apollo-link

What is apollo-link?

The apollo-link package provides a standard interface for modifying control flow of GraphQL requests and fetching GraphQL results. It is part of the Apollo ecosystem and allows developers to create a chain of link objects to handle GraphQL operations. This modular approach enables fine-grained control over the request/response cycle in a composable way.

What are apollo-link's main functionalities?

Middleware functionality

This feature allows developers to modify requests before they are sent to the server. In the code sample, a middleware link is created that injects an authorization token into the headers of each request.

import { ApolloLink } from 'apollo-link';

const middlewareLink = new ApolloLink((operation, forward) => {
  operation.setContext({ headers: { authorization: getAuthToken() } });
  return forward(operation);
});

Error handling

This feature allows developers to handle errors that occur during the GraphQL operation. The code sample demonstrates how to catch and process errors before they propagate further.

import { ApolloLink, Observable } from 'apollo-link';

const errorLink = new ApolloLink((operation, forward) => {
  return new Observable(observer => {
    const sub = forward(operation).subscribe({
      next: observer.next.bind(observer),
      error: error => {
        handleError(error);
        observer.error(error);
      },
      complete: observer.complete.bind(observer)
    });
    return () => sub.unsubscribe();
  });
});

Afterware functionality

This feature allows developers to modify responses returned from the server. In the code sample, an afterware link is used to check for server errors after a response is received but before it is processed further.

import { ApolloLink } from 'apollo-link';

const afterwareLink = new ApolloLink((operation, forward) => {
  return forward(operation).map(response => {
    checkForServerErrors(response);
    return response;
  });
});

Other packages similar to apollo-link

relay-runtime

Relay Runtime is a framework from Facebook for building data-driven React applications with GraphQL. It is similar to apollo-link in that it manages GraphQL data operations, but it is tightly integrated with React and uses a different set of abstractions and optimizations specific to React applications.

urql

Urql is a highly customizable and versatile GraphQL client, similar to apollo-link. It features a similar extensible approach with 'exchanges' which are analogous to Apollo's links. However, urql provides a more integrated caching mechanism and a simpler API, which might be preferred for smaller projects or those seeking simplicity.

README

apollo-link

Purpose

apollo-link is a standard interface for modifying control flow of GraphQL requests and fetching GraphQL results, designed to provide a simple GraphQL client that is capable of extensions. The targeted use cases of apollo-link are highlighted below:

• fetch queries directly without normalized cache
• network interface for Apollo Client
• network interface for Relay Modern
• fetcher for

Apollo Link is the interface for creating new links in your application.

Installation

npm install apollo-link --save

To use this package in a web browser or mobile app, you'll need a build system capable of loading NPM packages on the client. Some common choices include Browserify, Webpack, and Meteor +1.3.