Polar is an open-source, full-stack monetization platform designed for AI-era software developers. It serves as a Merchant of Record, handling checkout flows, subscriptions, usage-based billing, seats, discounts, and global tax compliance. The system is built as a monorepo with a Python/FastAPI backend (async SQLAlchemy, Dramatiq workers, PostgreSQL, Redis, Tinybird for event analytics), a Next.js web dashboard and customer portal, a React Native mobile app (Expo), a design system (Orbit), and multiple SDKs (TypeScript, Python) with a CLI tool. Architecture follows a service-oriented pattern with versioned REST APIs (2026-04, 2026-10, 2027-01), a repository pattern for persistence, benefit fulfillment strategies for integrations (Discord, GitHub, Slack, license keys), and framework adapters (Next.js, Nuxt, TanStack Start, Better Auth) for easy integration. The platform also includes an internal backoffice (HTMX/tagflow), observability (SLOs, invariants, Prometheus metrics), and merchant migration tooling from Stripe.
AutoGPT is a full-stack SaaS platform for building, deploying, and running AI agents. It provides a visual agent builder (blocks), a copilot chat interface that can generate and orchestrate agents from natural language, a marketplace for sharing agents and skills, team/experts management with scheduling and credential stacks, and a subscription billing system with Stripe. The architecture is a multi-service backend (FastAPI, PostgreSQL, Redis) with a Next.js frontend, supporting real-time WebSocket execution events, OAuth integrations with 45+ providers, and feature flags via LaunchDarkly and PostHog. The platform is dual-licensed (Polyform Shield for platform code, MIT for classic code) and supports both hosted SaaS and self-hosted deployment. The codebase is organized into two major areas: `autogpt_platform/` (the current platform) and `classic/` (legacy AutoGPT and Forge). The platform backend uses a block-based execution system where each integration (Google, Slack, GitHub, etc.) is a self-contained block with typed I/O schemas, cost tracking, and credential management. The frontend features a rich component library with dynamic form rendering (RJSF), responsive dialog/drawer patterns, and extensive state management via TanStack Query and Zustand. Key entry points include the copilot chat UI, the agent library, marketplace, team management, and admin panels. Primary data flows involve user input through the copilot or builder, which triggers graph execution via the execution manager, with events streamed over WebSocket. Credits are deducted per block execution, and billing is handled through Stripe subscriptions with trial management. Analytics events are sent to PostHog and DataFast for product metrics. The system relies on external dependencies including PostgreSQL (with pgvector), Redis, Stripe, LaunchDarkly, PostHog, Sentry, and numerous third-party APIs for integrations.
This repository is a full-stack, multi-tenant AI API gateway platform (named 'sub2api') that proxies requests to a wide range of AI model providers including OpenAI, Anthropic, Gemini, Grok, DeepSeek, and many others. It provides authentication (JWT, OAuth, API keys), rate limiting, token-level billing, subscription management, user and admin management, payment processing (Alipay, WeChat Pay, Stripe, Airwallex, EasyPay), content moderation, channel monitoring, and operational dashboards. The backend is written in Go using the Gin framework and Ent ORM, with PostgreSQL and Redis for data storage and caching. The frontend is built with Vue 3, TypeScript, and Pinia stores, offering both admin and user interfaces. The system is designed for production deployment with Docker, includes extensive unit and integration tests, and supports a plugin architecture for extensible OAuth transport. Despite its large codebase and many features, the architecture follows a clear service-repository pattern with well-isolated packages, making it a solid example of a complex API gateway implementation.
Akaunting is a self-hosted accounting and invoicing platform for small businesses and freelancers. It manages invoices, bills, banking transactions, transfers, reconciliations, multi-currency, multi-tax, recurring documents, and financial reports. It includes a customer portal for invoice viewing and payment, and supports multi-company, multi-user with role-based permissions. The system is highly internationalized with 40+ language locales. The architecture is built on a custom PHP framework (Illuminate) that extends Laravel conventions. It uses an event-driven architecture with domain events, listeners, and a job queue for async processing. The frontend combines Blade templates, Vue.js components, and Livewire for interactivity. Multi-tenancy is achieved through company scoping via a global query scope. The system is extensible via a module system with an app store, and includes a CLI for maintenance tasks. Key entry points include the installation wizard (controllers under Install/), authentication flows, and core CRUD controllers for sales, purchases, banking, and settings. Data flows typically involve controllers dispatching jobs that emit events, which listeners handle to update related entities, send notifications, or trigger side effects. Reports use a filter/group/rows event system for extensibility. Import/export is handled via Excel with multi-sheet support. External dependencies include PHP 8.1+, Node 18-20, and a database (MariaDB, MySQL, PostgreSQL, SQLite). It uses Laravel Mix for asset compilation, integrates with payment gateways, email services, and error trackers (Sentry, Bugsnag), and relies on a module marketplace at api.akaunting.com.
Pollinations is a large monorepo hosting a multi-modal generative AI platform. It provides a unified API gateway (gen.pollinations.ai) for text, image, video, audio, 3D, and embeddings generation, routing requests to dozens of providers (OpenAI, Anthropic, Google, Fal, Replicate, ElevenLabs, etc.) with fallback chains, caching, rate limiting, and billing. A separate enterprise service (enter.pollinations.ai) handles authentication (GitHub OAuth, API keys, device code flow), user dashboards, API key management, Stripe/Pollen billing, community-hosted model endpoints, and a quest/reward gamification system. The platform also includes a design system UI library (@pollinations/ui), a Node.js SDK, a CLI tool (polli-cli), an MCP server, and over 20 community apps (chat, AI dungeon master, image reimagine, Discord bots, etc.). Infrastructure includes Cloudflare Workers, Durable Objects, D1, R2, Tinybird for analytics, and GPU inference servers (Flux, Dreamshaper, Z-Image) deployed on Vast/IO.net. The architecture is service-oriented within a monorepo: each major service (gen, enter, media, pollinations.ai) is a separate Cloudflare Worker with its own wrangler.toml and dependencies. Shared packages (shared/, packages/) provide cross-cutting concerns like billing logic, model registry, auth middleware, HTTP error handling, and UI components. Data flows from user requests through middleware (auth, balance, cache, rate-limit, safety, model resolution) to provider-specific adapters, with usage events published to Tinybird for analytics and billing. The system is heavily tested with Vitest, including integration tests with Cloudflare Workers test environment and VCR snapshots. External dependencies include Cloudflare (Workers, D1, R2, KV, Durable Objects, Vectorize), Tinybird (analytics), Stripe (payments), GitHub (OAuth, quests), and numerous AI model providers. The codebase is predominantly TypeScript with some Python (Floret agent, GPU servers, social automation) and Rust (polli-core for Discord bot). While the core platform is well-maintained, there is significant drift in community apps and infrastructure scripts, and the sheer number of provider integrations creates maintenance surface area.
CoAI (formerly ChatNio) is a full-stack AI chat platform and LLM gateway that provides a unified chat interface and OpenAI-compatible API proxy for 16+ AI model providers (OpenAI, Anthropic Claude, Google Gemini, Midjourney, DeepSeek, etc.). The system is built with a Go backend (Gin framework) and a React/TypeScript frontend (Redux, Tailwind CSS, Radix UI), with MySQL and Redis for persistence and caching. It supports real-time WebSocket chat, streaming responses, conversation management, file upload, web search, image generation, and a plugin system (MCP). The platform includes a comprehensive billing and subscription system with elastic per-token pricing and tiered plans (Basic/Standard/Pro), along with an admin dashboard for user management, channel configuration, analytics (model usage, request volume, revenue, errors), and system settings. It also features invitation codes, redeem codes, affiliate marketing, and multiple payment gateway integrations (Stripe, WeChat Pay, etc.). Architecturally, the backend employs an adapter pattern for AI providers, a channel manager for load balancing and retries, and a WebSocket connection manager with rate limiting. The frontend is heavily componentized with shadcn/ui and includes i18n support (English, Chinese, Japanese, Russian). Deployment is supported via Docker Compose, Zeabur, and a Tauri desktop app. While the codebase is well-structured and feature-rich, it shows signs of technical drift such as copy-pasted processor files across adapters, hardcoded model constants, and a large dependency surface. The open-source version is Apache 2.0 licensed, with a commercial 'CoAI Pro' tier offering additional features.
PayKit is an open-source, embedded billing framework for TypeScript applications. It provides a core library (`paykitjs`) that handles subscription management, metered usage tracking, feature entitlements, and webhook processing – all backed by PostgreSQL and currently integrated exclusively with Stripe as the payment provider. The monorepo includes a CLI for project scaffolding and migration, a client SDK, a dashboard plugin (`@paykitjs/dash`), a documentation website (Next.js + Fumadocs), a demo application, and a webhook tunnel worker (Cloudflare Workers + Durable Objects). The architecture is modular: a core `createPayKit()` instance orchestrates services for customers, subscriptions, entitlements, products, and invoices, each following a service pattern with Drizzle ORM models. A plugin system allows custom endpoints, and a provider abstraction (currently only Stripe) handles payment operations. The codebase is well-structured with TypeScript end-to-end, extensive E2E testing against Stripe test clocks, and a clean separation of concerns. However, it is in alpha stage, has duplicated UI component libraries across apps, and is tightly coupled to Stripe – factors that limit immediate reuse without adaptation.
Invoice Ninja is a full-featured open-source invoicing and billing platform designed for small to medium businesses. It handles the complete lifecycle of invoices, quotes, credits, purchase orders, payments, and expenses. The system supports multi-tenant SaaS and self-hosted deployments with per-company database isolation, and integrates with over 20 payment gateways (Stripe, PayPal, Mollie, GoCardless, etc.). It provides client and vendor portals for self-service, along with extensive reporting, CSV export, and e-invoicing capabilities (Peppol, Verifactu, France e-reporting). The application follows a monolithic MVC architecture built on a custom PHP framework (Illuminate) similar to Laravel. It uses an event-driven architecture with domain events and listeners for side effects (email, webhooks, activity logging). Background jobs are queued for async processing. The frontend uses Blade templates, Livewire components, and a separate React app for some admin panels. The API is RESTful with OpenAPI documentation. Key entry points are HTTP controllers under app/Http/Controllers, with request validation classes. Core data flows involve creating entities through factories, persisting via repositories, and triggering events that listeners handle. Payment flows go through abstract payment drivers that encapsulate gateway-specific logic. The system also includes extensive reporting, CSV export, and e-invoicing (Peppol, Verifactu, France e-reporting). External dependencies include a relational database (MySQL/PostgreSQL), Redis for caching and queues, Elasticsearch for search, and various third-party APIs for payment processing, tax calculation (ZipTax), bank integration (Nordigen, Yodlee), email delivery (Mailgun, Postmark, Brevo), and e-invoicing gateways (Storecove, Qvalia).