Introduction to Clojure's STM

Clojure’s Software Transactional Memory @fronx at @cljugb 12/2011Thursday, December 15, 11

what’s it good for?Thursday, December 15, 11

it’s pretty much useless unless you have: concurrency shared state changes to stateThursday, December 15, 11

the pitch “It allows you to do coordinated change without the complexity of locking.” — Rich Hickey http://www.infoq.com/interviews/hickey-clojureThursday, December 15, 11

kind of like database transactions. but in memory.Thursday, December 15, 11

example shared state 1 9 5 7 3 10 4 2 6 8 changing state 4 4 threads a b c d e solution: do it atomically!Thursday, December 15, 11

user=> (def my-atom (atom 0)) #user/my-atom atoms (No STM here.) user=> @my-atom 0 user=> (swap! my-atom inc) 1 user=> @my-atom 1 user=> (swap! my-atom (fn [n] (* (+ n n) 2))) 4Thursday, December 15, 11

broken example (using atoms)Thursday, December 15, 11

changing state atomic (acts as one point in time) consistent (from valid state to valid state) isolated (changes are local until committed)Thursday, December 15, 11

sharing state mutate in STM only! coordinated independent synchronous ref atom asynchronous – agent receive message and returnThursday, December 15, 11

STM functions ref reference to a collection dosync transaction set-ref set new value alter set new value via a function commute set new value and don’t block ensure block writes by others io! IllegalStateExceptionThursday, December 15, 11

working and slightly different example (using STM)Thursday, December 15, 11

STM functions ref reference to a collection dosync transaction set-ref set new value alter set new value via a function commute set new value and don’t block ensure block writes by others io! IllegalStateExceptionThursday, December 15, 11

what to think about is it okay if others make changes in parallel? commute multi-ref constraints? ensureThursday, December 15, 11

how does it work? MVCC, snapshot isolation persistent data structures on-commit evaluation http://java.ociweb.com/mark/stm/article.htmlThursday, December 15, 11

Introduction to Clojure's STM

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