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System Design Masterclass · Web Content Delivery
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Visit amazon.com from two different accounts. The pages are completely different. Your recently viewed items, personalized recommendations, location-based deals, Prime membership status, cart count, and even the product order in search results are all unique to you.
Amazon cannot pre-build these pages with SSG. There are over 300 million active customer accounts. Each one sees a different page. The content is assembled per request based on who you are, where you are, and what you have done.
This is dynamic content delivery: serving pages where some or all of the content depends on the specific user, their location, their device, the current time, or real-time data that changes second by second. It is the hardest content delivery problem because you cannot cache a page that is different for every visitor.
Content is dynamic when it depends on at least one of these factors:
User identity. Logged-in state, subscription tier, past behavior, shopping cart. Netflix shows different rows of shows to different users. Spotify's Discover Weekly is unique per account.
Geographic location. Currency, language, legal restrictions, local pricing. A user in Tokyo sees prices in yen. A user in Germany does not see content restricted by EU regulations.
Time. Flash sales that start at midnight. Breaking news banners. "Posted 2 minutes ago" timestamps. Live sports scores.
A/B test variant. 5% of users see the new checkout flow. 95% see the old one. The variant assignment must be consistent per user (sticky sessions or cookie-based) but different between users.
Device or context. Mobile users get simplified layouts. Users on slow connections get lower-quality images. AMP pages for Google search results.
The more of these factors a page depends on, the harder it is to cache. A page that varies only by geo can be cached per-region (6-10 variants). A page that varies by user identity cannot be cached at all in a shared .
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