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The Enemy of the State

Joy Heron
October 06, 2017

The Enemy of the State

State can cause multiple problems within our systems. What are those problems, and what can we do about it?

Joy Heron

October 06, 2017
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  1. DDD Building Blocks > immutable > no lifecycle > constant

    identity > value can change over time ValueObject Entities Aggregates > extra grouping > transactional boundary @iamjoyclark
  2. Commodore 64 Basic Jens Bendisposto https://twitter.com/jbendisposto/status/904662544245297152 1 LET JAR =

    0 10 DATA "GRANNY", "MONSTER", "GRANNY", "CAT", "END" 20 READ A$ 100 IF A$="GRANNY" THEN GOSUB 1010 200 IF A$="MONSTER" THEN GOSUB 2010 300 IF A$="CAT" THEN GOSUB 3020 400 IF A$="END" THEN GOTO 10000 500 GOTO 20 1010 LET JAR = JAR + 4 1015 PRINT "YOU LOOK SKINNY, GET SOME COOKIES" 1020 RETURN 2010 IF JAR = 0 THEN PRINT "NO COOOKIE, ME SAD" 2020 IF JAR > 0 THEN PRINT "OM NOM NOM" 2030 LET JAR = 0 2040 RETURN 3020 PRINT "MEOW" 3030 LET JAR = JAR - 1 3040 RETURN 10000 END READY.
  3. Object-Oriented class CookieJar def initialize(initial_count) @count = initial_count end def

    remove_cookies(amount) if amount > @count @count = 0 else @count -= amount end end def add_cookies(amount) @count += amount end end Lucas Dohmen https://gist.github.com/moonglum/f763eb257a4ede22e3008d861225535c
  4. Functional (with MONADS!) takeCookies n jar = if n >

    jar then 0 else jar - n cat = modify (takeCookies 1) grandma = modify (+10) cookieMonster = put 0 workflow = do cat cookieMonster grandma cookieMonster run = execState workflow 5 Martin Kühl https://gist.github.com/joyclark/843932d96dbf99ab8438cb9de485d10f
  5. Functional (defn take [{cookies ::cookies} to-take] (if (< to-take cookies)

    {::cookies (- cookies to-take)} {::cookies 0})) (defn cookie-monster [cookie-jar to-eat] (take cookie-jar to-eat)) (defn kitten [cookie-jar] (take cookie-jar 1)) (defn grandma [{cookies ::cookies} nr-baked] (if (pos? nr-baked) {::cookies (+ cookies nr-baked)} {::cookies cookies})) Joy Clark https://github.com/joyclark/cookie-jar
  6. We are not alone! A friendly ghost with an appetite

    for cookies has started haunting the kitchen. And if the ghost reaches into the cookie jar at the same time as someone else… @iamjoyclark
  7. In an otherworldly dimension… In the kitchen… cookie_jar = CookieJar.new(5)

    cookie_jar.remove_cookies(3) ⋮ amount > @count => false ⋮ @count -= amount ⋮ cookie_jar.count_cookies => -2 cookie_jar.remove_cookies(4) ⋮ amount > @count => false ⋮ @count -= amount @iamjoyclark
  8. Object-Oriented: Locks public IList<Cookie> TakeCookies(int aNumberOfCookies) { var cookies =

    new List<Cookie>(); lock (_jarLid) { // take cookies out!! } return cookies; } Benjamin Wolf https://github.com/programming-wolf/CookieJar
  9. Object-Oriented: Locks class CookieJar @@jarLid = Mutex.new def remove_cookies(amount) @@jarLid.synchronize

    do if amount > @count @count = 0 else @count -= amount end end end end Lucas Dohmen https://gist.github.com/moonglum/f763eb257a4ede22e3008d861225535c
  10. In an otherworldly dimension… In the kitchen… cookie_jar = CookieJar.new(5)

    cookie_jar.remove_cookies(3) ## The lid is off for kitten! amount > @count => false @count -= amount cookie_jar.count_cookies => 2 cookie_jar.remove_cookies(4) ## The lid is off for kitten! ## We’ll wait until it’s back on ## The lid is off for ghost! amount > @count => false @count = 0 cookie_jar.count_cookies => 0 @iamjoyclark
  11. Functional (def cookie-jar (atom {::cookies 5})) (defn ghost [cookie-jar] (take

    cookie-jar 2)) (swap! cookie-jar grandma 5) (swap! cookie-jar kitten) (swap! cookie-jar ghost) @cookie-jar // => 7: retrieves current value of cookie-jar Joy Clark https://github.com/joyclark/cookie-jar
  12. In an otherworldly dimension… In the kitchen… (def cookie-jar (atom

    {::cookies 5})) (swap! cookie-jar grandma 5) @cookie-jar => {::cookies 8} (swap! cookie-jar ghost) swap! retrieve {::cookies 5} (grandma {::cookies 5} 5) save! {::cookies 10} swap! retrieve {::cookies 5} (ghost {::cookies 5}) atom changed, retry! retrieve {::cookies 10} (ghost {::cookies 10}) save! {::cookies 8} @iamjoyclark
  13. Event Sourcing > The state of the application is determined

    by a sequence of business events. > Reproducibility of the states @iamjoyclark
  14. Event Sourcing Cookie Jar created 1 eaten 4 eaten 6

    eaten 10 baked current state @iamjoyclark
  15. How to Attack Think about the problem you are trying

    to solve. Identify essential state and encapsulate it. Eliminate unnecessary state. @iamjoyclark
  16. How to Attack Prefer immutable values which are thread safe.

    Use locking and synchronized blocks. @iamjoyclark Look into what that your language offers for managing concurrency
  17. How to Attack @iamjoyclark Derive the state from the events

    or transactions which have taken place Make sure that you are logging all necessary information