Relaxed Currency and Consistency: How to Say “Good Enough” in SQL

1. Relaxed Currency and Consistency:How to Say “Good Enough” in SQL • H. Guo, P. Larson, R. Ramakrishnan, J. Goldstein • Presented by Mike Nie

2. Background-1 • Using asynchronous copies technique is common because they are good for: • scalability • performance • availability

3. Background-II • The way asynchronous copies technique works: • Replicate data over a number of database systems • User interact with replicas to balance the workload of the entire system • Data update over the entire system is not in the same scope the transactions; the system will propagate the update later for the purpose of better respond time

4. Motivation • Currently there is NO way for user to express “freshness” in SQL. • Database system can’t respond well because queries are not explicit, even though it can easily keep track how current its data is.

5. Currency and Consistency • Currency: how fresh the data is in replica database system • Consistency: whether or not two replica tables correspond to the same snapshot • e.g. book_copy and review_copy

6. C & C Syntax in SQL • A C & C Clause occurs last in SFW block and follow the same scoping rule as the Where clause • New clause can reference tables defined in current and outer SFW blocks

7. Single-Block Query

8. Multi-Block Query

9. Implementation-I • Integrate C & C in MTCache

10. Implementation-II • Normalizing C&C Constraints • Transform C&C clause to a set of tuples, e.g. C = {<b1,S1>,<b2,S2>,...,<bn,Sn>} • Compile-Time Consistency Checking • Integrate consistency checking into SQL optimizer

11. Implentation-III • Compile-time consistency checking(cont.) • e.g. checking if Reserves and Sailors belong to the same mutually consistent group.

12. Implementation-IV • Run-time currency Checking • whether exist a copy in locate that is satisfy currency bound • choose plan based on cost

13. Experiments • Query Optimization Experiments • Workload Distribution • Overhead of Currency Guards

14. Conclusion • Given such an explicit syntax user can specify C&C constraints in SQL • System can respond well given such an explicit requirement • Questions and Comments