocked?

Answer: No Guarantees

Unfortunately, there are no such guarantees. Even when the channel is not full, if two goroutines are started simultaneously to send to it, there's no assurance that the goroutine initiated first will execute first. The sequence of execution and message arrival order is unpredictable.

This is because the scheduler's behavior is non-deterministic. The runtime system manages goroutines and decides which ones to execute when, and it's not bound by any order constraints. Consequently, the messages may not arrive at their destinations in the order they were sent, especially after a channel becomes unblocked.

Implications

This lack of order preservation imposes limitations on scenarios where the message order is critical. If the order of messages is crucial, alternative approaches should be considered, such as using synchronized channels or other synchronization primitives to enforce the desired order.

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