cassandra day chicago 2015: cassandra consistency & tolerance (a morality guide for databases)
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Cassandra: Consistency & Tolerance (a morality guide for databases)Matt Kennedy, Architect
@thetweetofmatt
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?
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Georgetown
American
20 minutes
Worth the wait?
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Georgetown
American
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Lessons?
• High Availability means:• Have to maintain some redundancy• Need to distribute your stuff
• Distributed Stuff means:• Need to accommodate unreliable networks• Need to have strategies for different path costs• Plan for outright failures• Cooperation!
Also, don’t underestimate a squirrel with a Jesuit education.
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Eventual Consistency
Meh.
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http://nekrocodile-izumi.deviantart.com/art/Lazy-college-student-322186707
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Consistency vs. Consistency
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Tunable Consistency & High Availability
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Example assumes replication factor = 3
write
read
Quorum-write, RF=5
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Client
6-node cluster, rf=5 OK!
Quorum read RF=5
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Client
6-node cluster, rf=5
00101010
00101010
00101010 00101010
? ?
Read value: 00101010
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Linearizable Consistency
• AKA:• Compare & Set• Check & Set• Lightweight Transactions• CQL “IF” Statements
• Uses Paxos• Can specify local datacenter or all datacenters• (CASSANDRA-5797)
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Linearizable Consistency
• Put X in the database if X is not already in the database
• Set foo=bar if foo=baz
• Set foo=bar if boo=faz
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MONSTER MASH!!!
Alice Bob Create “Stinky”
C*
Create “Stinky”
Insert “Stinky” Insert “Stinky” Quorum insert
Quorum insert
Now Bob owns “Stinky” and Alice doesn’t know why she can’t see the monster she just made
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MONSTER MASH!!!
Alice Bob Create “Stinky”
C*
Create “Stinky”
Insert “Stinky” Insert “Stinky”
LWT Insert LWT insert
NO!
Consistency & Cassandra
• Tunable Consistency
• Linearizable Consistency
• Your App can tell C* what it needs
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THIS IS IMPORTANT
• The Consistency Mechanisms are King• Cassandra’s core strengths…• High Availability• Tolerance of node failures• Tolerance of datacenter failures• Tolerance of network failures between datacenters
• All due to the Consistency Mechanisms • Cassandra isn’t Cassandra without Eventual
Consistency
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Consistency is a Key Value!
• Get it? Key-Value?• It’s a NoSQL joke.
• Always-on Big Data demands multi-region DCs• Multi-region DCs must face the reality of network partitions
• Eventual Consistency enables applications built on Cassandra to do what they need to do correctly and flexibly while remaining constantly available.
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Thank You
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Tunable Consistency & High Availability
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write
read
Example assumes replication factor = 3
1.
2.
write
read
3.
Options 1 & 2 achieve strong consistency at the expense of availability. Option 3 requires only 2 replicas on write and 2 on read, so it achieves high availability & strong consistency.
Eventual Consistency The Netflix Experiment
• “Eventual Consistency != Hopeful Consistency”• Netflix, big C* user, performed an experiment
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http://planetcassandra.org/blog/post/a-netflix-experiment-eventual-consistency-hopeful-consistency-by-christos-kalantzis/
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C* Datacenters A Single Cluster
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One Datacenter
Peer-to-peer ring of nodes
Datacenters can go offline…
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For a variety of reasons
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C* Datacenters A Single Cluster
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One Datacenter
Peer-to-peer ring of nodes
C* Datacenters What happens when I lose one?
Los Angeles
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Chicago
New York
C* Datacenters What is possible?
Los Angeles
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Chicago
New York
Cloud
C* Datacenters Cloud Deployments
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Availability Zone 1
AWS
C* Datacenters Cloud Deployments
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Availability Zone 1
Availability Zone 2
AWS
C* Datacenters Cloud Deployments
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Availability Zone 1
Availability Zone 2
Google Compute
Engine
AWS
C* Datacenters Cloud Deployments
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Off-Cloud Migration
On-Prem
Start in Cloud
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