Serve the last good page when the server fails

How to do it

Add stale-if-error (RFC 5861) to the Cache-Control of public pages. In a front server or CDN you control, turn on its own setting, such as proxy_cache_use_stale in nginx. Never do this for signed-in pages, which stay out of shared caches. Keep an alert on the errors, because visitors no longer report them.

Example

proxy_cache_path /var/cache/nginx/pages keys_zone=pages:10m max_size=1g inactive=1d;
location / {
  proxy_cache pages;
  proxy_cache_valid 200 5m;
  proxy_cache_use_stale error timeout updating http_500 http_502 http_503 http_504;
  proxy_cache_background_update on;
  proxy_pass http://127.0.0.1:8080;
}

// Express: public pages only
res.set('Cache-Control', 'public, max-age=300, stale-while-revalidate=86400, stale-if-error=86400');

# Flask: public pages only
response.headers["Cache-Control"] = "public, max-age=300, stale-while-revalidate=86400, stale-if-error=86400"

How to check

Load a page through the cache, then stop the app. The same page still answers 200 from the cache, and a page never cached answers the 500 page.