Backend engineering API and database development. Actions: design API, implement endpoints, optimize database, create authentication, secure, cache, handle errors, validate input. Projects: REST API, GraphQL API, microservices, backend service, server development. Topics: RESTful API, HTTP methods, status codes, JWT, OAuth, authentication, authorization, RBAC, database indexing, query optimization, N+1 queries, caching, Redis, SQL injection, XSS, CORS, rate limiting, error handling, validation, bcrypt, password hashing.