Thursday, September 1, 2011

java concurrent hash map iterator consistency

As mentioned repeatedly in other blog posts, thread safety primarily means liveness + consistency. Consistency with reality.

For any collection, counting the size of the collection should reflect a _snapshot_. On a multiprocessor machine, if Thread A adds 3, while Thread B removes 100 items all within the same time window, over a 1000-element collection, all after I start counting, then I should eventually return a count of 1000 or 903. If I were to return 1003, then that's inconsistent with reality, since at no time is the collection 1003-long??

Now let's turn to iterators. If one thread is iterating while another thread write a structural change, one valid result (consistent) is the create-time snapshot of the underlier. Another valid result would be that snapshot + changes in the unseen items, assuming all the changes are in the unseen items. Personally, I see this as a consistent snapshot -- the snapshot taken after the changes.

What if the change happens ON a segment already visited? Iterator already "shipped" them to client, so I feel we had better ignore those changes and all future changes. Use the create-time snapshot instead??

Brian Goetz said -- Iterators returned by ConcurrentHashMap.iterator() will return each element once at most and will not ever throw ConcurrentModificationException, but may or may not reflect insertions or removals that occurred after creation. No table-wide locking is needed (or even possible) to provide thread-safety when iterating the collection. Note size() does need table-wide locking.

http://tech.puredanger.com/2009/02/02/java-concurrency-bugs-concurrentmodificationexception/ points that CHM iterator is _NOT_ a snapshot iterator.

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