The concepts, as well as the Clojure implementation, presented in this article, may be useful for those designing and implementing some of the "distributed wiki" and "load-balancing" capabilities of 2.X; also for "realtime group editing" features I've seen mentioned in the survey. Also, the concepts here could help in designing finer-grained, yet "federalized/distributed" locking for concurrent access e.g. which might be needed for "collaborative" or simultaneous editing of xwiki documents, as well as load-balanced wikis. This also isn't just "starry eyed theory" -- via JSR223, these features could be integrated into the Xwiki Java API, giving a "concurrent scripting face" to the Xwiki API running on multiple wiki hosts. (see http://groups.google.com/group/clojure/msg/ad811b69d448e3db re JSR-223 status of Clojure). Software Transactional Memory <http://java.ociweb.com/mark/stm/article.html> - Overview <http://java.ociweb.com/mark/stm/article.html#Overview> - Lock-based Concurrency<http://java.ociweb.com/mark/stm/article.html#Locks> - Actor-based Concurrency<http://java.ociweb.com/mark/stm/article.html#Actors> - Transactional Memory <http://java.ociweb.com/mark/stm/article.html#TM> - Software Transactional Memory<http://java.ociweb.com/mark/stm/article.html#STM> (STM) - Persistent Data Structures<http://java.ociweb.com/mark/stm/article.html#PersistentDataStructures> - STM Implementations<http://java.ociweb.com/mark/stm/article.html#STMImplementations> - Clojure Overview<http://java.ociweb.com/mark/stm/article.html#ClojureOverview> - Clojure Reference Types<http://java.ociweb.com/mark/stm/article.html#ClojureReferenceTypes> - Clojure Validators and Watchers<http://java.ociweb.com/mark/stm/article.html#ClojureValidatorsAndWatchers> - Clojure STM - High Level<http://java.ociweb.com/mark/stm/article.html#ClojureSTMHigh> - Clojure STM - Low Level<http://java.ociweb.com/mark/stm/article.html#ClojureSTMLow> - Conclusion <http://java.ociweb.com/mark/stm/article.html#Conclusion> - References <http://java.ociweb.com/mark/stm/article.html#References> Clojure has built-in support for Concurrency: ( http://java.ociweb.com/mark/clojure/article.html#Concurrency )

Managing concurrency with locks is hard. It requires determining which objects need to be locked and when they need to be locked. These decisions need to be reevaluated when the code is modified or new code is added. If a developer forgets to lock objects that need to be locked or locks them at the wrong times, bad things can happen. These include deadlocks<http://en.wikipedia.org/wiki/Deadlock> and race conditions <http://en.wikipedia.org/wiki/Race_condition>. If objects are locked unnecessarily, there is a performance penalty.

Support for concurrency is one of the main reasons why many developers

choose to use Clojure. All data is immutable unless explicitly marked as mutable by using the reference types Var<http://java.ociweb.com/mark/clojure/article.html#Vars> , Ref <http://java.ociweb.com/mark/clojure/article.html#Refs>, Atom<http://java.ociweb.com/mark/clojure/article.html#Atoms> and Agent <http://java.ociweb.com/mark/clojure/article.html#Agents>. These provide safe ways to manage shared state and are described in the next section titled "Reference Types<http://java.ociweb.com/mark/clojure/article.html#ReferenceTypes> ".

Niels http://nielsmayer.com