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Fear No More:Embrace Eventual Consistency Sean Cribbs @seancribbs

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Watch the video with slide synchronization on InfoQ.com! http://www.infoq.com/presentations /Embrace-Eventual-Consistency InfoQ.com: News & Community Site• 750,000 unique visitors/month• Published in 4 languages (English, Chinese, Japanese and Brazilian Portuguese)• Post content from our QCon conferences• News 15-20 / week• Articles 3-4 / week• Presentations (videos) 12-15 / week• Interviews 2-3 / week• Books 1 / month

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Presented at QCon San Francisco www.qconsf.comPurpose of QCon- to empower software development by facilitating the spread ofknowledge and innovationStrategy - practitioner-driven conference designed for YOU: influencers ofchange and innovation in your teams- speakers and topics driving the evolution and innovation- connecting and catalyzing the influencers and innovatorsHighlights- attended by more than 12,000 delegates since 2007- held in 9 cities worldwide

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Distributed Systems Experts

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FEAR Photo from Wikimedia

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ACID vs. BASE

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ACID vs. BASE • Strong consistency • Weak consistency • Isolation • Availability first • Focus on “commit” • Best effort • Nested transactions • Approximate answer • Conservative • Aggressive (pessimistic) (optimistic) Fox, Gribble, Chawathe, Brewer, Gauthier -Cluster-Based Scalable Network Services (SOSP97)

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ACID vs. BASE “Inconsistency is “Being unavailablethe worst thing that is the worst thing could happen.” that could happen.”

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Why BASE / EC?

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Why BASE / EC?• “Omniscience” is expensive and slow.

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Why BASE / EC?• “Omniscience” is expensive and slow.• Availability is often correlated to revenue.

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Why BASE / EC?• “Omniscience” is expensive and slow.• Availability is often correlated to revenue.• Failures happen all the time.

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Why BASE / EC?• “Omniscience” is expensive and slow.• Availability is often correlated to revenue.• Failures happen all the time. “Any sufficiently large system is in a constant state of partial failure.” Justin Sheehy, Basho CTO

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Why BASE / EC?• “Omniscience” is expensive and slow.• Availability is often correlated to revenue.• Failures happen all the time.• You’re probably doing it already.

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Safety &LivenessLeslie Lamport1977

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Safety

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Safety• “Bad things don’t happen”• Point-in-time identifiable

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Safety• “Bad things • mutual don’t happen” exclusion• Point-in-time • partial identifiable correctness • first-come, first-serve

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Liveness

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Liveness• “Good things eventually happen”• Always in future

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Liveness• “Good things • starvation eventually freedom happen” • termination• Always in future • guaranteed service

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ACID vs. BASE • Strong consistency • Weak consistency • Isolation • Availability first • Focus on “commit” • Best effort • Nested transactions • Approximate answer • Conservative • Aggressive (pessimistic) (optimistic) Fox, Gribble, Chawathe, Brewer, Gauthier -Cluster-Based Scalable Network Services (SOSP97)

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Peter Bailis Eventual consistency is not safe “...it’s easy to satisfy liveness without being useful... If all replicas return the value 42 in response to every request, the system is eventually consistent.”http://www.bailis.org/blog/safety-and-liveness-eventual-consistency-is-not-safe/

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Liveness of BASE• Convergence - “eventual delivery”• Responsiveness - “eventual service”• Resilience - “eventual recovery”• Consensus-free - “eventual progress”

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Safety of BASE• Durability - “accepted writes are not lost”• Integrity - “data is not corrupted”• Authenticity - “data is not forged”

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Real BASE Systems Photo from Wikimedia

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Domain Name Service• Federated, hierarchical database • How qconsf.com becomes 77.66.16.106• Layered system with caching

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Domain Name Service• Federated, hierarchical database • How qconsf.com becomes 77.66.16.106• Layered system with caching Diagrams from Wikimedia

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Domain Name Service• Federated, hierarchical database • How qconsf.com becomes 77.66.16.106• Layered system with caching Diagrams from Wikimedia

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DNS Liveness• Convergence - caches eventually expire• Consensus-free - local authority over subtree updates• Responsiveness - intermediaries can cache results and reply quicker• Resilience - authority servers can be replicated/ load-balanced

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DNS Safety• Authenticity - forgery prevented by DNSSEC

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BitTorrent• Peer-to-peer cooperative large-file transfer• Dynamic membership and block discovery through the “tracker” node• Epidemic effect http://computer.howstuffworks.com/bittorrent2.htm

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BitTorrent Liveness• Convergence - all peers that remain connected eventually become seeds• Resilience - loss of one peer doesn’t impede progress• Responsiveness - closer, faster peers tend to be preferred

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BitTorrent Safety• Integrity - each block is checksummed to prevent corruption

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The Web• Sparsely-connected graph of hypertext documents identified by URIs• Rich caching semantics: expiration, validation, control• Fluid evolution through uniform interface• Layered system (federated)

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Web: Liveness• Consensus-free - local documents can be changed, moved, removed without coordination• Convergence - caching semantics prevent unbounded staleness, redirection• Responsiveness - many parties can proxy, cache• Resilience - failure of one server doesn’t stop the system

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Web: Safety• Privacy & Authenticity - HTTPS/SSL/TLS• Integrity - POST responses don’t pollute caches

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Dynamo 1• Key-value store: distributed, replicated, partitioned 2• Client requests can go to any node• Low-latency at high percentiles 3• Many clones: Riak, Cassandra, Voldemort

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Dynamo: Liveness• Convergence - read-repair, hash-tree exchanges, vector-clocks• Resilience - hinted-handoff, sloppy quorums• Responsiveness - replication• Consensus-free - loose coordination, concurrent updates

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Dynamo: Safety• Authenticity - won’t serve data you didn’t store• Durability - confirmed writes are not lost

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ACID vs. BASE

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CONFLICT Photo by Associated Press

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SPECTRUM Photo by Associated Press

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hu g s!EmbraceEventualConsistency