1. CAP Theorem
The art of choice
@mlomnicki

2. CAP Theorem
by Eric Brewer in 2000
Proven in 2002 by Seth Gilbert & Nancy Linch

3. CAP Theorem
Consistency
Availability
Partition tolerance
At most 2 of 3 properties may be satisfied

4. CAP Theorem

5. RDBMS (single-site and cluster)
Consistency
Availability
Partition tolerance

6. Distributed RDBMS
Consistency
Availability
Partition tolerance

7. NoSQL
Consistency
Availability
Partition tolerance
...also DNS is AP

8. BigTable
Consistency
Availability
Partition tolerance
P is on GFS side

9. Prevayler/Madeleine
Consistency
Availability
Partition tolerance

10. NewSQL
Consistency
Availability
Partition tolerance
Impossible according to the theory
...but Stonebraker doesn't agree

11. Real life stories

12. What you think you have

13. What you really have

14. ..to be fair

15. The art of choice
Scalability over Consistency
Consistency over Scalability

16. Forfeit Partition Tolerance
Consistent system - easy
High Availability - hard
known & proven HA workarounds
Facebook - MySQL + memcache
Vertical scalability

17. Forfeit Consistency
High Availability
Horizontal scalability - better
"Eventually consistent" (BASE)
HA = race conditions
workarounds?

18. What you need
Extremely efficient
Reliable (in data sense)

19. ACID
Atomicity
Consistency
Isolation
Durability

20. BASE
Basically Available
Soft state
Eventual consistency

21. ACID
people don't care
...but think data is consistent anyway
you can't enforce consistency at
ActiveRecord level

22. BASE
consistency relaxed
to make horizontal scalability easier
inconsistencies handled by developer

23. beware of vendors
they don't always tell the truth
NoSQL is never ACID-compilant
NoSQL - lack of atomic operations
across documents/collections
Prevayler - poor isolation
HBase - poor durability

24. SQL is ACID
except MySQL with MyISAM
MVCC over locking
but must follow the rules
foreign keys, unique indexes
transaction isolation levels

25. Links
CAP preso
CAP proof
CAP explained
CAP and NoSQL
Comparision
Stonbraker on CAP
Sharding limits
more on CAP

26. Questions?