Apollo is a distributed configuration management system designed for microservices architectures. It provides a centralized web portal (AngularJS frontend with Spring Boot backend) for managing application configurations across multiple environments (DEV, FAT, UAT, PRO, etc.) and clusters. The system supports real-time configuration updates via long-polling, version management, grayscale (canary) releases, role-based access control, and comprehensive audit logging. It exposes REST APIs for programmatic management through an OpenAPI v1 layer and provides client SDKs for various languages. The architecture consists of three main services: apollo-portal (management UI and OpenAPI endpoints), apollo-adminservice (admin CRUD operations), and apollo-configservice (client-facing configuration delivery with long-polling notifications). These services share a common business logic module (apollo-biz) and persist data in MySQL databases (ApolloConfigDB and ApolloPortalDB). The portal communicates with admin services via REST, while clients poll the config service for updates. The system also includes a service registry module for discovery of admin service instances, supporting multiple discovery mechanisms (Eureka, Consul, Nacos, Zookeeper, database, Kubernetes). Key entry points include the portal web UI, OpenAPI v1 endpoints (/openapi/v1/), admin service REST APIs, and config service APIs for client configuration retrieval and notification. The primary data flow involves creating/modifying configurations through the portal or OpenAPI, which are stored in the database. Changes trigger release messages that are scanned by the config service to notify connected clients. Clients then fetch the latest configurations via the config service. The system also supports gray releases, branching, rollback, and configuration import/export. External dependencies include MySQL (primary database), optional service discovery mechanisms (Eureka, Consul, Nacos, Zookeeper, Kubernetes), LDAP/OIDC for authentication, and email/MQ services for notifications. The codebase is modular with clear separation of concerns, extensive test coverage, and a well-defined SPI (Service Provider Interface) pattern for extensibility.