@loopback/docs/site/tutorials/core/1-introduction.md

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---
lang: en
title: 'Introduction of the application scenario'
keywords: LoopBack 4.0, LoopBack 4, Node.js, TypeScript, OpenAPI
sidebar: lb4_sidebar
permalink: /doc/en/lb4/core-tutorial-part1.html
---

At the core of LoopBack 4, we provide a powerful
[Inversion of Control](../../Context.md) and
[Dependency Injection](../../Dependency-injection.md) container in TypeScript
that serves as the foundation to manage many types of collaborating artifacts as
building blocks for complex applications. In addition to great API and
Microservice capabilities offered by the LoopBack framework, these core modules
can be used independently as a platform to develop large scale Node.js projects
that require flexibility, extensibility, and composability.

To help our users leverage the core packages, we created a series of tutorials
to explain why such features matter and how to use them for your development.

## Sample scenario

Let's first introduce the business scenario for our series of tutorials.

We're going to build an fancy version of `hello world` with the following
requirements:

1. The service is capable of speaking different languages to greet people. For
   example:

   - English: Hello, Raymond!
   - Chinese: Raymond，你好！

2. It should be easy to add support for a new language, such as French.

3. A REST API is provided to access the service via
   `http://<server>:<port>/greet/<name>`. It will receive a response based on
   the `Accept-Language` http request header.

4. Caching is supported using the request url (`/greet/<name>`) and language as
   the key. Expiration is controlled by a configurable `ttl`.

5. The application can be run standalone to try the greeting service or as a
   server to expose REST APIs.

## High level design

Instead of creating a monolithic application, we would like to have good
separation of concerns by dividing responsibilities into a few artifacts. The
key building blocks are illustrated below based on features offered by
LoopBack 4. The code is being developed in the
[Greeter Extension Example](https://github.com/loopbackio/loopback-next/tree/core-tutorial/examples/greeter-extension).

![Greeting Scenario](../../imgs/tutorials/core/greeting-app.png)

## Key artifacts

We are going to decompose the application into two packages:

1. [example-greeter-extension](https://github.com/loopbackio/loopback-next/tree/core-tutorial/examples/greeter-extension)

   - GreetingService to greet by language and name
   - English and Chinese greeters to greet in English and Chinese respectively

2. [example-greeting-app](https://github.com/loopbackio/loopback-next/tree/core-tutorial/examples/greeting-app)

   - CachingService to provide caching facilities
   - GreetingController to expose REST APIs
   - CachingInterceptor to apply caching
   - CachingObserver to start/stop caching and sweep the store to remove expired
     entries

There are dependencies between such artifacts, such as:

- GreetingService -> English Greeter and Chinese Greeter
- GreetingController -> GreetingService
- CachingInterceptor/CachingObserver -> CachingService

Some of the artifacts can be configurable, either statically at startup time or
dynamically when the application is running.

---

Previous: [Index](./index.md)

Next: [Part 2 - Architectural challenges](./2-architecture.md)