In-depth Investigation of CDN: Part 2

In this lesson, we learn how content consistency can be achieved using different consistency mechanisms. We also learn about where we should deploy the proxy servers and the difference between CDN as a service and specialized CDN.

Content consistency in CDN

Data in the proxy servers should be consistent with data in the origin servers. There’s always a risk of users accessing stale data if the proxy servers don’t remain consistent with the origin servers. Different consistency mechanisms can be used to ensure consistency of data, depending on the push or pull model.

Periodic polling

Using the pull model, proxy servers request the origin server periodically for updated data and change the content in the cache accordingly. When content changes infrequently, the polling approach consumes unnecessary bandwidth. Periodic polling uses time-to-refresh (TTR) to adjust the time period for requesting updated data from the origin servers.

Time-to-live (TTL)

Because of the TTR, the proxy servers may uselessly request the origin servers for updated data. A better approach that could be employed to reduce the frequency of refresh messages is the time-to-live (TTL) approach. In this approach, each object has a TTL attribute assigned to it by the origin server. The TTL defines the expiration time of the content. The proxy servers serve the same data version to the users until that content expires. Upon expiration, the proxy server checks for an update with the origin server. If the data is changed, it gets the updated data from the origin server and then responds to the user’s requests with the updated data. Otherwise, it keeps the same data with an updated expiration time from the origin servers.

Leases

The origin server ...