while working on my actor model concurrency for freepascal i met the following problem.
Each actor has a mailbox, that is made of the following components :
A standard TObjectQueue wich holds the messages
A TMultiReadExclusiveWriteSynchronizer that avoids race conditions while acessing the TObjectQueue.
A TEventObject that is used to signalize when a new object is pushed into the queue. Usefull if the actor wants a message but the queue is empty. (Writer triggers the signal and the reader sleeps until the signal is received).
Well, the problem i hit is the following :
Each message has a transactionID value that is used to differentiate among transactions. This is usefull when you need a synchronous transfer to happen on the fully assynchronous actor model.
An actor sends a request message and waits for a response message, both with the same transaction id.
While developing this framework i set myself the task of making it with two guarantees that are not much common on other actor model implementations :
Guaranteed message order and Guaranteed message delivery.
well well, thats where i need a TObjectQueue with ability to dequeue/inspect arbitrary positions :
When the object issues request it has to keep looping (until timeout) and dequeueing the last message from the mailbox, inspecting its transaction id for a match and requeueing the same message back into the mailbox if theres no match.
Thats the problem. When messages are requeued i lose message order guarantees (Very usefull). They are still guaranteed to be delivered, but not in order anymore.
Finishing the long story, i need a TObjectQueue that can access/extract arbitrary positions so as to avoiding losing the order of message delivery.
Does anybody have one already done/debugged and in GPL license ?
Thanks in advance,
J. Aldo.