Comments on: CopyOnWriteArrayList concurrency fun

By: Jed Wesley-Smith

Jed Wesley-Smith — Mon, 16 Feb 2009 01:29:44 +0000

sorry, previous comment got stuffed up... Our package has both CopyOnWriteMap and CopyOnWriteSortedMap. These implementations can use any required implementation as the internal storage, and so support HashMap, LinkedHashMap, TreeMap etc. - as long as the "non-mutative" methods do not actually modify the internal structure, such as a LinkedHashMap with access ordering turned on.

By: Taylor Gautier

Taylor Gautier — Fri, 06 Feb 2009 10:30:03 +0000

@Alex – oh, I know, I said “related” :) The Clojure system reminds me of the NetApp filesystem, actually. And also some of the experimental ideas for a COWTreeMap Geert and I dreamt up so long ago…

By: Alex

Alex — Wed, 04 Feb 2009 13:27:40 +0000

@Taylor: The Clojure data structures differ in an important regard from the Java CopyOnWriteArrayList. On a write, COWAL copies the entire data structure, which makes writes suck. The Clojure data structures are “persistent” and use structural sharing. Instead of copying the entire data structure, they reuse almost all of the existing data structure, which means that writes are actually very efficient. The key thing of course is that the Clojure structures are always immutable which allows them to safely be reused in whole and part.

By: Taylor Gautier

Taylor Gautier — Wed, 04 Feb 2009 06:51:38 +0000

This looks somewhat related to both COWAL and COWMap : http://blog.higher-order.net/2009/02/01/understanding-clojures-persistentvector-implementation/

By: Alex

Alex — Tue, 03 Feb 2009 01:32:17 +0000

@Tim: Absolutely right as usual.