Distributed Database Management Systems: Lecture 21

Distributed Database Management Systems: Lecture 21. The main topics covered in this chapter include: types of fragmentation with horizontal fragmentation, vertical fragmentation; storing a separate copy of database at each of two or three or more sites;...

Distributed Database Management Systems

Lecture # 21

Fragmentation: One of the possible way we can implement

a DDBS

In the previous lectures

Types of Fragmentation

• Horizontal Fragmentation

• Vertical Fragmentation

Types of Horizontal

Fragmentation

1 Primary Horizontal Fragmentation

2 Derived Horizontal Fragmentation

Vertical Fragmentation

Vertical Fragmentation is relatively more complex Why ?

In today’s Lecture

Storing a separate copy of database

at each of two or

three or more sites

integrity routines (if

replicated data is refreshed

at scheduled intervals.)

–De-couples nodes

(transactions proceed

even if some nodes are

down.)

–Reduced network traffic at

prime time (if updates can

be delayed.)

• Disadvantages of

Replication

–Additional requirements

for storage space.

–Additional time for

update operations.

–Complexity and cost of

updating.

–Integrity exposure of getting

incorrect data if replicated

data is not updated

simultaneously.

• Therefore, better when

used for non-volatile data

Replication Architecture in SQL

Server

Replication uses a

publish-subscribe

model for distributing data

A Publication is group of related data and objects that we want to replicate together.

Publication

A Publisher is a server

that is the source of data

to be replicated

Publisher

A Subscriber is a server that receives the data

replicated by the

publisher

Subscriber

• The Subscriber

defines a

subscription to a

particular publication

• A Distributor is a server

that performs various tasks when moving articles from Publishers to Subscribers

• The actual tasks performed

depend on the type of

replication performed

SQL Server Replication Agents

Agents are the software components used in

replication

Types Of SQL Server Replication Agents

• Snapshot Agent: Runs at least once

in all replicas

different activities, mainly distributes publication to subscribers

transactional rep, reads log files of all servers involved

different servers made since last

snapshot

Push/Pull subscriptions; depends where the

distributor is running.

Replication Models

Central Publisher / Distributor

Publ/Dist Sever

Central Publisher / Remote Distributor

Publisher Remote

Distr

Replication Methods

• Snapshot Replication

• Transaction Replication.

• Merge Replication.

Snapshot Replication

• Preferred when

subscribers need only access

read-• Higher latency (working

without updated data)

low Bandwidth

• Used in all types,

initially

Summary