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Sky Keynote Preview: Origin Servers and CDNs
When does delivery from an origin server make sense, and when does a CDN—or a multi-CDN strategy—make sense? Read on for insights from Sky's principal streaming architect

This is the fourth of five articles examining the decision-making process behind the creation of Sky's state-of-the art live streaming services. See the bottom of this article for links to the earlier installments. The author will be delivering the keynote at the 2018 Streaming Forum in London.

Content is all around us and everywhere we look. One of the challenges of our modern-day lifestyle is that we are increasingly “time-poor” and want the convenience to watch what we want, when we want. This works perfectly for entertainment and movie content, although live sports will likely remain tied to the calendar. 

The power of the internet comes from its decentralised architecture. This provides a consistent delivery mechanism for TCP-IP packets to be routed and, if necessary, re-routed across the internet in the event of failure. The idea that we should not lose packets is very important, especially when you are watching a live streaming event. Popular sporting events coinciding with popular box set releases could create a perfect storm if they occur at the same time. This is fundamentally a scaling issue and one that requires both the origin and CDN to be scaled appropriately to cope with customer demand.

Cisco’s Visual Networking Index report states that by 2021, 80% of all internet traffic will be video, up from 67% in 2016. Live sporting events such as the 2018 Football World Cup will accelerate this trend, requiring an effective strategy to prevent failure. Using a multi-CDN strategy can help tremendously because after a certain point you will reach a tipping point and have to distribute the load across multiple CDNs to ensure that your performance metrics remain consistent.

What is an Origin Server?

In a low-traffic environment, the origin server acts as an HTTP web server within the live video workflow, temporarily storing content on the disk and responding to requests from CDNs. NGINX is an excellent choice, being both lightweight and capable of serving multiple gigabits of traffic.

When traffic levels reach the point where your origin servers can no longer handle the load, it is highly recommended to introduce a caching layer in front of the origin servers. In this way, they act much like the shock absorber in your car, smoothing out the road and making the journey more comfortable. Using this two-tiered approach significantly reduces the HTTP traffic load on your backend origin servers from your frontend caching servers. Typically, we see up to a 50% reduction in traffic levels for live content.

Why Do I Need a Content Delivery Network?

It’s basic maths. Let’s say that your top bitrate is 5Mbps and you have 10,000 concurrent customers watching different channels. In this example, you would need 50Gbps of bandwidth, which is costly and difficult to sustain consistently. This is where a CDN can help by placing itself between the customer and the origin servers. At Sky, we have the ability to automatically change CDNs based on a set of performance rules, all without our customers being aware of any issue. 

For a live streaming service, there are several features that should be considered. First, you must have a tiered cache hierarchy, to allow the load to be evenly distributed within the CDN and avoid overloading origin servers. Second, use sensible HTTP timing values to avoid false positives and errors. Third, excellent HTTP monitoring ensures both the origin and CDNs are performing optimally. Finally, you should consider who can access your origin and restrict that access to trusted CDN partners.

Looking to the future, CDNs allow us to continue to grow our business, and I’m really excited about the potential for new technologies such as HTTP2 to improve security.

Thank you for reading. For my last article, I will be discussing why we should all care about video quality.

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