Relaxed Currency and Consistency:

How to Say “Good Enough” in SQL

H. Guo, P. Larson, R. Ramakrishnan, J. Goldstein

Presented by Mike Nie

Background-1

•Using asynchronous copies technique is common because they are good for:

•scalability

•performance

•availability

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

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.

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

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

Single-Block Query

Multi-Block Query

Implementation-I•Integrate C & C in MTCache

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

Implentation-III•Compile-time consistency

checking(cont.)

• e.g. checking if Reserves and Sailors belong to the same mutually consistent group.

Implementation-IV• Run-time currency Checking

•whether exist a copy in locate that is satisfy currency bound

• choose plan based on cost

Experiments

•Query Optimization Experiments

•Workload Distribution

•Overhead of Currency Guards

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