router.ts

const server = Bun.serve({
  port: 3004,
  hostname: "0.0.0.0",
  async fetch(req) {
    // First check for header-based routing
    const appName = req.headers.get('fly-app-name');
    const machineId = req.headers.get('fly-machine-id');

    if (appName) {
      // Header-based routing
      let replayHeader = `app=${appName}`;
      if (machineId) {
        replayHeader += `;instance=${machineId}`;
      }

      return new Response(
        `Replaying request to: ${replayHeader} (via headers)`,
        {
          status: 200,
          headers: {
            'Content-Type': 'text/plain',
            'fly-replay': replayHeader
          }
        }
      );
    }

    // Fall back to hostname-based routing
    const url = new URL(req.url);
    const hostname = url.hostname;
    
    // Parse hostname pattern: <machine-id>.<app-name>.example.com or <app-name>.example.com
    const parts = hostname.split('.');
    
    if (parts.length < 3) {
      return new Response('Invalid hostname format. Use either <machine-id>.<app-name>.example.com or <app-name>.example.com, or set fly-app-name and optionally fly-machine-id headers', { 
        status: 400 
      });
    }

    // Remove .example.com from the end
    const domainParts = parts.slice(0, -2);
    
    let hostMachineId: string | undefined;
    let hostAppName: string;

    if (domainParts.length === 2) {
      // Format: <machine-id>.<app-name>
      [hostMachineId, hostAppName] = domainParts;
    } else if (domainParts.length === 1) {
      // Format: <app-name>
      hostAppName = domainParts[0];
    } else {
      return new Response('Invalid hostname format', { status: 400 });
    }

    // Construct fly-replay header
    let replayHeader = `app=${hostAppName}`;
    if (hostMachineId) {
      replayHeader += `;instance=${hostMachineId}`;
    }

    return new Response(
      `Replaying request to: ${replayHeader} (via hostname)`,
      {
        status: 200,
        headers: {
          'Content-Type': 'text/plain',
          'fly-replay': replayHeader
        }
      }
    );
  }
});

console.log(`🚀 Router server running at http://0.0.0.0:${server.port}`);