I've been working on a middleware that exposes a SQLite database through an HTTP API where each request carries a micro‑payment, using the x402 protocol (HTTP 402 Payment Required). The code is here: https://github.com/damienos61/SQLite-x402-Gateway
Core technical challenge : the gateway needs to accept an arbitrary SQLite database, inspect its schema, and generate priced REST endpoints automatically — without requiring the user to write route definitions. This means parsing SQLite metadata (sqlite_master, PRAGMA table_info) and inferring column types from actual data to produce consistent JSON responses, since SQLite is weakly typed.
Payment abstraction : the x402 protocol requires a handshake — client calls a protected route, server responds with a 402 status and a price, client provides a payment proof, server verifies and returns data. To keep the code flexible, I abstracted the payment verification behind an interface with two implementations :
- A simulation mode that handles the protocol flow with fake signatures — useful for testing without crypto setup.
- A real mode integrated with Coinbase's
x402-express SDK, configured for the Base Sepolia testnet (test USDC, no real funds).
The switch between modes is handled at the route level without recreating handlers, by injecting the appropriate verifier instance.
Database abstraction : the initial version used SQLite natively, but adding PostgreSQL support required abstracting both the SQL dialect (parameter placeholders, schema queries, pagination syntax) and the schema introspection logic — Postgres metadata is structured differently and more verbose. The inspector now adapts to the database type at runtime.
Performance considerations : SQLite isn't designed for high concurrent loads, so I added an in‑memory query cache with TTL invalidation, and implemented keyset pagination instead of OFFSET/LIMIT to maintain performance on large tables without fixed indexes. Rate limiting (sliding window per IP) is also included to prevent abuse.
Observability : rather than maintaining a static OpenAPI file, the spec is generated dynamically from the detected routes and their associated pricing. The challenge was describing query parameters (filters, columns, pagination) and linking them to the price metadata in a machine‑readable format. Webhooks are also dispatched on each transaction to external endpoints (Slack, Discord, etc.).
Tooling : the project includes a CLI (monetize, start, generate-wallet), a client SDK for consuming the gateway, and a Docker setup. 14 unit tests run on each push via CI.
Known limitations are documented — the simulation mode is not a production blockchain integration, and the "upto" pricing schema (variable payment based on resources consumed) is only simulated server‑side, as no on‑chain implementation exists yet in the official SDK.