[fpc-pascal] TObjectQueue with ability to dequeue arbitrary positions

Jorge Aldo G. de F. Junior jagfj80 at gmail.com
Fri Aug 24 20:57:40 CEST 2012

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.

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