System Architecture - DBMS

Database Management Systems (DBMS) are built to abstract data complexities and provide users a simplified and secure interaction layer. This tutorial explains how data is viewed at various abstraction levels, the role of schemas and instances, database languages, data models, and application interaction mechanisms. It also covers the role of a Database Administrator (DBA) and common DBMS architectures used in real-world applications.

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View of Data (3-Schema Architecture)

You should Know

What is a Schema??

Schema is nothing but Design. Often while developing (Backend / Full Stack) Application we will encouter the word schema. It is nothing but the design of the database, how will we store data in the Database for efficient retrieval.

Purpose:

• Provides abstraction between data storage and user interaction.
• Allows multiple users to access the same data with personalized views.

Levels of Abstraction:

1. Physical Level / Internal Level

• Describes how data is actually stored, in forms of bits/bytes, in which file format.
• Uses low-level structures like N-ary trees, compression, or encryption.
• Defines the physical schema.
• Goal: Enable efficient access to data via optimized storage strategies.

2. Logical Level / Conceptual Level

• Describes what data is stored and the relationships among data.
• Used by DBAs to define the logical schema.
• Users at this level do not require knowledge of physical storage.
• In General, when we say DB schema it refers to Logical Schema.
• Goal: Focus on ease of use and application design.

3. View Level / External Level

• Highest level of abstraction; provides different views to different users.
• Defines subschemas relevant to specific user groups.
• Enhances security by hiding sensitive parts of the database.

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Instances and Schemas

Concept	Description
Instance	Data stored in the database at a specific moment
Schema	Structural design or blueprint of the database
Types	Physical schema, Logical schema, View schema (subschemas)

• Schema defines the data structure and changes infrequently.
• Instances reflect the current data and change frequently.
• Logical schema is critical for application development.
• Physical data independence ensures schema changes don’t affect applications.

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Data Models

Definition:

A data model is a collection of conceptual tools for describing:

• Data
• Relationships
• Semantics
• Constraints

Examples:

• Entity-Relationship (ER) Model
• Relational Model
• Object-Oriented Model
• Object-Relational Model

These are used to design databases at the logical level.

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Database Languages

Language	Purpose	Examples / Notes
DDL	Define schema & structure	CREATE TABLE, ALTER, DROP
DML	Query & manipulate data	INSERT, SELECT, DELETE, UPDATE

• In practice, most systems use a unified language like SQL for both DDL and DML.
• Query language is a subset of DML for data retrieval.

Example: DML Operations

SELECT * FROM Students;
INSERT INTO Students VALUES (1, 'Akash', 'CSE');
UPDATE Students SET name = 'A. Halder' WHERE id = 1;
DELETE FROM Students WHERE id = 1;

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Accessing Database from Application Programs

How It Works:

• Applications written in languages like PHP, C++, Java, JavaScript, TypeScript send queries to the DB.
• These use APIs to send DDL/DML statements and retrieve results.

Common APIs:

API	Platform	Language
ODBC	Microsoft	C/C++
JDBC	Java-based	Java
Drizzle/Prisma	JavaScript / TypeScript based	JavaScript / TypeScript

Example:

• Amazon may store its customer data in any-language and can use an ORM to fecth it from the DB
• A blog System like https://akashhalder.in/blogs/explore is getting powered by this type of interface(ORM)

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Database Administrator (DBA)

The DBA manages and controls access to both data and DBMS operations.

Functions:

1. Schema Definition

2. Storage & Access Method Selection

3. Modifying Schema & Physical Organization

4. Authorization Management

5. Routine Maintenance

   • Backups
   • Security patching
   • Software upgrades

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DBMS Application Architectures

T1 Architecture – Single-Tier

• DBMS, application, and client reside on the same machine.

T2 Architecture – Two-Tier

• Client sends queries to the DB server using APIs.
• Business logic may reside partly on the client.

T3 Architecture – Three-Tier (Used in Web Apps)

• Client → App Server → DB Server
• App server handles business logic and prevents direct DB access.
• Improves security, scalability, and data integrity.

Advantages of T3

Advantage	Description
Scalability	Supports distributed application servers
Data Integrity	App server ensures consistent transaction logic
Security	Clients have no direct access to DB → safer