Real-time with GraphQL Subscriptions

While Queries and Mutations cover data fetching and modification, many modern applications require real-time updates. GraphQL Subscriptions provide a way for clients to listen to events from the server and receive new data as it becomes available, typically over a persistent connection like WebSockets.

What are Subscriptions?

Subscriptions are a type of GraphQL operation that allows a client to subscribe to specific events on the server. When such an event occurs, the server pushes data to the subscribed client. This is in contrast to queries, which are a pull-based mechanism (client requests data when it needs it).

This push-based model is ideal for applications such as:

• Live chat applications
• Real-time notifications
• Live sports score updates
• Collaborative editing tools
• Streaming financial data (similar to how AI financial tools process live market data)

How Subscriptions Work

A subscription operation looks very similar to a query. The client sends a subscription string to the server, specifying the event it's interested in and the data it wants to receive when that event occurs. For instance, a client might subscribe to new comments on a specific blog post.

                
                    subscription OnNewComment($postId: ID!) {
                        newComment(postId: $postId) {
                            id
                            content
                            author {
                                name
                            }
                        }
                    }
                
            

When a new comment is added to the specified post, the server would send a payload to the client containing the id, content, and author details of the new comment. The underlying transport is usually WebSockets, which maintains a long-lived connection between the client and server. This is crucial for technologies like IoT, as detailed in The Impact of 5G on IoT.

Defining Subscriptions in the Schema

Like queries and mutations, subscriptions are defined as fields on a special root type in your schema, typically called Subscription.

                
                    type Subscription {
                        newComment(postId: ID!): Comment
                        postLiked(postId: ID!): Post # Assuming a Post type and a like event
                    }

                    type Comment {
                        id: ID!
                        content: String!
                        author: User!
                    }

                    type User {
                        id: ID!
                        name: String!
                    }

                    type Post {
                        id: ID!
                        title: String!
                        likes: Int!
                    }
                
            

The resolver function for a subscription field is different from query and mutation resolvers. Instead of returning data directly, it returns an AsyncIterator which the GraphQL execution engine uses to push events to the client.

Subscriptions complete the picture of GraphQL's interaction model, enabling rich, dynamic, and real-time user experiences. With an understanding of queries, mutations, and subscriptions, you're well-equipped to build powerful applications. The next step is to explore the vast GraphQL Tools and Ecosystem that support development.