Recently helped a friend to deal with a tricky online problem, his e-commerce site's pictures and static resources load fast and slow, especially during peak hours, some areas of the user feedback directly can not open. At first, I thought the server bandwidth is not enough, after the upgrade, the problem remains. I took a look at the background access logs, good guy, the traffic source IP a variety of, but back to the source request is concentrated in one or two machines - obviously CDN scheduling problems.
These days, even CDNs have to “prevent teammates”. Many teams think that buying a high-defense CDN will be all right, but do not know that if the node scheduling is not intelligent, the user experience is torn, or directly to the attacker opened the back door. Today we will talk about how to play the intelligent scheduling of high-defense CDN, especially how to automatically select the optimal node according to the user's location.
First of all, I would like to say a tearful lesson: do not trust those CDN service providers that only roughly allocate nodes according to geographic location. I found that the so-called “intelligent scheduling” of some service providers is only a simple mapping based on the country/province information of the user's IP, or even a static configuration. For example, all the users in South China are pointing to the Guangzhou node, the result is that the node was broken that night, the entire South China access to all the detours to the United States - this scheduling is simply negative optimization.
Real intelligent scheduling must dynamically combine a variety of factors: user location is only the first layer, but also real-time calculation of the node load, network congestion, the quality of the path back to the source, and even including whether the current attack. As an example, the CDN5 vendor's approach is a bit tricky: they use BGP Anycast + EDNS-client-subnet technology, in the DNS resolution stage through the IP prefix precision positioning user's autonomous domain (AS), while parallel detection of multiple POP node latency and packet loss rate, and finally select a comprehensive optimal node to return to the resolution results.
But DNS layer scheduling is not enough. Attackers have long since learned to directly resolve node IPs and then target strikes. High-defense CDNs must be able to do secondary scheduling when HTTP requests arrive at edge nodes - this is the stage for intelligent load balancing. For example, CDN07's practice is to calculate the client's RTT (round-trip delay) at the TCP handshake stage, and if it finds that the current node's latency suddenly spikes, it immediately kicks the user to a healthier neighboring node through a 302 redirect or HTTP/3 ORIGIN frame.
Here's a comparison of the scheduling latency (in ms) that I recorded when I pressure tested CDN07 last year:
See the difference? Intelligent scheduling is literally a lifesaver in abnormal scenarios. Especially when it comes to DDoS attacks, a good scheduling system can quickly divert attack traffic to the cleaning center, while normal users are dispatched to unaffected nodes.
Specifically for scheduling based on user location, I highly recommend edge computing + machine learning for prediction. For example, 08Host's solution deploys lightweight probes at each POP node to continuously collect network quality data across geographic locations, and then predicts latency trends for the next few minutes using a timing prediction model (e.g., LSTM). When a user request arrives, the scheduling system directly calls the prediction model API to get a list of optimal nodes, rather than relying on a static mapping table.
This is a simplified version of the logic of the scheduling algorithm that they have made public:
Of course, the actual production environment is much more complex than this, but also consider session retention, failover, cost control and so on. For example, the e-commerce user login request must keep the session sticky, not because of the scheduling jump nodes lead to session loss - this time can not simply look at the network quality.
Plus a pit: some CDN service providers in order to save costs, will be the “optimal node” secretly replaced by the “cheapest node”. I have encountered a vendor to the Shanghai user's request scheduling to the Inner Mongolia node, just because the node bandwidth is cheap. Later, we added a cost weighting restriction in the scheduling strategy to solve the problem.
Finally, a practical suggestion: if you are selecting a high-defense CDN, be sure to test their scheduling accuracy. My usual method is to run continuous ping and HTTP tests with cloud hosts in multiple regions at the same time to observe the node switching strategy in different time periods and under different network conditions. A good CDN should be able to do second switching and user senseless, such as CDN5 and CDN07 are good in this regard, while 08Host wins in cost-effective, suitable for teams with limited budget.
Intelligent scheduling is not a silver bullet, but it is really the core of the value of high-defense CDN. The next time you see those colorful node maps in the CDN console, you might as well ask: How do you define “optimal”? The other side of the answer is likely to determine the user experience ceiling of your business in the next year.
