The design of a Database Management System highly depends on its
architecture. It can be centralized or decentralized or hierarchical.
DBMS architecture can be seen as single tier or multi tier. n-tier
architecture divides the whole system into related but independent n
modules, which can be independently modified, altered, changed or
replaced.
In 1-tier architecture, DBMS is the only entity where user directly
sits on DBMS and uses it. Any changes done here will directly be done
on DBMS itself. It does not provide handy tools for end users and
preferably database designer and programmers use single tier
architecture.
If the architecture of DBMS is 2-tier then must have some
application, which uses the DBMS. Programmers use 2-tier architecture
where they access DBMS by means of application. Here application tier
is entirely independent of database in term of operation, design and
programming.
3-tier architecture
Most widely used architecture is 3-tier architecture. 3-tier
architecture separates it tier from each other on basis of users. It is
described as follows:
[Image: 3-tier DBMS architecture]
Database (Data) Tier: At this tier, only database
resides. Database along with its query processing languages sits in
layer-3 of 3-tier architecture. It also contains all relations and
their constraints.
Application (Middle) Tier: At this tier the application
server and program, which access database, resides. For a user this
application tier works as abstracted view of database. Users are
unaware of any existence of database beyond application. For
database-tier, application tier is the user of it. Database tier is not
aware of any other user beyond application tier. This tier works as
mediator between the two.
User (Presentation) Tier: An end user sits on this
tier. From a users aspect this tier is everything. He/she doesn't know
about any existence or form of database beyond this layer. At this
layer multiple views of database can be provided by the application.
All views are generated by applications, which resides in application
tier.
Multiple tier database architecture is highly modifiable as almost
all its components are independent and can be changed independently.
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