Introduction to Clojure's STM
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This document discusses Clojure's software transactional memory (STM) which allows coordinated changes to shared state without locking. STM transactions are atomic, consistent, and isolated like database transactions. An example shows threads A-E changing shared state atomically using STM functions like dosync, set-ref, and alter instead of atoms which are not transactional. STM uses MVCC and snapshot isolation under the hood to enable parallel changes without blocking.
![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)))
4
Thursday, December 15, 11](https://image.slidesharecdn.com/clojure-stm-111214175555-phpapp02/85/Introduction-to-Clojure-s-STM-7-320.jpg)
Introduction to Clojure's STM
- 1. 遺鉛看逮顎姻艶s Software Transactional Memory @fronx at @cljugb 12/2011 Thursday, December 15, 11
- 2. whats it good for? Thursday, December 15, 11
- 3. its pretty much useless unless you have: concurrency shared state changes to state Thursday, December 15, 11
- 4. the pitch It allows you to do coordinated change without the complexity of locking. Rich Hickey http://www.infoq.com/interviews/hickey-clojure Thursday, December 15, 11
- 5. kind of like database transactions. but in memory. Thursday, December 15, 11
- 6. 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
- 7. 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))) 4 Thursday, December 15, 11
- 8. broken example (using atoms) Thursday, December 15, 11
- 9. 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
- 10. sharing state mutate in STM only! coordinated independent synchronous ref atom asynchronous agent receive message and return Thursday, December 15, 11
- 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 dont block ensure block writes by others io! IllegalStateException Thursday, December 15, 11
- 12. working and slightly different example (using STM) Thursday, December 15, 11
- 13. 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 dont block ensure block writes by others io! IllegalStateException Thursday, December 15, 11
- 14. what to think about is it okay if others make changes in parallel? commute multi-ref constraints? ensure Thursday, December 15, 11
- 15. how does it work? MVCC, snapshot isolation persistent data structures on-commit evaluation http://java.ociweb.com/mark/stm/article.html Thursday, December 15, 11