Interpretation of NR_BUFFER_FULL

[ericthegeek]

Originally Posted by Thomas

do I get you right that only the last message per PID + SD + CC needs to be available in the message queue?

Yes, that is my opinion.

Consider the following sequence:
Using the menu interface on the Fixture itself, a user changes the DMX address of a fixture to 300.

The fixture adds a "GET_RESPONSE DMX_START_ADDRESS 300" to the message queue.

Then, 15 minutes later (having never retrieved the queued message), the console does a "SET DMX_START_ADDRESS 301" followed by a "GET DMX_START_ADDRESS"

Because it's still busy processing the SET, the fixture ACK_TIMERs the GET. When it's ready, it adds "GET_RESPONSE DMX_START_ADDRESS 301" to the queue.

Herein lies the problem. You now have two different "GET_RESPONSE DMX_START_ADDRESS" messages in the queue. One for 300 and one for 301.

Since the console received an ACK_TIMER, it now does a "GET QUEUED_MESSAGE". If the responder has implemented a true queue, it receives a "GET_RESPONSE DMX_START_ADDRESS 300". This data is 15 minutes out of date.

If the responder implemented a true queue, any time you did a GET QUEUED_MESSAGE you'd have to completely drain the queue of all messages before you could act on any of the messages because there *might* be newer data waiting later in the queue. On a busy responder that's generating lots of queued messages you may never be able to drain the queue.

Thus, it's my opinion that responder should always return the most current data in response to a GET QUEUED_MESSAGE. Also, if the same message has been added to the queue multiple times, it only needs to be returned once in response to a GET QUEUED_MESSAGE until the data changes again.

Originally Posted by Thomas

This would result in that a queue that has the size of (nPID * nSD * 2) never reports NR_BUFFER_FULL except if status messages are not supported. Would this be according to the standard?

Status Messages and Queued Messages behave differently. What I've said applies to Queued Messages. Some of the same concerns may apply to Status Messages as well. Frankly, I don't think the best practices for status messages have been fully worked out yet, especially since the addition of STATUS_CLEARED in E1.20-2010.

Originally Posted by Thomas

I don't find a hint that allows to replace messages in the queue.

There's no text directly supporting this in the standard. It's entirely my opinion of best practices for how queued messages should work based on operational experience.