CombineKit

Key Metrics

Overview

CombineKit is a PDF processing platform that allows users to merge multiple PDF documents into a single file while enforcing freemium usage restrictions. The project explores SaaS architecture patterns including authentication, quota enforcement, binary file processing, and usage metering through a production-inspired full-stack design.

Problem

Document processing applications often require secure file handling, authenticated access, usage control, and scalable backend processing. Building such a platform involves challenges around binary file management, quota enforcement, authentication workflows, and SaaS business logic.

Solution

Developed a PDF merging platform where authenticated users can upload multiple PDF files, merge them server-side, download the resulting document, and have their monthly usage tracked automatically through MongoDB-based quota enforcement.

Architecture

Full-stack architecture built with Next.js App Router and API Routes. Clerk handles authentication and JWT validation, MongoDB stores user usage data, and pdf-merger-js performs server-side document processing. Usage quotas are enforced through atomic MongoDB operations before merged PDFs are streamed back to authenticated users.

Challenges

• Handling binary file uploads and downloads.
• Implementing secure PDF processing workflows.
• Designing freemium quota enforcement.
• Managing MongoDB connection lifecycles.
• Tracking usage through atomic database operations.
• Handling server-side file processing in a serverless environment.
• Managing Blob and Object URL cleanup on the client.

Lessons Learned

• Next.js App Router full-stack architecture.
• JWT authentication workflows using Clerk.
• MongoDB atomic update operations.
• Binary HTTP response handling.
• Serverless connection management patterns.
• Freemium SaaS quota system design.
• Secure file upload and processing techniques.
• Event loop implications of CPU-intensive operations.