Travis API is the backend REST API for the Travis CI continuous integration platform. It provides multiple API versions (v0/v1/v2/v3) that expose resources such as builds, jobs, repositories, users, organizations, cron schedules, caches, logs, environment variables, SSL keys, and billing/subscription data. The application is built on Ruby with Sinatra for HTTP routing and ActiveRecord for PostgreSQL persistence. It uses Redis for caching, feature flags, and background job queues (Sidekiq). External integrations include GitHub OAuth, a billing service, a remote VCS service (GitHub/GitLab/Bitbucket/Assembla), a logs API, artifact manager, scanner service, and cloud object storage (S3/GCS). The codebase follows a service-oriented architecture for the V3 API, with clear separation into services, queries, models, renderers, and permission classes. Legacy V1/V2 endpoints coexist, increasing maintenance surface. Authentication supports user tokens, application tokens, and log tokens, with fine-grained permissions checked against an external authorizer or local models. The system also handles enterprise deployments with custom licensing and user suspension. Notable engineering patterns include a generic renderer framework with representation modes (minimal/standard), a pagination system with Redis count caching, an encrypted column implementation, a config obfuscator for secure variables, and a feature flag system using Redis.