Overview

With HTTP callbacks, a subgraph server uses a callback URL provided by the router to send new subscription data as soon as it becomes available. This process occurs without maintaining a long-lived, resource-intensive connection—instead, the subgraph "checks in" periodically with the router.

Process at a glance

There are three primary roles involved in this process: the router, the subgraph, and the emitter.

The router receives the query, and breaks it up to fetch data from each responsible subgraph. In addition, it tells the subgraph(s) that provide subscription data how to "call back" when new data is available.

The emitter is the mechanism that checks in with the router periodically and "emits" new subscription data. (This can be a separate service—or the subgraph itself.)

The subgraph's roles & responsibilities

• Receive subscription operation from router
• Establish connection with the router
• Retrieve and return data
• Manage periodic check-ins

The subgraph also needs to be running a framework that has implemented support for the HTTP Callback Protocol. This comes out of the box with Apollo Server.

The router's roles & responsibilities

• Receive operations from clients, break them up across the graph, and return the response
• Set the HTTP callback configuration:
  • Define path and port
  • How often the subgraph should check in
  • Which subgraphs support subscriptions