Cloudflare serverless worker: Failover to secondary server if main server is down

gistfile1.txt

export default {
    async fetch(request) {
        const primaryUrl = 'https://api.example.com';
        const fallbackUrl = 'https://api2.example.com';

        const url = new URL(request.url);
        const path = url.pathname + url.search;

        // Define CORS headers
        const corsHeaders = {
            'Access-Control-Allow-Origin': '*',
            'Access-Control-Allow-Methods': 'GET, HEAD, POST, OPTIONS',
            'Access-Control-Allow-Headers': 'Content-Type',
        };

        // Handle OPTIONS preflight requests
        if (request.method === 'OPTIONS') {
            return new Response(null, {
                headers: corsHeaders
            });
        }

        // Helper function to add CORS headers to a response
        const addCors = (response) => {
            // Recreate the response to modify headers (original might be immutable)
            const newResponse = new Response(response.body, response);
            Object.keys(corsHeaders).forEach(key => {
                newResponse.headers.set(key, corsHeaders[key]);
            });
            return newResponse;
        };

        // Helper function to handle the fetch logic
        async function fetchUpstream(targetUrl) {
            const response = await fetch(targetUrl + path, request);

            // If origin responds with a redirect (301/302), return it with CORS headers
            if ([301, 302].includes(response.status)) {
                return new Response(null, {
                    status: response.status,
                    headers: {
                        'Location': response.headers.get('Location'),
                        ...corsHeaders
                    }
                });
            }

            return response;
        }

        // Try primary
        try {
            const response = await fetchUpstream(primaryUrl);
            if (response.ok) {
                return addCors(response);
            }
        } catch (e) {
            console.error('Primary failed:', e);
        }

        // Fallback: try the same logic on secondary
        try {
            const response = await fetchUpstream(fallbackUrl);
            // Return the response (even if it's an error like 404) with CORS headers
            return addCors(response);
        } catch (e) {
            console.error('Fallback failed:', e);
        }

        return new Response('Service unavailable', {
            status: 503,
            headers: corsHeaders
        });
    }
};