This article details our midstream multi-CDN switcher ranks CDN and how this ranking is used to decide which CDN to use for video segment downloads during a session.
Global CDN Scores
Global CDN Scores are simply calculated as:
Global Score = QOS Score * Business Score
In practice this will mean that:
- if 2 CDNs have the same QOS score, the Business Score will determine which CDN has the highest Global Score.
- The opposite is true as well: if 2 CDNs have the same Business Score, the QOS Scores will be left to determine who has the highest Global Score.
- If CDN-A has a Business Score that is twice that of CDN-B. Then for CDN-B to have the highest Global Score, it will need to have a QOS Score that is at least twice as high as CDN-A QOS Score.
The business score for a single CDN can be different in two different CDN lists. This allows you to force a certain level of priority on a CDN based on your own business requirements.
Let's say that I am using 3 different CDNs:
- CDN A: The cheapest price of all my CDNs, but can be unreliable and has failed me in the past. I want to use it as long as its QOS is decent but switch to my fallback CDNs if I start to see QOS problems or errors on CDN A.
- CDN B: More expensive than CDN A, but cheaper than CDN C: I want to use it as my go-to fallback when CDN A starts to have issues.
- CDN C: the most expensive but most reliable of my CDNs. I only want to use it if both CDN A and B have QOS issues. I use it to guarantee a good QOS to my viewers no matter what happens with CDN A and B.
In that scenario I could set Business Scores for my CDNs:
- CDN A: Business Score of 4.
- CDN B: Business Score of 2.
- CDN C: Business Score of 1.
This way my viewers will source segment from CDN A unless:
- CDN A has errors, in which case I will try to source segment from CDN B.
- CDN A's bandwidth is half of CDN B, in which case I will try to source segment from CDN B.
When trying to source segment from CDN B, I will only go to CDN C if:
- CDN B has errors
- CDN B's bandwidth is half of CDN C.
The Business Score for a CDN can be changed at any moment and will be reflected on new sessions less than 1 minute after you update it in our dashboard. This can allow you to modify CDN priority if you reach one of your CDN commits during the month.