Hazelcast Ringbuffer readManyAsync returns empty results

Question

I am trying to read N items from RingBuffer using readManyAsync, but always returns an empty result set. If I use readOne, I get the data.

I am using readManyAsync as indicated in the documentation. Is there any other way to do this?

Enviroment:

• Java 8
• Hazelcast 3.5.3

Example:

Ringbuffer<String> buffer = this.hazelcastInstance.getRingbuffer("testBuffer"); buffer.add("a"); buffer.add("b"); buffer.add("c"); Long sequence = buffer.headSequence(); ICompletableFuture<ReadResultSet<String>> resultSetFuture = buffer.readManyAsync(sequence, 0, 3, null); ReadResultSet<String> resultSet = resultSetFuture.get(); System.out.println("*** readManyAsync *** readCount: " + resultSet.readCount()); int count = 0; for (String s : resultSet) { System.out.println(count + " - " + s); count++; } System.out.println("*** readOne ***"); for (int i = 0; i < 3; i++) { System.out.println(i + " - " + buffer.readOne(i)); } 

Output:

 *** readManyAsync *** readCount: 0 *** readOne *** 0 - a 1 - b 2 - c 

Answer 1

You are satisfied with getting zero results:

buffer.readManyAsync (sequence, 0, 3, null);

Try changing 0 to 1.

buffer.readManyAsync (sequence, 1, 3, null);

Now the call will be blocked until at least 1 result is achieved.

You can probably make things more efficient by requesting more than three items. In most cases, getting data is cheap, but io / operation planning is expensive. So try to get as far as you can. Therefore, try to get as many results as possible. 100 ... or 1000 (which is the maximum).

Answer 2

Ok, but how do you use readManyAsync in a non-blocking way, from minCount to 0?

I made a minimal test case, and I really can't figure it out. I posted a support topic here:

https://groups.google.com/forum/#!topic/hazelcast/FGnLWDGrzb8

As an answer: I am using readManyAsync with a timeout, for example:

 try{ buffer.readManyAsync(sequence, 1, 3, null).get(500, TimeUnit.MILLISECONDS); } catch (TimeoutException e){ // We timed out, shame, let move on } 

This is the only way to make an elegant non-blocking thread, but while reading the document, I really thought minCount = 0 would do the trick.