Performance, Scalability, and Caching
Performance questions test the caching layers and their invalidation. Know full-page cache (FPC) with Varnish in front, the private content mechanism (customer-specific blocks loaded via customer-data), and block cache with cache tags and lifetimes. Understand cache types (bin/magento cache:status), the difference between the default cache backend and Redis for backend and session, and how cache tags let a save invalidate only affected pages.
Traps: assuming FPC caches personalized content (it does not; private content is fetched client-side), forgetting that cache tags must be declared or invalidation is too broad or too narrow, and confusing Redis roles (backend cache vs session vs FPC). Also know indexer mode effects on catalog performance, the role of the catalog_product_flat tables, and that asynchronous indexers reduce write-time cost but introduce staleness.
Scalability concepts: horizontal scaling of web nodes behind a load balancer, shared session and cache in Redis, search in Elasticsearch/OpenSearch, and media in shared storage or CDN. Know that Cloud uses a three-node architecture with separate database, Redis, and search services.
Worked example
Requirement: "A category page shows stale prices after a price update." Check whether the price indexer runs by schedule (staleness window) and whether the FPC tag for the product/category was invalidated. If a custom block renders prices, it must declare the product cache tag so a product save invalidates it; otherwise the block stays cached. Rule of thumb: every cached block should declare the narrowest tag set that covers its data.