What is N-Tier Architecture? How is it structured and how does it work?

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## What is N-Tier Architecture?

**N-Tier Architecture** divides a system into separate deployment tiers, usually running on different servers or groups of servers and communicating over a network. A common arrangement has three tiers: web, business and data.

The key distinction is that **a layer separates code responsibilities, while a tier separates deployment locations**. One deployment tier can contain several code layers. Creating three folders within an application does not make it a three-tier system.

## Structure of a three-tier system

- **Web tier:** Provides the web interface, receives user requests and passes processing work to the business tier.
- **Business tier:** Applies application rules, such as validating orders or handling leave requests. Data-access code can also live in this tier.
- **Data tier:** Runs the database that stores information and serves queries from the business tier.

The tiers are deployed separately, but a tier does not have to be a single server. For example, the web tier can run multiple instances to serve many users at once.

## How does a request move through the tiers?

**Consider a leave-management system:** An employee submits a request on the website. The web tier receives the information and calls the business tier's API over the network. The business tier retrieves the employee's allowance from the database, checks the request and saves it for approval if valid.

The result returns to the web tier for display. In this design, the browser does not connect directly to the database; the business tier handles requests to read and write data.

If the web tier is overloaded, additional web servers can be added without immediately adding business servers. However, if the database is the bottleneck, increasing web capacity will not address that cause.

## Benefits

- **Resources can be allocated by tier:** Capacity can be increased where it is needed instead of across every part of the system.
- **Clearer access controls:** Database access can be restricted to the business tier, reducing external exposure.
- **Separate tier deployments:** Operations teams can update or maintain a tier as needed, provided communication with the other tiers stays compatible.

## Limitations

- **Additional latency and network failures:** Calls between tiers can be slow or fail, and the application must handle those situations.
- **More operational work:** Multiple deployment locations need configuration, monitoring and protection.
- **Features are not necessarily separated:** HR, leave management and reporting may still share a business tier and require a coordinated release.

## When should you use it?

N-Tier Architecture suits systems that need separate resource management or access controls for web, business processing and data. It is also common in traditional enterprise systems. For a small application, add a tier only when the benefit justifies the extra communication and operational complexity.

What is N-Tier Architecture?

Structure of a three-tier system

How does a request move through the tiers?

Benefits

Limitations

When should you use it?

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