NOOB Question - Get / lock / update (unserializable transaction)

I'm attempting to use Prevayler as a journalling mechanism for a distributed queueing mechanism.
I have a producer and 'n' consumers across the network.


If i send a message i
1) add it to my prevalent system (chat queue) via a transaction
2) send the message
3) remove it via a transaction

this works, no issues.

my issue is when i have no consumers.  i must queue up the messages and when a consumer comes online, i need to send all the queued messages.

since my producer has multiple threads i cannot do a :
1) query for queue 'head' (in a query tx)
2) send message
3) remove message via tx

because Thread 1 after executing #1 could pause, where in Thread 2 exeutes #1. they both have the same message object now and i will get dups.


I temporary fixed the issue by synchronizing on the prevalentSystem() as that's what execute() syncs on (per the doc).


What i thought i could do, or what i'd like to do is to have a 'send message transaction' that would :
1) send the message in executeOn()
2) mark the message sent
all in one tx.

I can't do this however because after de-serialization i have no 'output queue'.

This is understood and that's fine.

what i need to do though is to have a nice clean way to bundle the three operations in a threadsafe manner in the prevayler.

I need prevayler to have a method like 'executeSafe(PrevaylerCallback);

interface PrevaylerCallback {
	void executeWith(Prevayler prevayler) ;
}
executeSafe would synchronize appropriately and then allow me to run my query (tx) / send / update (tx) safely.

so, for example i would have a 
prevayler.executeSafe(new PrevaylerCallback { 
  void executeWith(Prevayler prevayler) {
    while (true) {
      String message = prevayler.query(new Query() { // get queue 'top' });
      if (message == null) break;
      channel.send(message);
      prevayler.execute(new RemoveMessageTransaction(message));
    }
  }
});


Am i going about this the wrong way?  Is there something there that would help me already?  Do i need to fork the code and add the PrevaylerCallback?

Thanks for any help.

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