Do buffered channels maintain order?

Question

In Go, do buffered channels have any order guarantee?

For example: you have two goroutines A & B that share a channel. A pushes data onto the channel while B reads from it. Are you guranted that B will read data In the same order that A put it into the channel?

I understand that if there are multiple producers or consumers the order may be non-deterministic, but I'm specifically asking about having just 1 producer and 1 consumer.

Answer 1

"Are you guaranteed that B will read data In the same order that A put it into the channel?"

Yes. The order of data is guaranteed.
But the delivery is guaranteed only for unbuffered channels, not buffered.
(see second section of this answer)

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You can see the idea of channels illustrated in "The Nature Of Channels In Go" from William Kennedy (Feb. 2014): it shows how the order or read/write is respected.
See also Channels:

Receivers always block until there is data to receive.

• If the channel is unbuffered, the sender blocks until the receiver has received the value.
• If the channel has a buffer, the sender blocks only until the value has been copied to the buffer; if the buffer is full, this means waiting until some receiver has retrieved a value.

Unbuffered Channels

Buffered Channel

^{Image source: Ardan labs - William Kennedy}

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The same William Kennedy details the Guarantee Of Delivery aspect in "The Behavior Of Channels" (Oct 2017)

Do I need a guarantee that the signal sent by a particular goroutine has been received?

^{Image source: Again, Ardan labs - William Kennedy}

The three channel options are Unbuffered, Buffered >1 or Buffered =1.

• Guarantee

  • An Unbuffered channel gives you a Guarantee that a signal being sent has been received.
    • Because the Receive of the signal Happens Before the Send of the signal completes.

• No Guarantee

  • A Buffered channel of size >1 gives you No Guarantee that a signal being sent has been received.
    • Because the Send of the signal Happens Before the Receive of the signal completes.

• Delayed Guarantee

  • A Buffered channel of size =1 gives you a Delayed Guarantee. It can guarantee that the previous signal that was sent has been received.
    • Because the Receive of the First Signal, Happens Before the Send of the Second Signal completes.