Distributed DBMS - Unit 3 - Distributed DBMS Architecture

UNIT-3
Distributed DBMS Architecture

Outlines…
• Models- Autonomy, Distribution, Heterogeneity
• DDBMS Architecture – Client/Server, Peer to peer, MDBS
1/11/2017 2Prof. Dhaval R. Chandarana

DBMS STANDARDIZATION
• Based on components.
The components of the system are defined together with the
interrelationships between components. A DBMS consists of a
number of components, each of which provides some functionality.
• Based on functions.
The different classes of users are identified and the functions that
the system will perform for each class are defined. The system
specifications within this category typically specify a hierarchical
structure for the user classes.

• Based on data.
The different types of data are identified, and an architectural
framework is specified which defines the functional units that will
realize or use data according to these different views. This approach
(also referred as the data logical approach) is claimed to be the
preferable choice for standardization activities.

ANSI / SPARC ARCHITECTURE
• The ANSI / SPARC architecture is claimed to be based on the data
organization. It recognizes three views of data: the external view,
which is that of the user, who might be a programmer; the internal
view, that of the system or machine; and the conceptual view, that of
the enterprise.
• For each of these views, an appropriate schema definition is required.

• At the lowest level of the architecture is the internal view, which deals
with the physical definition and organization of data.
• At the other extreme is the external view, which is concerned with
how users view the database.
• Between these two ends is the conceptual schema, which is an
abstract definition of the database. It is the „real world” view of the
enterprise being modeled in the database.

DBMS STANDARDIZATION ANSI / SPARC ARCHITECTURE
• The ANSI-SPARC Architecture, where ANSI-SPARC stands for American National Standards
Institute, Standards Planning And Requirements Committee, is an abstract design standard for a
Database Management System (DBMS), first proposed in 1975. The ANSI-SPARC model however
never became a formal standard.

DBMS STANDARDIZATION ANSI / SPARC ARCHITECTURE
• The square boxes represent processing functions, whereas the hexagons
are administrative roles.
• The arrows indicate data, command, program, and description flow,
whereas the „I”-shaped bars on them represent interfaces.
• The major component that permits mapping between different data
organizational views is the data dictionary / directory (depicted as a
triangle), which is a meta-database.
• The database administrator is responsible for defining the internal schema
definition.
• The enterprise administrator’s role is to prepare the conceptual schema
definition.
• The application administrator is responsible for preparing the external
schema for applications.

ARCHITECTURAL MODELS FOR DISTRIBUTED
DBMSs
The systems are characterized with respect to:
(1) the autonomy of the local systems,
(2) their distribution,
(3) their heterogeneity.

DBMSs - AUTONOMY
• Autonomy refers to the distribution of control, no data. It indicates
the degree to which individual DBMSs can operate independently.
• Three alternatives:
• tight integration (A0)
• semiautonomous systems (A1)
• total isolation (A2)

DBMSs - AUTONOMY
• Tight integration.
A single-image of the entire database is available to any user who wants to
share the information, which may reside in multiple databases. From the
users’ perspective, the data is logically centralized in one database.
• Semiautonomous systems.
The DBMSs can operate independently. Each of these DBMSs determine
what parts of their own database they will make accessible to users of
other DBMSs.
• Total isolation.
The individual systems are stand-alone DBMSs, which know neither of the
existence of the other DBMSs nor how to communicate with them.

DBMSs - DISTRIBUTION
• Distributions refers to the distributions of data. Of course,
we are considering the physical distribution of data over
multiple sites; the user sees the data as one logical pool.
• Two alternatives:
• No distribution (D0)
• client / server distribution (D1)
• peer-to-peer distribution (full distribution) (D2)

DBMSs - DISTRIBUTION
• Client / server distribution.
The client / server distribution concentrates data management duties
at servers while the clients focus on providing the application
environment including the user interface. The communication duties
are shared between the client machines and servers. Client / server
DBMSs represent the first attempt at distributing functionality.
• Peer-to-peer distribution.
There is no distinction of client machines versus servers. Each
machine has full DBMS functionality and can communicate with other
machines to execute queries and transactions.

DBMSs - HETEROGENEITY
• Heterogeneity may occur in various forms in distributed
systems, ranging form hardware heterogeneity and
differences in networking protocols to variations in data
managers.
• Representing data with different modeling tools creates
heterogeneity because of the inherent expressive powers and
limitations of individual data models. Heterogeneity in query
languages not only involves the use of completely different data
access paradigms in different data models, but also covers differences
in languages even when the individual systems use the same data
model.

DBMSs - ALTERNATIVES
• The dimensions are identified as: A (autonomy), D (distribution) and
H (heterogeneity).
• The alternatives along each dimension are identified by numbers as:
0, 1 or 2.
A0 - tight integration D0 - no distribution
A1 - semiautonomous systems D1 - client / server systems
A2 - total isolation D2 - peer-to-peer systems
H0 - homogeneous systems
H1 - heterogeneous systems

(A0, D0, H0)
If there is no distribution or heterogeneity, the system is a set of
multiple DBMSs that are logically integrated.
(A0, D0, H1)
If heterogeneity is introduced, one has multiple data managers that
are heterogeneous but provide an integrated view to the user.
(A0, D1, H0)
The more interesting case is where the database is distributed even
though an integrated view of the data is provided to users (client /
server distribution).

(A0, D2, H0)
The same type of transparency is provided to the user in a fully
distributed environment. There is no distinction among clients
and servers, each site providing identical functionality.
(A1, D0, H0)
These are semiautonomous systems, which are commonly
termed federated DBMS. The component systems in a federated
environment have significant autonomy in their execution, but
their participation in the federation indicate that they are willing
to cooperate with other in executing user requests that access
multiple databases.

(A1, D0, H1)
These are systems that introduce heterogeneity as well as autonomy,
what we might call a heterogeneous federated DBMS.
(A1, D1, H1)
System of this type introduce distribution by pacing component
systems on different machines. They may be referred to as
distributed, heterogeneous federated DBMS.
(A2, D0, H0)
Now we have full autonomy. These are multi database systems
(MDBS). The components have no concept of cooperation. Without
heterogeneity and distribution, an MDBS is an interconnected
collection of autonomous databases.

(A2, D0, H1)
These case is realistic, maybe even more so than (A1, D0,
H1), in that we always want to built applications which
access data from multiple storage systems with different
characteristics.
(A2, D1, H1) and (A2, D2, H1)
These two cases are together, because of the similarity of
the problem. They both represent the case where
component databases that make up the MDBS are
distributed over a number of sites - we call this the
distributed MDBS.

DISTRIBUTED DBMS ARCHITECTURE
• Client / server systems - (Ax, D1, Hy)
• Distributed databases - (A0, D2, H0)
• Multidatabase systems - (A2, Dx, Hy)

CLIENT / SERVER SYSTEMS
• This provides two-level architecture which make it easier to manage
the complexity of modern DBMSs and the complexity of distribution.
• The server does most of the data management work (query
processing and optimization, transaction management, storage
management).
• The client is the application and the user interface (management the
data that is cached to the client, management the transaction locks).

DISTRIBUTED DBMS
ARCHITECTURE
• This architecture is quite
common in relational
systems where the
communication between
the clients and the
server(s) is at the level of
SQL statements.

• Multiple client - single server
From a data management perspective, this is not much different from
centralized databases since the database is stored on only one machine
(the server) which also hosts the software to manage it. However, there are
some differences from centralized systems in the way transactions are
executed and caches are managed.
• Multiple client - multiple server
In this case, two alternative management strategies are possible: either
each client manages its own connection to the appropriate server or each
client knows of only its “home server” which then communicates with
other servers as required.

PEER-TO-PEER DISTRIBUTED SYSTEMS
• The physical data organization on each machine may be
different.
• Local internal scheme (LIS) - is an individual internal schema
definition at each site.
• Global conceptual schema (GCS) - describes the enterprise view
of the data.
• Local conceptual schema (LCS) - describes the logical
organization of data at each site.
• External schemas (ESs) - support user applications and user
access to the database.

• In these case, the ANSI/SPARC model is extended by the addition of
global directory / dictionary (GD/D) to permits the required global
mappings. The local mappings are still performed by local directory /
dictionary (LD/D). The local database management components are
integrated by means of global DBMS functions. Local conceptual
schemas are mappings of global schema onto each site.

In these case, the ANSI/SPARC
model is extended by the addition
of global directory / dictionary
(GD/D) to permits the required
global mappings. The local
mappings are still performed by
local directory / dictionary (LD/D).
The local database management
components are integrated by
means of global DBMS functions.
Local conceptual schemas are
mappings of global schema onto
each site.

DISTRIBUTED
DBMS
ARCHITECTURE
PEER-TO-PEER
DISTRIBUTED SYSTEMS
• The detailed components
of a distributed DBMS.
• Two major components:
 user processor
 data processor

User processor
• user interface handler - is responsible for interpreting user commands
as they come in, and formatting the result data as it is sent to the user,
• semantic data controller - uses the integrity constraints and
authorizations that are defined as part of the global conceptual schema to
check if the user query can be processed,
• global query optimizer and decomposer - determines an execution
strategy to minimize a cost function, and translates the global queries in
local ones using the global and local conceptual schemas as well as global
directory,
• distributed execution monitor - coordinates the distributed
execution of the user request.

Data processor
• local query optimizer - is responsible for choosing the best
access path to access any data item,
• local recovery manager - is responsible for making sure that the
local database remains consistent even when failures occur,
• run-time support processor - physically accesses the database
according to the physical commands in the schedule generated
by the query optimizer. This is the interface to the operating
system and contains the database buffer (or cache) manager,
which is responsible for maintaining the main memory buffers
and managing the data accesses.

MDBS ARCHITECTURE
Models using a Global Conceptual Schema (GCS)
The GCS is defined by integrating either the external schemas of
local autonomous databases or parts of their local conceptual
schemas. If the heterogeneity exists in the system, then two
implementation alternatives exists unilingual and multilingual.
Models without a Global Conceptual Schema (GCS)
The existence of a global conceptual schema in a Multidatabase
system is a controversial issue. There are researchers who even
define a Multidatabase management system as one that
manages “several databases without the global schema”.

MDBS ARCHITECTURE - models using a GCS

• A unilingual multi-DBMS requires the users to utilize possibly different data
models and languages when both a local database and the global database
are accessed.
• Any application that accesses data from multiple databases must do so by
means of an external view that is defined on the global conceptual schema.
• One application may have a local external schema (LES) defined on the
local conceptual schema as well as a global external schema (GES) defined
on the global conceptual schema.

• An alternative is multilingual architecture, where the basic philosophy
is to permit each user to access the global database by means of an
external schema, defined using the language of the user’s local DBMS.
• The multilingual approach obviously makes querying the databases
easier from the user’s perspective. However, it is more complicated
because we must deal with translation of queries at run time.

MDBS ARCHITECTURE - models without a GCS

• The architecture identifies two layers: the local system layer and the
Multidatabase layer on top of it.
• The local system layer consists of a number of DBMSs, which present to the
Multidatabase layer the part of their local database they are willing to
share with users of the other databases. This shared data is presented
either as the actual local conceptual schema or as a local external schema
definition.
• The Multidatabase layer consist of a number of external views, which are
constructed where each view may be defined on one local conceptual
schema or on multiple conceptual schemas. Thus the responsibility of
providing access to multiple databases is delegated to the mapping
between the external schemas and the local conceptual schemas.

• The MDBS provides a layer of
software that runs on top of these
individual DBMSs and provides
users with the facilities of
accessing various databases.
• Fig. represents a no distributed
multi-DBMS. If the system is
distributed, we would need to
replicate the Multidatabase layer
to each site where there is a local
DBMS that participates in the
system.

Distributed DBMS - Unit 3 - Distributed DBMS Architecture

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Distributed DBMS - Unit 3 - Distributed DBMS Architecture