Convex is a reactive backend platform made by Convex, Inc. A backend is the server side of an application: the database and the code that reads and writes it. In Convex both are written in TypeScript, and "reactive" means that when data changes, the app screens that show it receive the new data at once.
A developer writes server functions in TypeScript next to the app code, and client libraries connect the web or mobile app to them. Convex runs the functions with ACID transactions (all-or-nothing updates that keep data consistent), so concurrent writes do not corrupt data, and it caches and scales them automatically. The platform also includes file storage, text and vector search, scheduled jobs (crons), authentication, and open-source components for tasks such as multi-step workflows and AI agents.
The backend is open source on GitHub and can be self-hosted with Docker or a prebuilt binary. The hosted cloud service has a free plan with usage-based billing and paid plans priced per developer per month.