Scaling Twitter with Cassandra

Scaling Twitter with Cassandra Ryan King Storage Team

bit.ly/chirpcassandra ryan@twitter.com @rk

Legacy • vertically & horiztonally partitioned mysql • memcached (rows, indexes and fragments) • application managed

Legacy Drawbacks • many single-points-of-failure • hardware-intensive • manpower-intensive • tight coupling

Apache Cassandra • Apache top level project • originally developed at Facebook • Rackspace, Digg, SimpleGeo, Twitter, etc.

Why Cassandra? • highly available • consistent, eventually • decentralized • fault tolerant • elastic • flexible schema • high write throughput

What is Cassandra? • distributed database • Google's BigTable's data model • Amazon's Dynamo's infrastructure

Cassandra Data Model • keyspaces • column families • columns • super columns

Cassandra Infrastructure • partitioners • storage • querying

Partitioners • order-preserving • random • custom

Storage • commit log • memtables • sstables • compaction • bloom filters • indexes • key cache • row cache

Querying • get • multiget • range • slice

Consistency

Consistency • N, R, W

Consistency • N, R, W • N = number of replicas

Consistency • N, R, W • N = number of replicas • R = read replicas

Consistency • N, R, W • N = number of replicas • R = read replicas • W = write replicas

Consistency • N, R, W • N = number of replicas • R = read replicas • W = write replicas • send request, wait for specified number

Consistency • N, R, W • N = number of replicas • R = read replicas • W = write replicas • send request, wait for specified number • wait for others in background and perform read- repair

Consistency Levels • ZERO • ONE • QUORUM • ALL

Strong Consistency • If W + R > N, you will have consistency • W=1, R=N • W=N, R=1 • W=Q, R=Q where Q = N / 2 + 1

Eventuality • Hinted Handoff • Read Repair • Proactive Repair (Merkle trees)

Potential Consistency

Potential Consistency • causes • write-through caching • master-slave replication failures

Example

Read Repair • send read to all replicas • if they differ, resolve conflicts and update (in background)

Hinted Handoff • A wants to write to B • B is down • A tells C, "when B is back, send them this update"

Proactive Repair • use Merkle trees to find inconsistencies • resolve conflicts • send repaired data • triggered manually

Parallel Deployment

How we’re moving? • parallel deployments • incremental traffic shifting

Parallel Deployment 1. build new implementation 2. integrate it alongside existing 3. ...with switches for dynamically move/mirror traffic 4. turn up traffic 5. break something 6. Fix it 7. GOTO 4

http://nosql.mypopescu.com	3397
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Scaling Twitter with Cassandra

by ryansking on Apr 16, 2010

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