facebook/relay

Relay is a JavaScript framework created by Facebook (Meta) for building data-driven React applications. It solves a common problem in web apps: managing how your user interface fetches and updates data from a server. Without a tool like Relay, developers must manually write code to request the right data at the right time, track what's loaded and what isn't, and keep the UI in sync when data changes. Relay uses GraphQL, a query language that lets you describe exactly what data a component needs, and handles all the fetching logic automatically. The key idea is "colocation": each UI component declares its own data requirements right next to its display code, rather than in a central data-fetching layer. Relay then bundles those individual declarations into efficient network requests, fetching only the data actually needed. It also handles data mutations (updating data on the server), with built-in support for optimistic updates, where the UI updates instantly while the server request is still in flight, making the app feel fast. You would use Relay when building large React applications that talk to a GraphQL API and where careful, efficient data fetching matters. It's particularly well-suited to complex apps with many components each needing different slices of data. The framework is MIT licensed and used in production at Meta and by the broader community.