What is a Modular Monolith and how does it work?

Question

IT Interview Practice · Accepted Answer

## What is a Modular Monolith?

**A Modular Monolith organizes an application into separate business modules that are packaged and deployed together.** Each module owns a set of responsibilities and defines how other modules can use it.

For example, a sales application might have product, inventory, and order modules. They remain part of one application; separating them helps keep the code manageable as features grow.

## How are modules organized?

A module typically has two parts:

- **Public interface:** Operations available to other modules, such as checking stock or reserving items for an order. These can be functions within the same program, without a network API.
- **Internal implementation:** The module's business rules and data access code. Other modules use its public interface instead of reaching into these details.

Modules can share a database, but ownership of each group of data should be clear. Creating separate folders while allowing every module to modify everyone else's data provides little useful separation.

## Example: placing an order

The order module calls an operation exposed by the inventory module to reserve items. Inventory checks availability, reserves the items if there is enough stock, and returns a result so order processing can continue. The order module does not update the stock table itself.

If inventory management changes, developers mainly update the inventory module while keeping the operation used by orders stable. The entire application is still deployed together for a release.

## Benefits and limitations

- **Easier maintenance:** Clear ownership of functionality and data helps locate the code involved in a fix or feature.
- **Simple deployment:** Operations manages one application, and modules can collaborate through local calls.
- **No independent module scaling:** Adding resources for one module usually means running more copies of the whole application.
- **Boundaries require upkeep:** Calls into another module's internals or direct changes to its data can tightly couple the modules.

## When should you use it?

A Modular Monolith fits an application with several business areas that the team still wants to develop and operate together. A sales management system, for instance, can separate inventory, orders, and customers without deploying each part independently.

Module boundaries also help when considering a future service extraction, but that move still requires changes to communication and data management.

What is a Modular Monolith?

How are modules organized?

Example: placing an order

Benefits and limitations

When should you use it?

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