Thoughts on consistency models

cap theorem• Eric Brewer (ex-Inktomi)• Proved by Lynch and Gilbert

cap theoremIt is impossible in the asynchrounous network model to implement a read/write object that garantuees the following properties: - Availability - Atomic consistency in fair transactions Or: If the network is broken, your database won’t work

AP vs CP• Real choices are • Available - Partition • Consistent - Partion

AP• Multiple Nodes participate in writes• System will be Eventually Consistent • Storage System guarantees if there are no new updates, all reads will eventually return the same, last updated valueExamples:- DNS- ASync replication- MongoDB with Slave-OK- Memcache

eventual consistency Master Slave Slave Client ClientAsuming update 1,2,3,4,5Client will expect 1,2,2,2,3,4,5,5,5

eventual consistency Master Slave Slave Client ClientHowever, we could get this: 1,2,2,4,2,5

eventual consistency• Monotonic read consistency• Pin client to certain slave / app server • Failover still fails

multi masterDynamo modelR - number of servers to read fromW - number of servers to get response fromN - Replication FactorR + W > N has nice properties

multi masterExample 1 Example 2R + W <= N R +W > NR=1 R=2 R =1W=1 W=1 W=2N=5 N=2 N=2Possibly Stale Data ‘Consistent’ DataHigher Availability

R +W > N If R + W > N you can’t both have fast local reads and writes

network partitions

trivial network partition

network write possibilities• deny all writes • read fully consistent data• allow writes on one side • allow reads on other side (stale)• allow writes on both sides • give up consistency

multiple writer strategies • Last one wins • vector clocks • Insert • insert often means: • if (!exist(x)) set(x) • exist is hard to implement in eventually consistent systems

deleteop1: set joe, age 40op2: delete joeop3: set joe, 41- consider switching 2 and 3- tombstone: remember delete and apply last opwins

multiple writer strategies• programmatic merge • store ops instead of state • replay operations • did I get the last one ?• Commutative operations • conﬂict free • anything that’s foldable

CP• Sometimes we need global state• Unique - constraints• User registration• ACL changes

Finallyuptime(CP + average developer)>=uptime(AP + average developer)Where uptime is the system is up and non-buggy

Thoughts on consistency models

by rogerbodamer

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