Mercurius is an open-source library for building real-time GraphQL servers in Node.js. It provides a simple and flexible way to build GraphQL servers by providing a set of tools and features for handling real-time subscriptions.
One of the key features of Mercurius is that it is built on top of the popular WebSockets protocol and it allows the client to subscribe to real-time updates from the server. It also supports batching and caching of queries and mutations, which allows for efficient and fast data transfer between the client and server.