What is Eventual Consistency?

Replicas are not updated atomically, but eventually converge.

Conflict Resolution

{:foo 1 :bar 2}
{:baz 1}
{:bar 7}

(require 'knockbox.core)
(knockbox.core/resolve [a b c d])

(require 'knockbox.core)
(require '[knockbox.sets :as kbsets])

(def original (into (kbsets/lww)
              #{:mug :kettle}))

(def a (disj original :kettle))
(def b (conj original :coffee))

(def c (conj original :coffee-roaster))
;; this one wins because its
;; timestamp is later
(def d (disj original :coffee-roaster))

(println (knockbox.core/resolve
  [a b c d]))
;; => #{:coffee :mug}

(def original
  (into (kbsets/observed-remove)
        #{:crust :sugar :egg}))

(def a (conj original :blueberries))
(def b (conj original :blueberries))

(def c (disj a :blueberries))

(println (knockbox.core/resolve [b c]))
;; => #{:crust :sugar :egg :blueberries}

(require '(knockbox core registers))

(def a (knockbox.registers/lww 1))
(def b (knockbox.registers/lww 2))
(def c (knockbox.registers/lww 3))

(.value a)
;; => 1

(.value (knockbox.core/resolve [c b a]))
;; => 3

        Coordinated Synchronous
Ref     Yes         Yes
Atom    No          Yes
Agent   No          No

Avout

Protocol

(resolve [a b])
(gc [this time items])

(conj a :foo)
(assoc b :bar [1 2 3])

A comprehensive study of Convergent and Commutative Replicated Data Types

Shapiro, Preguiça, Baquero, Zawirski

http://bit.ly/crdt-paper

#{:foo :bar}

#{:bar :baz}

(clojure.set/union ...)

but what about...

#{:foo :bar}

(disj #{:bar :qux} :bar)

{:adds #{:foo :qux}
 :dels #{:bar}}

better, but can't add something back

This is where time comes in... (logical or physical)

{:adds {:foo #{"3359ce86" ...}}
 :dels {:foo #{"4ae98f10"}}}

Observed-Remove Set

(defbox Account
  (:name       LWWRegister
               :required true)
  (:address    LWWRegister
               :required true)
  (:followers  ObservedRemoveSet
               :default (lww-set)))

(def person (Account.))

(-> person
  (update-in [:followers] conj :bar)
  (assoc-in  [:name]      "Reid")
  (assoc-in  [:address]   "27 Main"))

Garbage Collection

with no coordination

Deleted/old item comes back

(gc [this
     gc-max-seconds
     gc-max-items])

Counters are hard

{:a 35
 :b 43
 :c 125
 :d 93}

Uses Midje

(= (knockbox.core/resolve ?z)
   (knockbox.core/resolve
     (reverse ?z)))

(= ?x (deserialize (serialize ?x))