Install

npm install apollo-link

yarn add apollo-link

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.

The client sends a request as a method call to a link and can recieve one or more (in the case of subscriptions) responses from the server. The responses are returned using the Observer pattern.

Apollo Link Chain

Results from the server can be provided by calling next(result) on the observer. In the case of a network/transport error (not a GraphQL Error) the error(err) method can be used to indicate a response will not be recieved. If multiple responses are not supported by the link, complete() should be called to inform the client no further data will be provided.

In the case of an intermediate link, a second argument to request(operation, forward) is the link to forward(operation) to. forward returns an observable and it can be returned directly or subscribed to.

 forward(operation).subscribe({
  next: result => {
    handleTheResult(result)
  },
  error: error => {
    handleTheNetworkError(error)
  },
});

Implementing a basic custom transport

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

export class CustomApolloLink extends ApolloLink {
  request(operation /*, forward*/) {
    //Whether no one is listening anymore
    let unsubscribed = false;

    return new Observable(observer => {
      somehowGetOperationToServer(operation, (error, result) => {
        if (unsubscribed) return;
        if (error) {
          //Network error
          observer.error(error);
        } else {
          observer.next(result);
          observer.complete(); //If subscriptions not supported
        }
      });

      function unsubscribe() {
        unsubscribed = true;
      }

      return unsubscribe;
    });
  }
}