When your server is down

If your WordPress server stops responding, Edge Caching can keep serving the pages it has cached. Whether a given visitor sees your site or an error page depends on two things: whether Always Online is enabled, and whether the page they ask for is in the cache.

At a glance

The page a visitor requests	Always Online enabled	Always Online disabled
Cached less than an hour ago	Served normally	Served normally
Cached more than an hour ago, within the stale period	Served from cache	Error page
Not in cache, or older than the stale period	Error page	Error page
Never cached: logged-in users, cart, checkout, /wp-admin	Error	Error

The stale period is the Always Online setting that decides how long an expired page remains available: 24 hours by default, up to 48 hours.

Why most visitors are unaffected

Popular pages are refreshed every hour, so at any moment they are in the cache. When your server fails, Edge Caching keeps serving them from the cache for as long as the stale period allows, and tries to refresh them in the background. As soon as your server answers again, pages are refreshed and visitors receive current content.

The visitors who do see an error are the ones requesting a page nobody has viewed recently, or a page that is never cached. Turning on scheduled cache warming keeps more pages in the cache and shrinks the first group.

What counts as “down”

Edge Caching treats your server as failing when it:

• refuses the connection, or does not accept it within 5 seconds;
• does not start answering within 60 seconds;
• stops sending data for more than 10 seconds;
• answers with a server error (any 5xx status), for example a PHP fatal error or a database connection failure.

A slow page that answers within these limits is not a failure.

The error page

When a page cannot be served, the visitor sees a maintenance page saying the site is temporarily down. Its HTTP status is 503 when your server could not be reached, or your server’s own 5xx status when it answered with an error. The page is never cached, so each new request tries your server again.

Example

Your site has Always Online enabled with the default 24-hour stale period. Your database fails at 14:00 and is fixed at 18:30.

Page	Last cached	During the outage
Home page	13:40	Served from cache all afternoon
A blog post	09:15	Served from cache
An article nobody has read in three days	Not in cache	Error page until 18:30
Checkout	Never cached	Error page until 18:30

The health check

While Always Online is enabled, Edge Caching checks your home page every 10 seconds and expects an HTTP 200. This check drives the server status shown in the admin and the origin down alerts you can configure under Settings → Alerts. It does not decide what visitors see: that is determined by each request, as described above.

Home pages that redirect

If your home page redirects, for example from example.com to www.example.com, the check fails even though your site works, and alerts are sent. Make sure the domain configured in Edge Caching is the one your home page answers on directly.

Terms

Term	Meaning here
Grace	Varnish’s name for the stale period: how long an expired object can still be delivered. 10 seconds when Always Online is disabled.
Stale copy	An expired page served while a fresh one cannot be obtained.
Background fetch	The refresh Varnish starts when it serves a stale copy. If it fails, the stale copy is kept and the next request retries.
Health probe	The periodic check of your home page: GET / every 10 seconds, 5-second timeout, healthy when 3 of the last 5 checks succeed.