tailer.nextIndex()

[Deepesh Mehta]

Hi

We currently use CQ Version 3 and trying to migrate to CQ Version 4. I have noticed that tailer.nextIndex() method is no longer supported.

This was extremely useful method for us to navigate the queue for presence of more data without actually reading the data.

I see in CQ Version 4 you can set the direction of the navigation using TailerDirection but which method will be called to navigate the queue?

Thanks & Regards

Deepesh Mehta

[Peter Lawrey]

Hi. 

   How is this used? Why don't you want to read the excerpt if it is there?

  We could ado a method to do this but first I would like to understand what it is used for.

Best Regards, Peter. 

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[Deepesh Mehta]

Hi Peter

We have use cases where in client wants to iterate the CQ from last known valid index.

In that situation we run a loop on presence of tailer.nextIndex() which returns back true if more data is present and also changes the tailer to point to next valid index.

During cycle roll over which by default was daily in Vanilla CQ this method proved very efficient instead of incrementing index by 1 and checking if it is valid.

 Something like below code.

protected static void searchForwards(Visitor visitor, long indexStart,

ExcerptTailer tailer) {

boolean keepGoing = true;

long index = AtsReadWriteUtil.findFirstDataAfterIndexStart(tailer,

indexStart);

if (index > NOT_FOUND) {

keepGoing = visitor.visit(index, tailer);

}

while (tailer.nextIndex() && keepGoing) {

keepGoing = visitor.visit(tailer.index(), tailer);

}

}

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[Peter Lawrey]

Hi,

   You can now use the ExcerptTailer.toEnd() method to wind to the end of the queue. (And toStart to wind to the start if you need to) If you change the direction to BACKWARD you can read backwards from the end.

Regards,

   Peter.

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[Deepesh Mehta]

Hi Peter

Post setting of direction which method will allow me to move the tailer to read the index (backward or forward)? . Will by calling index() method on tailer move the index to its next or previous value based on direction?

Thanks & Regards

Deepesh Mehta

[Rob Austin]

this will read the next document based on direction, 

try (DocumentContext dc = tailer.readingDocument()) {
     
}

please look at the test cases, net.openhft.chronicle.queue.impl.single.SingleChronicleQueueTest for further examples

[Peter Lawrey]

When you call index(N) the next message you read should N regardless of the direction of reading.

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