What is caching?
To understand how caches work, consider real-world caches of food and other supplies. When explorer Roald Amundsen made his return journey from his trip to the South Pole in 1912, he and his men subsisted on the caches of food they had stored along the way. This was much more efficient than waiting for supplies to be delivered from their base camp as they traveled. Caches on the Internet serve a similar purpose; they temporarily store the 'supplies', or content, needed for users to make their journey across the web.
What is CDN caching?
A CDN, or content delivery network, caches content (such as images, videos, or webpages) in proxy servers that are located closer to end users than origin servers. (A proxy server is a server that receives requests from clients and passes them along to other servers.) Because the servers are closer to the user making the request, a CDN is able to deliver content more quickly.
Think of a CDN as being like a chain of grocery stores: Instead of going all the way to the farms where food is grown, which could be hundreds of miles away, shoppers go to their local grocery store, which still requires some travel but is much closer. Because grocery stores stock food from faraway farms, grocery shopping takes minutes instead of days. Similarly, CDN caches 'stock' the content that appears on the Internet so that webpages load much more quickly.
How does content become cached?
When a user requests content from a website using a CDN, the CDN fetches that content from an origin server, and then saves a copy of the content for future requests. Cached content remains in the CDN cache as long as users continue to request it.
What is a cache hit? What is a cache miss?
A cache hit is when a client device makes a request to the cache for content, and the cache has that content saved. A cache miss occurs when the cache doesn't have the requested content. In the case of a cache miss, a CDN server will pass the request along to the origin server, then cache the content once the origin server responds, so that subsequent requests will result in a cache hit.
Where are CDN caching servers located?
CDN caching servers are located in data centers all over the globe. Cloudflare has CDN servers in 200 cities spread out throughout the world in order to be as close to end users accessing the content as possible. A location where CDN servers are present is also called a data center.
How long does cached data remain in a CDN server?
When websites respond to CDN servers with the requested content, they attach information to the content that will let the servers know how long to store it. This information is stored in a part of the response called the HTTP header, and it specifies for how many seconds, minutes, or hours content will be cached. This is known as the Time-To-Live (TTL). When the TTL expires, the cache removes the content. Some CDNs will also purge files from the cache early if the content is not requested for a while, or if a CDN customer manually purges certain content.
How do other kinds of caching work?
Web browser caching takes place when a browser saves a copy of files from a website on the user device's hard drive. When a webpage is cached, the browser only needs to load new or updated pieces of a page, which enables browsers to deliver pages quickly even if an Internet connection is slow. Browsers store these files until their TTL expires or until the hard drive cache is full. Users can also clear their browser cache if desired.
DNS caching takes place on DNS servers. The servers store recent DNS lookups in their cache so that they don't have to query nameservers and can instantly reply with the IP address of a domain.
Search engines may cache webpages that frequently appear in search results in order to answer user queries even if the website they are attempting to access is temporarily down or unable to respond.
How does Cloudflare use caching?
Cloudflare offers a CDN with 200 PoPs distributed internationally. Cloudflare offers free CDN caching services, while paid CDN customers are able to customize how their content is cached. The network is Anycast, meaning the same content can be delivered from any of these data centers. A user in London and a user in Sydney can both view the same content loaded from CDN servers only a few miles away.