Agent Development Guide

This document provides guidance for LLMs working on the @tursodatabase/serverless TypeScript driver.

Project Overview

This is a fetch() API-compatible serverless database driver for Turso Cloud that implements the SQL over HTTP protocol (internally called "hrana"). It's designed for serverless and edge compute environments like Cloudflare Workers and Vercel Edge Functions.

Key Features

• HTTP-based SQL execution using the v3 cursor endpoint for streaming
• Native streaming API with Connection/Statement pattern
• LibSQL compatibility layer for drop-in replacement
• TypeScript-first with full type safety
• Edge-optimized using only fetch() API

Architecture

Core Files Structure

src/
├── connection.ts    # Connection class and connect() function
├── statement.ts     # Statement class with get()/all()/iterate() methods
├── protocol.ts      # Low-level SQL over HTTP protocol implementation
├── compat.ts        # LibSQL API compatibility layer
├── compat/index.ts  # Compatibility layer exports
└── index.ts         # Main package exports

Package Exports

• Main API: @tursodatabase/serverless - Native streaming API
• Compatibility: @tursodatabase/serverless/compat - LibSQL-compatible API

Native API Design

Connection/Statement Pattern

import { connect } from "@tursodatabase/serverless";

const client = connect({ url, authToken });
const stmt = client.prepare("SELECT * FROM users WHERE id = ?", [123]);

// Three execution modes:
const row = await stmt.get();        // First row or null
const rows = await stmt.all();       // All rows as array  
for await (const row of stmt.iterate()) { ... } // Streaming iterator

Key Classes

Connection

• Purpose: Database connection and session management
• Methods: prepare(), execute(), batch(), executeRaw()
• Internal: Manages baton tokens, base URL updates, cursor streaming

Statement

• Purpose: Prepared statement execution with multiple access patterns
• Methods: get(), all(), iterate()
• Streaming: iterate() provides row-by-row streaming via AsyncGenerator

Protocol Layer

• Purpose: HTTP cursor endpoint communication
• Key Function: executeCursor() returns streaming cursor entries
• Protocol: Uses v3 cursor endpoint (/v3/cursor) with newline-delimited JSON

LibSQL Compatibility Layer

Purpose

Provides drop-in compatibility with the standard libSQL client API for existing applications.

Key Differences

• Entry Point: createClient() instead of connect()
• Import Path: @tursodatabase/serverless/compat
• API Surface: Matches libSQL client interface exactly
• Config Validation: Only supports url and authToken, validates against unsupported options

Supported vs Unsupported

// ✅ Supported
const client = createClient({ url, authToken });
await client.execute(sql, args);
await client.batch(statements);

// ✅ Supported: remote encryption key for encrypted Turso Cloud databases
createClient({ url, authToken, remoteEncryptionKey: "base64-encoded-key" });

// ❌ Unsupported (throws LibsqlError)
createClient({ url, authToken, encryptionKey: "..." }); // Validation error
await client.transaction(); // Not implemented
await client.sync(); // Not supported for remote

Protocol Implementation

SQL over HTTP (v3 Cursor)

• Endpoint: POST /v3/cursor
• Request: JSON with baton, batch steps
• Response: Streaming newline-delimited JSON entries
• Entry Types: step_begin, row, step_end, step_error, error

Session Management

• Baton Tokens: Maintain session continuity across requests
• Base URL Updates: Handle server-side redirects/load balancing
• URL Normalization: Convert libsql:// to https:// automatically

Testing Strategy

Integration Tests

integration-tests/
├── serverless.test.mjs  # Native API tests
└── compat.test.mjs      # Compatibility layer tests

Test Requirements

• Environment Variables: TURSO_DATABASE_URL, TURSO_AUTH_TOKEN
• Serial Execution: All tests use test.serial() to avoid conflicts
• Real Database: Tests run against actual Turso instance

Running Tests

npm test  # Runs all integration tests
npm run build  # TypeScript compilation

Development Guidelines

Code Organization

• Single Responsibility: Each file has a clear, focused purpose
• Type Safety: Full TypeScript coverage with proper imports
• Error Handling: Use proper error classes (LibsqlError for compat)
• Streaming First: Leverage AsyncGenerator for memory efficiency

Key Patterns

• Protocol Abstraction: Keep protocol details in protocol.ts
• Compatibility Isolation: LibSQL compatibility in separate module
• Row Objects: Arrays with column name properties (non-enumerable)
• Config Validation: Explicit validation with helpful error messages

Performance Considerations

• Streaming: Use iterate() for large result sets
• Memory: Cursor endpoint provides constant memory usage
• Latency: First results available immediately with streaming

Common Tasks

Adding New Features

1. Protocol: Add to protocol.ts if it requires HTTP changes
2. Connection: Add to connection.ts for connection-level features
3. Statement: Add to statement.ts for statement-level features
4. Compatibility: Update compat.ts if LibSQL compatibility needed
5. Tests: Add integration tests for new functionality

Debugging Issues

1. Check Protocol: Use executeRaw() to inspect cursor entries
2. Validate Config: Ensure URL/auth token are correct
3. Test Streaming: Compare all() vs iterate() behavior
4. Review Errors: Check for LibsqlError vs generic errors

Extending Compatibility

1. Research LibSQL: Check resources/libsql-client-ts for API patterns
2. Validate Config: Add validation for unsupported options
3. Map Interfaces: Convert between LibSQL and native formats
4. Test Coverage: Ensure compatibility tests cover new features

Important Notes

Security

• No Secret Logging: Never log auth tokens or sensitive data
• Validation: Always validate inputs, especially in compatibility layer
• Error Messages: Don't expose internal implementation details

Compatibility

• Breaking Changes: Avoid breaking the native API
• LibSQL Parity: Match LibSQL behavior exactly in compatibility layer
• Version Support: Document which libSQL features are supported

Edge Cases

• Large Results: Test with large datasets to verify streaming
• Network Issues: Handle connection failures gracefully
• Protocol Evolution: Be prepared for protocol version updates

Future Considerations

Potential Enhancements

• Transaction Support: Interactive transactions in compatibility layer
• Prepared Statement Caching: Cache prepared statements
• Connection Pooling: Multiple concurrent connections
• Protocol Negotiation: Support multiple protocol versions

Monitoring

• Performance Metrics: Track query latency and throughput
• Error Rates: Monitor protocol and application errors
• Resource Usage: Memory and CPU usage in serverless environments

This guide should help future contributors understand the architecture and maintain consistency across the codebase.