Iterator of TMap, TSet

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Lixing

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Feb 28, 2013, 3:05:25 PM2/28/13
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Hi,

I found the ScalaSTM Homepage says 'We have chosen to make TMap.View.iterator and TSet.View.iterator return an iterator over an atomic snapshot of the collection. The iterator method acts as if it has made a copy of the entire collection and returned an iterator over the copy. Concurrent updates can proceed uninhibited.'

I am wondering if this method is reasonable because I think the copy of the iterator may not represent the current keyset in the object. For example, after you copy the iterator, a transaction adds a key in TMap, and then you return the iterator. This iterator is old.

BTW, is it possible to share the implementation code for TMap iterator? I only found the interface in the project.

Thanks

Nathan Bronson

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Feb 28, 2013, 3:39:00 PM2/28/13
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Lixing,

The TMap implementation is part of the ScalaSTM source, it is in https://github.com/nbronson/scala-stm/blob/master/src/main/scala/scala/concurrent/stm/skel/HashTrieTMap.scala . The heavy lifting is all in a superclass that is shared between the set and the map: https://github.com/nbronson/scala-stm/blob/master/src/main/scala/scala/concurrent/stm/skel/TxnHashTrie.scala

If you are iterating outside a transaction it is not possible to provide both a consistent view of the map and the latest view of the map, since once iteration has begun the code using the iterator will have already seen part of the old state. It is certainly a design tradeoff to choose one or the other, but my experience is that the consistent snapshot is a more powerful and useful primitive. It is hard to implement (and very difficult to implement efficiently) at a level higher than the map itself. The freshness of the iterator, on the other hand, can be implemented either by rechecking or by running the iterating code inside a transaction.

 - Nathan


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Nathan Grasso Bronson
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Lixing

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Mar 1, 2013, 3:05:54 PM3/1/13
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Thanks. In that case,  I am confused how to make an atomic snapshot of the collection by avoiding the conflict of making a copy current key and value while another transaction is trying to modify the key and value.
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Nathan Bronson

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Mar 1, 2013, 3:29:43 PM3/1/13
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You are right that a transaction that modifies the map and a transaction that makes a snapshot will conflict. The snapshot can outlive its creating transaction or be created outside a transaction entirely, in which case it can be stale.

 - Nathan


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