introduction to clojure's stm

Clojure’s

SoftwareTransactionalMemory

@fronx at @cljugb 12/2011

Thursday, December 15, 11

what’s it good for?


it’s pretty much useless

unless you have:

concurrency

shared state

changes to state


the pitch

“It allows you to docoordinated changewithout the complexityof locking.”

— Rich Hickeyhttp://www.infoq.com/interviews/hickey-clojure


http://www.infoq.com/interviews/hickey-clojure

http://www.infoq.com/interviews/hickey-clojure

kind of likedatabase transactions.

but in memory.


example

ab c d

e

1 9 5 7 3 10 42 6 8

4

threads

shared state

changing state

4

solution: do it atomically!


atomsuser=> (def my-atom (atom 0))

#'user/my-atom

user=> @my-atom

0

user=> (swap! my-atom inc)

1

user=> @my-atom

1

user=> (swap! my-atom (fn [n] (* (+ n n) 2)))

4

(No STM here.)


broken example(using atoms)


changing state

atomic (acts as one point in time)

consistent (from valid state to valid state)

isolated (changes are local until committed)


sharing state

coordinated independent

synchronous

asynchronous

ref atom

– agent

mutate inSTM only!

receive messageand return


STM functions

ref

dosync

set-ref

alter

commute

ensure

io!

reference to a collection

transaction

set new value

set new value via a function

set new value and don’t block

block writes by others

IllegalStateException


working and slightly different example

(using STM)


STM functions

ref

dosync

set-ref

alter

commute

ensure

io!

reference to a collection

transaction

set new value

set new value via a function

set new value and don’t block

block writes by others

IllegalStateException


what to think about

is it okay if others make changes in parallel?

commute

multi-ref constraints?

ensure


how does it work?

MVCC, snapshot isolation

persistent data structures

on-commit evaluation

http://java.ociweb.com/mark/stm/article.html




introduction to clojure's stm

Technology

function commute

alter set

valid state

ref set

atom

stm

thursday

illegalstateexceptionthursday