GraphQL Subscription Fields

What are GraphQL Subscription Fields?

GraphQL subscription fields enable real-time data streaming between a client and a server. Unlike queries and mutations, which are one-time operations, subscriptions keep an open connection, allowing the server to push updates to the client as they occur. This is particularly useful for applications requiring real-time updates, such as live sports scores, chat applications, or stock market feeds.

Defining Subscription Fields in GraphQL Schema

Subscriptions are defined in the GraphQL schema using the subscription keyword. Each subscription field specifies the type of data that will be streamed to the client. Here's an example of a simple subscription schema:

Implementing Resolver Logic for Subscriptions

Resolvers for subscription fields are different from those used in queries and mutations. They often involve setting up a PubSub mechanism to publish events and manage active subscriptions. The resolver function typically returns an AsyncIterator to handle the continuous stream of data. Here's a basic example using Apollo Server:

Example: Subscribing to New Messages

To subscribe to new messages, the client sends a subscription request to the server. Once the connection is established, the server will push any new messages to the client as they occur. Below is an example of a subscription query a client might use:

Handling Subscription Events

Once a subscription is active, the server can publish events that match the subscription's criteria. In our example, whenever a new message is created, it should trigger an event:

Conclusion

GraphQL subscriptions provide a powerful way to handle real-time data streaming in your applications. By defining subscription fields in your GraphQL schema and implementing resolver logic with a PubSub mechanism, you can efficiently manage and deliver live updates to clients. In the next post, we will explore the PubSub system in greater detail.