Directus

Directus is an open-source headless CMS and backend-as-a-service that allows developers to manage complex content structures, handle digital assets, and control user permissions. It provides instant REST and GraphQL APIs for any SQL database and features a visual automation builder for data workflows. Directus aims to provide a production-ready backend and UI for various custom builds, from simple websites to complex applications.

Directus is a headless CMS and backend-as-a-service that allows developers to connect to any SQL database and instantly get REST and GraphQL APIs. It offers a visual data modeling interface, granular policy-based authentication, and a visual automation builder for workflows. While it provides a 'Studio' for managing content and data, it's not a drag-and-drop UI builder for building full applications in the same vein as Retool, but rather for managing the data and content itself. It supports self-hosting and offers extensive customization and extension capabilities.