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Concurrency Control
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2016-02-14
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Why Concurrency Control is required? Formal Concepts Serial, realizable, equivalent schedules 4 invalid schedules Concurrency control – a DBMS component Isolation levels ANSI SQL isolation levels Snapshot Isolation Locking Basics Deadlocks Index locking Optimistic locking Multi-Version Concurrency Control
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Concurrency Control 1
Ch. 4 – Concurrency Control
Ref.
- Chapters 20, 21 (Kifer)
-
Chapters 16, 17 (Ramakrishnan)
-
http://technet.microsoft.com/en-us/
library/jj856598.aspx
Concurrency Control 2
Outline
•
Why Concurrency Control is required?
•
Formal Concepts
–
Serial, realizable, equivalent schedules
–
4 invalid schedules
•
Concurrency control – a DBMS component
•
Isolation levels
–
ANSI SQL isolation levels
–
Snapshot Isolation
•
Locking
–
Basics
–
Deadlocks
–
Index locking
–
Optimistic locking
•
Multi-Version Concurrency Control
Introduction 3
Why Concurrent Xacts?
•
Multiprogramming system:
–
Only one program is executed by the CPU at one
time
–
Multiple programs are active in the system
•
waiting for execution (time sharing).
•
waiting for I/O actions to be carried out
(asynchronous I/O).
•
Data sharing:
–
I/O cost may be reduced if the same page of data
inside the buffer is shared among concurrent
xacts.
Concurrent
Xacts
Concurrency Control 4
8
7
6
5
6
5
4
3
9
8
7
6
5
4
3
2
Dispatcher
4
1
5
4
2
3
Dispatcher is responsible
for serialization of
operations from all
concurrent Xacts.
Buffer Manager & System Log
For each requested operation from a Xact, the dispatcher sends to the
Buffer manager, the information about:
•
Xact ID,
•
intended operation (e.g., read/write),
•
associated data object (e.g., the rowID)
is recorded on the System Log. All entries are ordered in time sequence.
5
Buffer
Manager
Transaction 1
Buffer
Pool
System
Log
Transaction n
Dispatcher
…
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