consistency without borders

Peter Alvaro, Peter Bailis, Neil Conway, Joseph M. HellersteinUC Berkeley

The transaction concept DEBIT_CREDIT: BEGIN_TRANSACTION; GET MESSAGE; EXTRACT ACCOUT_NUMBER, DELTA, TELLER, BRANCH FROM MESSAGE; FIND ACCOUNT(ACCOUT_NUMBER) IN DATA BASE; IF NOT_FOUND | ACCOUNT_BALANCE + DELTA < 0 THEN PUT NEGATIVE RESPONSE; ELSE DO; ACCOUNT_BALANCE = ACCOUNT_BALANCE + DELTA; POST HISTORY RECORD ON ACCOUNT (DELTA); CASH_DRAWER(TELLER) = CASH_DRAWER(TELLER) + DELTA; BRANCH_BALANCE(BRANCH) = BRANCH_BALANCE(BRANCH) + DELTA; PUT MESSAGE ('NEW BALANCE =' ACCOUNT_BALANCE); END; COMMIT;

An application-level contract

Write Read

Application

Opaquestore

Transactions

Pervasive distribution

CAP

partial failure

asynchrony

Research on consistency

Write Read

Application

Opaquestore

Consistency models

Assert: balance > 0

causal?PRAM?delta?fork/join?red/blue?release?SC?

(translation)

R1(X=1) R2(X=1) W1(X=2) W2(X=0)W1(X=1) W1(Y=2) R2(Y=2) R2(X=0)

Meanwhile, in industry…

Application

Opaquestore

Custom solutionsAssert: balance > 0

(prayer)

Write Read

Distributed consistency: staying relevant

• Is this an important problem?• Is academia disconnected from

reality?• OK, what now?

Goal: help programmers write correct applications.Today: some promising approaches

Case study: a graph

Partitioned, for scalability

Replicated, for availability

Problem: deadlock detection

Task: Identify strongly-connected components

Waits-for graph

Problem: garbage collection

Task: Identify nodes not reachable from Root.

Root

Refers-to graph

Correctness

Deadlock detection• Safety: No false positives-• Liveness: Identify all deadlocksGarbage collection• Safety: Never GC live memory!• Liveness: GC all orphaned memory

Partition

Root

Consistency at the extremes

Linearizable key-value store?

Custom solutions?

Efficient Correct

Consistency across the stack

Object-level consistencyCapture semantics of data structures that• allow greater concurrency • maintain guarantees (e.g. convergence)

Insert Read

Convergentdata structure(e.g., Set CRDT)

Object-level consistency

Insert Read

CommutativityAssociativityIdempotence

ReorderingBatchingRetry/duplication

Tolerant to

Application

Convergentdata structures

Object-level consistency

? ?

GC Assert: No live nodes are reclaimed

Assert: Graph replicasconverge

Flow-level consistency

Flow-level consistencyCapture semantics of data in motion• Asynchronous dataflow model• component properties system-wide guarantees

Flow-level consistencyOrder-insensitivity (confluence)

output set = f(input set)

{ }

{ }=

Flow-level consistencyConfluence is compositional

output set = f g(input set)

Graph queries as dataflowConfluent

Coordinate here

Language-level consistencyDSLs for distributed programming?• Capture consistency concerns in the

type system

Language-level consistency

CALM Theorem:

Monotonic confluent

Conservative, syntactic test for confluence

Language-level consistencyDeadlock detector

Garbage collector

nonmonotonic

correctreusable

efficientintuitive

Where we’ve been;where we’re headed

Remember• Consistency is an application-level

property

• Correctness and performance are compatible

• Meet programmers on their home turf

• Build bridges!

Queries?