Consider supporting FIFO semantics on the Priority Channel for tie breaks [INT-2031]

[spring-operator]

Oleg Zhurakousky opened INT-2031 and commented

Currently if Comparator is not provided and PRIORITY header is not set the place of the Message in the Priority channel is undetermined based on the implementation fo PriorityBlockingQueue. However I am suspecting that a typical use case would treat PriorityChannel as a FIFO QueueChannel with prioritization capabilities, sort of like an extension.

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Issue Links:

• Message store option is not available for priority-queue channel [INT-1870] #5858 Message store option is not available for priority-queue channel

• Change Message PRIORITY header to be of type Long (currently Integer) [INT-2112] #6096 Change Message PRIORITY header to be of type Long (currently Integer)
  ("is superseded by")

[spring-operator]

Oleg Zhurakousky commented

Since we can't fall back on LinkedBlockingQueue (use by QueueChannel) since we do have default Comparator which relies on the 'priority' header, it seems like the only way it coud be done is by checking if the Message does NOT have 'priority' header and inject one (after calculating it) using somewhat questionable technique giving the fact that Message is immutable.

if (message.getHeaders().getPriority() == null){	
	Map innerMap = (Map) new DirectFieldAccessor(message.getHeaders()).getPropertyValue("headers");
	innerMap.put(MessageHeaders.PRIORITY, this.calculateDefaultPriorityHeader());
}

However it is in spirit of the PriorityChannel, so I do believe we can make an exception here.

Anyway, it s ready for the review #62