Introduction to Data Persistence

Introduction to Data Persistence in C#

Why We Need Persistent Data

When a program runs, it uses RAM (Random Access Memory) to store data that it needs while executing.

RAM is fast but volatile --- meaning that all data stored in it is lost when:

• The application closes
• The computer shuts down or restarts
• The process crashes

Therefore, if our application needs to remember data between runs --- for example, user preferences, scores in a game, or saved records --- we need a way to persist this data to a non-volatile storage medium (e.g., hard drive or SSD).

---

Ways to Persist Data

1. Saving Data to Files

The simplest form of persistence is writing data to files on disk.
In C#, we can use classes from the System.IO namespace such as:

• File
• StreamWriter / StreamReader
• FileStream

Example (writing text to a file):

File.WriteAllText("data.txt", "Hello, world!");

Example (reading text from a file):

string content = File.ReadAllText("data.txt");
Console.WriteLine(content);

We can also use structured Data Formats such as CSV, JSON, or XML for better organization and interoperability.

---

2. Using Databases

A database is a more structured and powerful way to persist and manage data. It allows multiple users or applications to access and modify data concurrently and provides mechanisms for:

• Querying data efficiently
• Enforcing relationships between data
• Ensuring data integrity

There are two main categories of databases:

a) Relational Databases (SQL)

Examples: - SQLite - MySQL - PostgreSQL - Microsoft SQL Server

They store data in tables with rows and columns, and data is accessed using SQL (Structured Query Language).

Example of a simple SQL table definition:

CREATE TABLE Students (
    Id INTEGER PRIMARY KEY,
    Name TEXT,
    Grade INTEGER
);

b) NoSQL Databases

Examples: - MongoDB - Redis - CouchDB

These are often used for unstructured or semi-structured data, offering more flexibility in data storage models such as documents, key-value pairs, or graphs.

---

3. The Common Ground

Even though databases appear to “magically” store and retrieve data, they too must write to persistent storage (usually disk). Whether we save data as a file or store it in a database, ultimately the data ends up on some form of persistent medium to survive program termination.

---

💡 Key takeaway: RAM is temporary; persistence means saving data in a non-volatile way so it can be restored later.