Distributed Database Management Systems: Lecture 28

Distributed Database Management Systems: Lecture 28. The main topics covered in this chapter include: serializability theory; serializability theory in DDBS; equivalent schedules; serializable schedule; fragmented databases; replicated databases;...

Distributed Database Management Systems

Lecture 28

In the previous lecture

• Types of Transaction

• Transaction in DDBS

• Serial Transactions

• Conflicting Ops/Tr.

In this Lecture

• Serializability Theory

• Serializability Theory in

DDBS.

Equivalent Schedules

Two schedules S1, S2 defined over same T are equivalent if they have same effect on the database, that is, leave

database in same final state

• Formally, if for each pair of

conflicting operations Oij and

Okl (i ≠ k) if Oij <1 Okl then

Oij <2 Okl

• The phenomena is also

called conflict equivalence

Serializable Schedule

If it is conflict equivalent

to a serial schedule,

i.e., the final state in

which it leaves the

database is equivalent

to a serial

• The function of the

Fragmented Databases

• The serializability is

straight forward

• Local transaction are

independent of each other; each concerns local data.

transactions-Replicated Databases

Read(x)

x = x + 5 Write(x) Commit

T2:

Read(x)

x = x*10 Write(x) Commit

• All values of replicated

data should be same

1.Local Schedule same

2.Conflicting Ops in same relative order on all sites.

• Logical and physical

data items

• User issues Ops on

logical data items

• Replica control maps

to physical ones-

Concurrency Control Algorithms

topology-• Synchronization primitive

is the most common

• Locking and Ordering

• Pessimistic & Optimistic.

• Pessimistic approach

synchronizes

transactions early

• Optimistic do this late

in execution life cycle

of transactions

• Optimistic

–Locking-based

–Timestamp based.

ordering-Locking based Concurrency Control

• Basic idea is that data

• Two major types of

locks, read lock and write lock

• Transaction need to

apply lock first

• Locking is job of

DDBMS, not the user

• Scheduler is the Lock

Manager

• TM and LM interact.