Performance, Caching, and Search
Acquia Cloud performance questions center on the cache layers: browser/CDN, Varnish (page cache), and application caches (Drupal render/dynamic page cache, Redis/Memcache for cache bins, and views/entity caches). Know the order: request hits CDN → Varnish → web server → Drupal. A cache miss at each layer costs time; a question may ask which layer to clear for a symptom.
Varnish serves anonymous pages and is bypassed for authenticated users unless configured. Cache tags and contexts (Drupal 8+) drive invalidation; misconfigured tags cause stale content. Redis or Memcache offloads cache bins from the database-good for large sites. Traps: assuming Varnish caches logged-in users (it generally does not), or that clearing Drupal cache clears Varnish (it does not; clear both).
Search on Acquia Cloud is typically Acquia Search (Solr). Know that each environment connects to its own Solr core/index, that reindexing is required after config or field changes, and that the search server connection is environment-specific (a classic parity issue). A trap is pointing Stage at the Prod Solr core-never do this; use environment-specific config.
Worked example: editors report new content not appearing for anonymous users. Check Varnish (purge/clear), then CDN, then Drupal cache tags. If only logged-in users see it, suspect Drupal dynamic page cache rather than Varnish.
Also know PHP version and opcache, image styles/derivative caching, and that Acquia Cloud provides performance insights. Questions often ask which single action fixes a stale anonymous page-usually a Varnish purge, not a full reindex.