What is GraphQL? — Query Language for APIs Explained

Definition

GraphQL is a query language for APIs and a server-side runtime for executing queries, developed by Facebook and open-sourced in 2015. Unlike REST APIs that expose multiple fixed endpoints, a GraphQL API exposes a single endpoint and lets clients specify exactly what data they want in their query. This eliminates over-fetching (getting more data than needed) and under-fetching (needing multiple requests to assemble a complete response).

How It Works

A GraphQL API is built around a strongly-typed schema that defines all possible queries, mutations, and types. A client sends a query document to the /graphql endpoint over HTTP POST, specifying exactly which fields to return and for which nested relationships. The server validates the query against the schema, executes resolver functions to fetch the data, and returns a JSON response matching the query's shape. Mutations follow the same syntax but declare intent to modify data. Subscriptions use a persistent connection (typically WebSocket) to push real-time updates.

Common Use Cases

• ▸Building flexible data APIs for mobile apps where bandwidth matters
• ▸Aggregating data from multiple microservices into a single request
• ▸Powering frontend data layers with tools like Apollo Client and urql
• ▸Generating TypeScript types automatically from a GraphQL schema
• ▸Building developer-friendly APIs where clients dictate response shape

Example

query GetUser($id: ID!) {
  user(id: $id) {
    name
    email
    posts(limit: 5) {
      title
      publishedAt
    }
  }
}

# Response only contains the fields you asked for

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