Allow 404s to be cacheable by dynamic page cache

X-Drupal-Cache-Max-Age: 0 (Uncacheable)

This doesn't apply to the dynamic page cache, only the internal page cache. There is a separate header for that:

X-Drupal-Dynamic-Cache

It does look like that's not cacheable with the standard profile though.

📌 Block status code visibility condition should use a status code cache context Active is related.

Can you check whether the same problem exists with 11.1?

If this problem was discovered on a version of Drupal that is not 11.x, add that information in the issue summary and leave the version at 11.x. In Drupal core changes are made on on 11.x (our main development branch) first, and are then back ported as needed according to the Core change policies → . Also mentioned on the version → section of the list of issue fields documentation.

OK I double checked what's going on.

The subrequest itself is cached in the dynamic page cache, e.g. this is what the cids look like in cache_dynamic_page_cache when you're logged out and browse to /abcdef

| response:[request_format]=html:[route]=system.40435786c7117b4e38d0f169239752ce71158266ae2f6e4aa230fbbb87bd699c0e3                                                                                                                                               |
| response:[cookies:big_pipe_nojs]=:[languages:language_interface]=en:[request_format]=html:[route]=system.40435786c7117b4e38d0f169239752ce71158266ae2f6e4aa230fbbb87bd699c0e3:[session.exists]=0:[theme]=olivero:[url7cOhVamWazkPEBofpmp46A6Nx9YVZ6IP27D-joYI6AY |

What doesn't get cached in dynamic_page_cache is request for the 404 page itself, e.g. /abcdef.

However, caching this would not really work in the context of dynamic_page_cache - it caches by route rather than per path, and by definition /abcdef can't be routed, if it could it wouldn't be a 404.

If we cached by path in this case, then there is no particular reason it couldn't be cached, but the internal page cache module (page_cache) already successfully caches the 404 response by path. This means the dynamic_page_cache would have a very low cache hit rate. Similarly, because the response sends cacheable http headers, varnish or a CDN can cache the HTML at the edge too. Once it's in either the internal page cache or an edge cache, then it won't hit dynamic_page_cache anyway.

Did some quick profiling of what it looks like with xhprof, and found 📌 Avoid preloading routes in subrequests Active which would be quick win in this situation though. There might be other things we can optimize away too.