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

Consumer does this:

1) transaction: {mark next message as "taken"}
2) send message
3) transaction: {mark message as "sent"}

On recovery after system crash:
For every "taken" but not "sent" message, one of these:
- Drop it. OR
- Query the destination system to see it if already has this message
and resend if not. OR
- Send it potentially redundantly.

Klaus


On Thu, Jun 21, 2012 at 10:24 PM, Trever Shick <[email protected]> wrote:
> 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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-- 
Valeu, Klaus.

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Exclusive live event will cover all the ways today's security and 
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