Does a CDN replace my application's Redis or memcached layer?

No, they cache different things at different distances. The CDN caches full HTTP responses near users and mostly helps shared, public content. Redis caches query results and objects next to your application servers and helps every request, including personalized ones. Mature systems run both: CDN for the shared outer layer, Redis for the per-request inner layer.

Push CDN or pull CDN, and when does the difference matter?

Pull is the default: edges fetch from origin on first request and cache, which fits most sites since only requested content occupies edge storage. Push means uploading content to the CDN ahead of demand, which suits large predictable files like video releases or game patches where the first user should not wait for an origin fetch. Many products, like Netflix preloading titles overnight, are push in spirit.

How do I cache pages when users can be logged in?

Split the response. Serve the parts identical for everyone, the page shell and content, from the CDN, and fetch personal fragments like the username and cart count with a separate uncached API call. Never let responses containing personal data or Set-Cookie headers into the shared cache; mark them private or no-store, and key the cache bypass off the session cookie.

Cachingchapter 7 of 8 · 4 lessons

CDN (Content Delivery Network)

Cache static assets at edge servers close to users worldwide.

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Every copy so far has lived in your infrastructure. Your users do not.

Someone in Pune loading an image from your servers in Virginia waits about 250 milliseconds for it to cross the planet, and no amount of caching in your own data centre recovers a single one of them. The distance is the cost. So put the copy near the user instead, in a network of servers you rent rather than run, which brings back every problem a cache has plus one more: this one is spread across hundreds of cities you do not control.

Lessons

4 in this chapter

How a CDN WorksDNS steers each user to a nearby edge server, which serves from cache or fetches from your origin once.3 min
Cache Keys and InvalidationThe cache key defines what counts as the same object, and versioned URLs beat purges.3 min
Static vs Dynamic ContentAnything identical for all users belongs at the edge; everything else can still ride the CDN's rails.2 min
Edge ComputeRunning code inside the PoP handles the personalization that pure caching cannot.3 min