[Cache] Adds information about smart edge revalidation (#19486)

* Adds information about smart edge revalidation

* Apply suggestions from code review

Co-authored-by: marciocloudflare <[email protected]>

---------

Co-authored-by: marciocloudflare <[email protected]>

Author: angelampcosta

src/content/docs/cache/concepts/revalidation.mdx

@@ -6,9 +6,11 @@ pcx_content_type: concept
 
 import { GlossaryTooltip } from "~/components"
 
-Revalidation is a caching mechanism that involves checking the [freshness](/cache/concepts/retention-vs-freshness/) of cached data before serving it to a client or user. If a cached object is no longer [fresh](/cache/concepts/retention-vs-freshness/#freshness-ttl) and Cloudflare receives a request for it, a request is made to the origin to revalidate the object in the Cloudflare cache.
+Revalidation is a caching mechanism that checks the [freshness](/cache/concepts/retention-vs-freshness/#freshness-ttl) of cached data before serving it to users. If a cached object is no longer fresh and Cloudflare receives a request for it, the system makes a request to the origin to revalidate the object in the Cloudflare cache. By using headers like `If-Modified-Since` and `ETag`, Cloudflare validates content without fully re-fetching it. When these headers are missing, Smart Edge Revalidation generates a `Last-Modified` header, ensuring efficient updates and delivery of fresh content while reducing origin traffic.
 
-## Example scenarios
+## Revalidation towards origin
+
+For stale (expired TTL) content, Cloudflare will send a revalidation request to the origin. If the stale content is still valid Cloudflare will set a new TTL. If the content is expired, then the origin will provide new fresh content to replace the old.
 
 Consider the following example scenarios.
 
@@ -19,3 +21,7 @@ One-thousand (1,000) requests arrive simultaneously at Cloudflare's network, and
 ### Example 2
 
 One-thousand (1,000) requests arrive simultaneously at a single Cloudflare data center, and the requested asset is not in Cloudflare's cache (a cache miss). These requests will use a <GlossaryTooltip term="cache lock">cache lock</GlossaryTooltip> to communicate with your origin. This means that only the first request will go to origin to fetch the asset. The remaining 999 requests wait for the first request to fetch the data, after which the response is [streamed](https://blog.cloudflare.com/introducing-concurrent-streaming-acceleration/) to all the waiting requests. The cache lock ensures that Cloudflare only sends the origin one request at a time for a given asset from a location in Cloudflare's network, preventing the origin from getting too much traffic.
+
+## Smart revalidation towards users
+
+When both [`Last-Modified`](https://datatracker.ietf.org/doc/html/rfc7232?cf_history_state=%7B%22guid%22%3A%22C255D9FF78CD46CDA4F76812EA68C350%22%2C%22historyId%22%3A15%2C%22targetId%22%3A%226C8153BAEF7BC0C5A331E28F8BCF1ABA%22%7D#section-2.2) and [`Etag`](https://datatracker.ietf.org/doc/html/rfc7232?cf_history_state=%7B%22guid%22%3A%22C255D9FF78CD46CDA4F76812EA68C350%22%2C%22historyId%22%3A13%2C%22targetId%22%3A%226C8153BAEF7BC0C5A331E28F8BCF1ABA%22%7D#section-2.3) headers are absent from the origin server response, Smart Edge Revalidation will use the time the object was cached on Cloudflare's global network as the `Last-Modified` header value. When a browser sends a revalidation request to Cloudflare using `If-Modified-Since` or `If-None-Match`, our global network can answer those revalidation questions using the `Last-Modified` header generated from Smart Edge Revalidation. In this way, our global network can ensure efficient revalidation even if the headers are not sent from the origin.