graphql-sse VS fetch-event-source

The GraphQL over SSE in its simplest form IMHO does not require an additional client library. Setting up a subscription in the browser is as simple as doing the following:

There is already a JavaScript implementation for server + client with support for multiplexing from the same maintainer of graphql-ws: graphql-sse

Actually WebSocket is not the only protocol for executing GraphQL subscriptions. There is also a graphql-over-sse protocol, which is much more lightweight, and uses Server Sent Events. A popular implementation of it is the graphql-sse library, which is maintained by the same person as graphql-ws. The setup and API of both libraries is pretty similar.

graphql-sse is a reference implementation of the GraphQL over Server-Sent Events Protocol aiming to become a part of the GraphQL over HTTP standard.

The EventSource Web API for handling SSEs (built into most modern browsers) unfortunately supports only GET requests, which was quite limiting when a POST request with larger body JSON data was required. As an alternative, you can use the Fetch API or a ready library like Microsoft’s Fetch Event Source.

Aforementioned limitations are taken care with a specialised SSE client (inspired by the awesome @microsoft/fetch-event-source) and two separate connection modes: the HTTP/1 safe "single connection mode" that uses a single SSE connection for receiving events with separate HTTP requests dictating the behaviour, and the HTTP/2+ "distinct connections mode" that uses distinct SSE connections for each GraphQL operation, accommodating the parameters in the request itself.