Data persistence and concurrency

 

Data Persistence

Definition

Data persistence refers to the ability of a system to store data permanently, so that it remains available even after the application is closed or the system is restarted.

In databases, persistent data is stored on non-volatile storage such as hard disks or cloud storage.

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Importance of Data Persistence

• Prevents data loss

• Ensures data availability over time

• Allows recovery after system failures

• Essential for business and user applications

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Data Persistence in Databases

• Data is stored in files or database storage

• Changes are written to disk using logs

• Supports backup and recovery

Examples:

• User records in a database

• Transaction history

• Logs and reports

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Challenges in Data Persistence

• Managing large volumes of data

• Ensuring durability in distributed systems

• Maintaining performance while writing to disk

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2. Concurrency

Definition

Concurrency refers to the ability of a database system to handle multiple users or operations accessing data at the same time.

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Need for Concurrency

• Multiple users may access the database simultaneously

• Improves system utilization

• Increases application performance

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Concurrency Problems

When multiple transactions run at the same time, problems may occur such as:

• Lost update

• Dirty read

• Non-repeatable read

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Concurrency Control

Databases use various techniques to manage concurrency:

• Locking mechanisms

• Transaction isolation levels

• Timestamp-based control

Relational databases strictly enforce concurrency control using ACID properties.

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3. Data Persistence and Concurrency in NoSQL

• NoSQL databases focus on high availability and scalability

• Some NoSQL systems relax strict concurrency rules

• Many NoSQL databases follow eventual consistency

• Concurrency is managed using:

  • Versioning

  • Timestamps

  • Conflict resolution

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4. Comparison: RDBMS vs NoSQL

Aspect	RDBMS	NoSQL
Data Persistence	Strong, disk-based	Strong but distributed
Concurrency	Strict control	Relaxed or eventual
Consistency	Strong consistency	Eventual consistency
Scalability	Vertical	Horizontal