Possible to set incoming message processed

[Jerry Shea]

Hi all,

I am using CQ 3.6.1.8 (IndexedChronicle) and would like to understand whether it is safe to mark an incoming message as processed while the chronicle is being appended to by another process.

The Chronicle Queue FAQ says that you cannot safely write to the same Chronicle from multiple processes but Peter has said before that you can set a message as processed after it has been read to ensure you don't process it twice.

Is this safe to do when the chronicle is being appended to by another process?

Thanks, Jerry

 

[Peter Lawrey]

Hello Jerry,

I have deleted the old FAQ.  The current one is

https://github.com/OpenHFT/Chronicle-Queue/blob/master/docs/FAQ.adoc

> Is this safe to do when the chronicle is being appended to by another process?

The ExcerptTailer/Excerpt can alter an existing Excerpt whether it is being appended or modified by another thread or process on the same machine.

For modifying the entry in a thread safe manner, I suggest using compareAndSetInt or compareAndSetLong to ensure the change is atomic.  You might want to have the Appender pad the message with 4/8 bytes so you can perform this update later.  I suggest putting in a timestamp so you can also see when a message was processed.

     if (tailer.compareAndSetLong(offset, System.nanoTime())) {

        // updated successfully

     }

 If you have multiple consumers you want to load balance between you can give each one an id like 1,2,3 and 

     if (tailer.compareAndSetInt(offset, workerId)) {

        // work is assigned to me

     } else {

        // some one else got there first

     }

Regards,

  Peter.

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[Jerry Shea]

Thanks Peter,

that is great to hear - my tests had shown that this did seem to behave reliably but I'm very pleased to have it confirmed.

Cheers, Jerry