In the previous post, we learned that Domain Modeling is not about classes.

It is about understanding:

• business behavior
• business rules
• ownership
• lifecycle
• correctness

Now we begin with the most fundamental building block of any domain model:

Entities

Almost every real system revolves around entities.

Orders.

Bookings.

Rides.

Payments.

Users.

Understanding entities correctly is one of the biggest differences between beginner and strong LLD thinking.

The Simplest Definition

An Entity is:

an object that has a unique identity and must be tracked over time.

The important phrase is:

tracked over time

Because identity is what makes an entity special.

Example: An Order

Imagine an e-commerce system.

A customer places an order.

Order #123

Today:

CREATED

Tomorrow:

PAID

Later:

SHIPPED

Then:

DELIVERED

Notice something important.

Many attributes changed:

• status
• shipment information
• timestamps

But it is still:

Order #123

The identity remains the same.

That is why Order is an Entity.

Identity Is More Important Than Data

Beginners often focus on fields:

Order {
 id
 amount
 status
}

But entities are not defined by fields.

They are defined by identity.

Even if all fields change:

Amount changes
Status changes
Address changes

The business still considers it the same Order.

Identity survives change.

Real-Life Analogy

Think about a person.

You may change:

• hairstyle
• phone number
• address
• job

But you remain the same individual.

Why?

Because your identity is independent of those attributes.

Entities behave similarly.

Entity = Continuity Through Time

This is one of the most important mental models.

Entities represent:

continuity through change.

Example:

Ride Sharing System

Ride #567

Initially:

REQUESTED

Then:

DRIVER_ASSIGNED

Then:

IN_PROGRESS

Then:

COMPLETED

The ride evolves.

But it remains the same ride.

That continuity is what makes it an entity.

How to Identify Entities

Ask this question:

Does the business care about this object's identity?

If yes, it is probably an entity.

Examples:

User

Business tracks specific user.

Entity.

Order

Business tracks specific order.

Entity.

Booking

Business tracks specific booking.

Entity.

Ride

Business tracks specific ride.

Entity.

Payment

Business tracks specific payment.

Entity.

Another Important Question

Ask:

If two objects contain exactly the same data, are they still different?

Example:

Order #101
Amount: ₹500

Order #102
Amount: ₹500

Same amount.

Same fields.

Still different orders.

Therefore:

Entity.

Entity Lifecycle Matters

Entities rarely stay static.

They evolve.

Example:

Booking

CREATED
→ CONFIRMED
→ CANCELLED

Order

CREATED
→ PAID
→ SHIPPED
→ DELIVERED

Ride

REQUESTED
→ ASSIGNED
→ STARTED
→ COMPLETED

This lifecycle is often more important than the fields themselves.

Entities Protect Business Meaning

Consider a banking system.

Account balance changes constantly.

₹10,000
₹15,000
₹8,000
₹25,000

The amount changes.

But the account remains the same account.

The business tracks:

Account #A123

not merely its current balance.

Identity carries business meaning.

Common Beginner Mistake

Many developers think:

Every noun becomes an entity.

Wrong.

Example:

Address
Money
Coordinates
Distance

These are important.

But the business usually does not track their identity.

They describe something else.

These often become Value Objects.

We will cover them in the next post.

Entity Responsibilities

A strong entity is not merely a data holder.

Bad design:

Order {
 id
 status
 amount
}

Good design:

Order {
 pay()
 cancel()
 ship()
}

Why?

Because the entity owns its lifecycle.

Business rules should stay close to the entity whenever possible.

Weak LLD Thinking

Entity = Database Row

Strong LLD Thinking

Entity = Business Object With Identity and Lifecycle

This small mindset shift changes the quality of your design dramatically.

Examples of Entities Across Systems

Ride Sharing

• Rider
• Driver
• Ride

BookMyShow

• Show
• Booking

E-Commerce

• User
• Cart
• Order

Banking

• Customer
• Account
• Transaction

Food Delivery

• Order
• Restaurant
• Delivery Partner

All of them have one thing in common:

the business cares about who they are, not just what data they contain.

The Most Important Insight

Fields can change.

State can change.

Behavior can change.

But identity remains.

That identity is what turns an ordinary object into an Entity.

And once you start recognizing entities correctly, domain models become much easier to design.

One-Line Takeaway

An Entity is a business object whose identity matters more than its current data, allowing it to be tracked consistently throughout its lifecycle.