The Cosmo Router is a fully open-source Federation V1/V2 compatible router that supports event-driven subscriptions in federated GraphQL. Unlike traditional WebSocket connections, which can be complex to implement and have drawbacks such as statefulness, inefficiency, and compatibility issues with enterprise environments, the Cosmo Router provides an alternative solution using Server-sent Events (SSE). SSE connections are stateless, making them more efficient and compatible with enterprise environments. The Cosmo Router supports three protocols: graphql-ws, Server-Sent Events (SSE), and subscriptions-transport-ws. It enables real-time data updates, reduces network overhead, and improves performance by minimizing the number of connections and state information required. Additionally, it allows for self-hosting, scalability, and serverless deployments, making it an ideal solution for enterprise environments.