พื้นฐาน GraphQL

Welcome to GraphQL! It's a powerful query language for APIs and a runtime for fulfilling those queries with your existing data.

Think of it as asking for exactly what you need, nothing more, nothing less. This helps make your APIs efficient and flexible.

Traditionally, REST APIs often require multiple requests to fetch all necessary data, leading to "over-fetching" or "under-fetching".

• Over-fetching: Getting more data than you need.
• Under-fetching: Needing multiple requests to get all data.

GraphQL solves this by letting clients define the exact data structure they want in a single request.

At the heart of every GraphQL API is its schema. This schema acts as a contract between the client and the server.

It defines all the data types and operations (queries, mutations, subscriptions) that clients can perform. It's like a blueprint for your API.

GraphQL schemas are written using the Schema Definition Language (SDL). It's a simple, human-readable syntax.

SDL allows you to specify your API's capabilities and data structures clearly. It's language-agnostic, meaning it works with any backend language.

The most fundamental component of a GraphQL schema is the Object Type. It represents a type of object you can fetch from your service.

Each object type has fields, which are specific pieces of data it can hold. Fields can also be other object types!

Scalar types are the basic building blocks that resolve to a single value, like String, Int, Float, Boolean, and ID (a unique identifier).

List types allow a field to return a list of a certain type, indicated by []. The ! means a field is Non-Null, so it must always return a value.

Queries are how clients request data from a GraphQL server. You specify the exact fields you want to receive.

This ensures you only get the data you ask for, avoiding over-fetching common in REST APIs.

Fields in GraphQL can take arguments, allowing you to pass parameters to customize the data fetched. This is useful for filtering or finding specific items.

Arguments are defined in the schema and used by the client when making a query.

While queries are for reading data, mutations are for writing, updating, or deleting data on the server.

Like queries, mutations also specify what data to return after the operation, ensuring immediate feedback on changes.

Subscriptions are a special type of operation that allows clients to receive real-time updates from the server when specific events occur.

This is perfect for features like live chat, notifications, or real-time dashboards. It typically uses WebSockets.

Great job! You've learned the core concepts of GraphQL:

• It's a query language for APIs, offering precise data fetching.
• The schema defines the API's capabilities using SDL.
• You use queries to read, mutations to write, and subscriptions for real-time data.

Next, we'll see how to integrate GraphQL into a NestJS application!