How to implement a multi-CDN strategy

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There are several factors to consider in a multi-CDN delivery solution.

First, is it VoD or Live? HLS (and DASH) have a second URL option (base URL in DASH), for the client to determine when to choose that Fallback URL. If playback falls back to the second URL, that fallback experience to the viewer, could have had some buffering, or bitrate downshifts triggering that player decision.

Although stream playback recovers/continues, the user experience could have and likely was impacted. Here a second CDN in the multi-CDN deployment was accessed by the client. There is no intelligence here, in the provider selection. Typically the (perceived) most reliable CDN gets that first spot, and the backup CDN gets the Fallback position (second URL) in the manifest/MPD.

In Live, you have the opportunity to provide intelligent CDN selection on every manifest/MPD refresh. If your multi-CDN selection layer has intelligence, access to performant metrics, in real time, that manifest can now point (directed) to the alternate CDN. This requires a level of manifest management on the session level, so that the m3u8 retains the proper historical CDN selection so as not to break playback for that session (in most if not all cases).

There are client solutions, DNS solutions, and cloud solutions that are neither client (sdks), or DNS based. You get to decide how you want integration to be managed and how much work your team can/can't invest in your solutions ongoing level of effort.

Why is most important to consider is the viewer experience, and how playback can best be delivered to avoid buffering, downshifts, the things that cause a viewer to abandon your content and possibly not come back.

If a CDN is performant, and N+1 users are now beginning to watch a stream on that providers network, capacity could be (often is) an issue. Continuing to send users to that CDN may be a sub-optimal experience. Metrics measuring playback determine that bitrates are dropping, buffering increasing, and serve new requests with an alternate CDN providing a better playback experience.

Video is a tightly controlled series of events. We work with chunks of 10s, 6s, 2s, for large buffers, and fast start times. Continually trying to balance the benefits of both.

With an SDK client based solution, you have engineering effort to keep up with OS/hardware updates, testing new code in SDKs, and then pushing out across several platforms, players, etc. Can be daunting.

With DNS, you have TTLs to manage, while lower is better, faster for that next user, there is no mid-stream switching with intelligence once the client is pulling manifests from a specific provider.

With a cloud based solution, each individual stream/user/device is measured and Can selection performed in real time for Live, and for _each_ request on VoD.

Disclaimer, I work at DLVR, and formerly Cedexis. = ]

Nikolay from Streamroot.io here, co-author of the article.

Yes Conviva provides a service that can give you information about the QoS for the CDN by aggregating data from their customers (they provide a video analytics solution), but it doesn't make the switching (nor on the server side or on the client side), so the video player would need to implement its own logic themselves.

The solution from Streamroot can use this kind of APIs like Conviva Precision, or the one from its competitors like Youbora and Cedexis, and the real value it adds is the client-side switching capability to the players, so it's quite complementary to those solutions.

And indeed the devil is in the details, that's why we built this client-side SDK so the customers don't have to implement all the logic themselves on each platform and device. It was easier for us as we already have SDKs and plugins for most players for our P2P hybrid delivery solution.

For VoD I like the approach where you use a fast and reliable CDN for the first seconds and in the background buffer the rest of the video from a cheap location/CDN.

This works if you download video faster than real time which is almost always the case. That way you get the best of both worlds.

Is this a bot?

Firstly, I'm not solving anything. I'm explaining why fallback URLs are not equivalent to CDNs.

You don't use a CDN because your site doesn't work, you use it because it's faster.

Secondly, no, doing an occasional speed test, using data you'd be downloading anyway, then selecting an endpoint between speedtests does not increase bandwidth usage by 3x.

Baffled.