@loopback/docs/site/Concepts.md

Documentation for LoopBack 4

---
lang: en
title: 'Key concepts'
keywords: LoopBack 4.0, LoopBack 4
tags:
sidebar: lb4_sidebar
permalink: /doc/en/lb4/Concepts.html
summary:
---

LoopBack 4 introduces some new concepts that are important to understand:

- [**Application**](Application.md): The central class for setting up all of
  your module’s components, controllers, servers and bindings. The Application
  class extends [Context](Context.md) and provides the controls for starting and
  stopping its associated servers.

- [**Server**](Server.md): An implementation for inbound transports/protocols
  such as REST (http, https), gRPC (http2) and graphQL (http, https). It
  typically listens on a specific endpoint (protocol/host/port), handles
  incoming requests, and then returns appropriate responses.

- [**Context**](Context.md): An abstraction of states and dependencies in your
  application that LoopBack uses to manage everything. It's a global registry
  for everything in your app (configurations, state, dependencies, classes and
  so on).

- [**Dependency Injection**](Dependency-injection.md): The technique used to
  separate the construction of dependencies of a class or function from its
  behavior to keep the code loosely coupled.

- [**Controller**](Controllers.md): A class that implements operations defined
  by the application’s REST API. It implements an application’s business logic
  and acts as a bridge between the HTTP/REST API and domain/database models. A
  Controller operates only on processed input and abstractions of backend
  services / databases.

- [**Route**](Routes.md): The mapping between your API specification and an
  Operation. It tells LoopBack which Operation to `invoke()` when given an HTTP
  request.

- [**Sequence**](Sequence.md): A stateless grouping of
  [Actions](Sequence.md#actions) that control how a Server responds to requests.

- [**Model**](Model.md): The definition of an object in respect to the
  datasource juggler. The `@loopback/repository` module provides special
  decorators for adding metadata to TypeScript/JavaScript classes to use them
  with the legacy implementation of DataSource Juggler. In addition,
  `@loopback/repository-json-schema` module uses the decorators' metadata to
  build a matching JSON Schema.

- [**Repository**](Repositories.md): A type of service that represents a
  collection of data within a DataSource.

- [**Decorator**](Decorators.md): The pattern used to annotate or modify your
  class declarations and their members with metadata.

- **Component**: A package that bundles one or more Loopback extensions.
  - See [Using components](Using-components.md) and
    [Creating components](Creating-components.md) for more information.