Hazelcast IList does not honour List behaviour (ordering, methods)

[häxx]

Hello folks.

I am concerned that I am missing something rather basic.
When getting a distributed List from Hazelcast, similar to the code
samples shown on the wiki and hazelcast.com sites, the behaviour
differs from the expected behaviour of java.util.Lists.

This seems very basic, but I can't see that I'm doing anything wrong.
Attaching a small testcase which fails in two ways:

a) The ordering of the elements in IList does not match the expected
{1, 2, 3}. Instead, the elements are unordered, similar to an ISet
implementation.
b) The List.get() method calls throw exceptions (more specifically
java.lang.UnsupportedOperationException
at com.hazelcast.impl.BaseCollection.get(BaseCollection.java:36))

What am I missing here?

@Test
public void validateOrderingInDistributedList() {

// Assemble
final HazelcastInstance cacheNode =
Hazelcast.newHazelcastInstance(HazelcastCache.readConfigFile(configFile));
final List<String> unitUnderTest =
cacheNode.getList("testList");

// Act
unitUnderTest.add("1");
unitUnderTest.add("2");
unitUnderTest.add("3");

// Assert
Assert.assertEquals(3, unitUnderTest.size());
Assert.assertEquals("1", unitUnderTest.get(0));
Assert.assertEquals("2", unitUnderTest.get(1));
Assert.assertEquals("3", unitUnderTest.get(2));
}

[Fuad Malikov]

Hi,

Hazelcast List does not preserve order. This is basically for performance reasons. It is very costly to order the items.

May be we should have documented this much better.

Fuad

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@fuadm

[Andy Burgess]

.... or, more appropriately perhaps, what Hazelcast calls a List shouldn't implement the List interface. In my opinion, the Hazelcast.getList(...) method should instead be Hazelcast.getBag(...) or Hazelcast.getMultiSet(...) and the returned type should directly implement the Collection interface. See http://download.oracle.com/javase/1.4.2/docs/api/java/util/Collection.html

As an aside, surely you could implement a proper ordered List, no matter how costly, because aren't you currently doing just that for Queue? As long as it's documented as costly, the choice of whether or not to use it would rest with the end-user.

Comments?

Regards,
Andy.

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[Talip Ozturk]

Nice feedback. Makes sense. We cannot jump in and change things right
now but we should address this issue in the future.

http://twitter.com/oztalip

[häxx]

Hello all,

I believe we should do one of 2 things:

a) Implement the java.util.List - including its behaviours - fully. If
the implementation implies a performance hit - document it clearly so
people can decide for themselves.
b) Drop the java.util.List interface from the IList implementation,
and move to Collection instead.

Personally, I would prefer the former choice.
I also believe we can implement the list interface on top of a Queue
implementation, since that implementation does not use a Map under the
covers. It seems that Queue elements *are* ordered, judging from the
implementation.

[Talip Ozturk]

Yes. Very possible. Queue is ordered.

http://twitter.com/oztalip

[Dan Syrstad]

I just ran into this too in Hazelcast 2.5. IQueue.iterator() has the same ordering problem. It seems that this has not been addressed yet. Any updates?
-Dan

[darri...@bifrostengine.com]

Recent documentation claims and testing shows order is preserved: http://docs.hazelcast.org/docs/3.5/manual/html/list.html. I let this old post mislead me for awhile, so I thought I'd leave a breadcrumb for the next person.

[Peter Veentjer]

Correct. Hazelcast 3 IList has been completely rewritten. Ordering is preserved.

On Tue, Feb 2, 2016 at 1:54 AM, <darri...@bifrostengine.com> wrote:

Recent documentation claims and testing shows order is preserved: http://docs.hazelcast.org/docs/3.5/manual/html/list.html. I let this old post mislead me for awhile, so I thought I'd leave a breadcrumb for the next person.

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