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CS 245 Notes 12 1
CS 245: Database System
Principles
Notes 12: Distributed Databases
Hector Garcia-Molina
CS 245 Notes 12 2
Distributed Databases
data
DBMS
data
DBMS
data
DBMS
data
DBMS
Distributed Database System
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Advantages of a DDBS
- Modularity
- Fault Tolerance
- High Performance
- Data Sharing
- Low Cost Components
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Issues
- Data Distribution
- Exploiting Parallelism
- Concurrency and Recovery
- Heterogeneity
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Parallelism: Pipelining
- Example:
- T 1 SELECT * FROM A WHERE cond
- T 2 JOIN T 1 and B
A (^) (with index)B
select (^) join
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Parallelism: Concurrent Operations
- Example: SELECT * FROM A WHERE cond
A where A.x < 10
select (^) select
A where 10 A.x < 20
select
A where 20 A.x
merge data location isimportant...
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Join Processing
- Example: JOIN A, B over attribute X
A 1 A 2 B 1 B 2
A.x < 10 (^) A.x 10 B.x < 10 (^) B.x 10
join strategy
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Join Processing
- Example: JOIN A, B over attribute X
A 1 A 2 B 1 B 2
A.z < 10 (^) A.z 10 B.z < 10 (^) B.z 10
join strategy
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Concurrency & Recovery
ATM
Bank Mainframe
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2PC: ATM Withdrawl
- Mainframe is coordinator
- Phase 1: ATM checks if money available; mainframe checks if account has funds (money and funds are “reserved”)
- Phase 2: ATM releases funds; mainframe debits account
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Replicated Data Mangement
- Key to fault-tolerance, durability
- Illustrates transaction processing issues
- Various concurrency control/recovery algorithms available
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Primary Copy Algorithm
- Updates run at primary site
- Backups repeat writes; backups allow “out-of-date” reads Primary Site A 3 B 8 C 4 D 25
Backup Site 1 A 3 B 8 C 4 D 25
Backup Site 2 A 3 B 8 C 4 D 25
5 6
9 7 T1: A:5; C: T2: B:9; C: 7 propagate in order
5 6
9 7
5 6
