Why Does Self-Built CDN Implement "Ordered Backhaul"?
1. Summary
Many people use self-built CDNs to accelerate websites, but only when facing high traffic peaks do they realize how terrifying the backhaul pressure can be. This is especially true for news, e-commerce, and promotional event websites. When a hot event happens, traffic can skyrocket like an explosion. If backhaul control is not properly set up, CDN nodes will dump a large number of requests all at once to the origin server, causing your server to collapse instantly. The database won’t even have time to respond, and the website crashes with 500 errors or even a blank screen.
This is where "ordered backhaul" becomes meaningful — it doesn't make the CDN faster; it keeps the origin server alive.
Here, I’ll explain in simpler terms how self-built CDN achieves "ordered backhaul."
2. What is Ordered Backhaul?
In simple terms, ordered backhaul is like a "throttle valve" added to the CDN.
When users access the CDN node → the node finds the content is not cached → the node needs to retrieve it from the origin server.
If too many of these "backhaul" requests happen at once, the origin server can be overwhelmed. Therefore, the CDN adds a queuing mechanism at the node to control the number of concurrent backhaul requests. This ensures the origin server can handle the pressure smoothly without crashing.
To summarize: Ordered backhaul is like a buffer layer between the origin server and the sudden surge in traffic.
3. How Does Ordered Backhaul Work?
Ordered backhaul actually involves three key mechanisms, each acting like a "fuse" to ensure the origin server isn't overloaded.
3.1 Dynamic Threshold Allocation: Allocate "backhaul quotas" to each node appropriately
Many people don’t know that CDN doesn’t assign a fixed backhaul limit to each node; it dynamically adjusts it based on real-time conditions.
Here’s how it works:
Each edge node sends feedback about its backhaul load to the policy center based on the last second or a recent period of time.
The policy center analyzes the data to determine which regions have high traffic and which nodes are under pressure.
The "backhaul quota" (number of concurrent connections or requests) is then dynamically redistributed to each node based on traffic hot spots, like a cashier opening more lanes when the line is too long.
The benefits of this approach are clear:
High-traffic regions get more quota, while lower-traffic nodes don’t waste resources. The origin server’s load is more evenly distributed.
3.2 Control Concurrency: Backhaul requests don’t go through just whenever
When a user request reaches the CDN node and requires backhaul, the CDN first checks whether the current "backhaul concurrency" exceeds the threshold.
The strategy usually follows these rules:
Below threshold → backhaul directly.
Below threshold but with requests already queued → prioritize requests in the queue first.
Above threshold → block direct backhaul, place requests in the waiting queue.
If the queue is too long or requests timeout → return a custom error page or a friendly message.
It’s like the security check at the subway:
When there are too many people, they don’t let everyone rush in at once but set up barriers to allow people to pass through one by one.
The benefits include:
The origin server won’t be overwhelmed by simultaneous requests.
Users won’t face an immediate crash, but instead, wait in an orderly manner or receive a friendly page.
This is why e-commerce and news websites can stay stable even when a sudden surge in traffic occurs.
3.3 Backhaul Priority: Not everyone gets priority
CDNs like NetEase, Alibaba, and Tencent support setting backhaul priorities to ensure that, when resources are limited, more important users get served first.
Ways to set priority include:
By region (e.g., prioritize Beijing, Shanghai, Guangzhou, or overseas regions).
By user level (e.g., priority for members, loyal users).
By file type (e.g., prioritize HTML over images, transaction interfaces over static resources).
By business channel (e.g., prioritize transaction parts over promotional images in e-commerce).
For example, if you run an e-commerce event, you want to prioritize ensuring users can see products and place orders rather than loading a bunch of promotional banners.
This feature is like a "traffic distribution tool" for e-commerce websites.
4. Which Websites Benefit from Ordered Backhaul?
It’s clear: Any website that might experience traffic bursts in a short period of time must use this feature.
Common scenarios include:
News websites (sudden surges of visits due to hot events)
E-commerce events (e.g., Double 11, Black Friday, New Year Sales)
Online courses, live-streaming, and event pages
Game events and server launches
Lottery or flash sale businesses
In short: Any website that might be overwhelmed by traffic spikes should use ordered backhaul. Without it, the origin server won’t be able to handle it and will fail.
5. What Are the Benefits of Ordered Backhaul?
Summarizing, ordered backhaul offers four key benefits for websites:
5.1 Prevents the Origin Server from Being Overwhelmed, Greatly Enhances Stability
The pressure on the origin server is controllable, and the system won’t crash when traffic surges suddenly. This feature is a lifesaver, especially for origin servers with limited load capacity.
5.2 More Rational Resource Allocation, Prevents Wasting Server Performance
CDNs dynamically distribute backhaul thresholds based on the origin server’s capacity, ensuring that the origin server is "just right" in terms of utilization. It’s neither idle nor overloaded.
5.3 Allows "Important Users" to Get Priority Service, Boosting Revenue
For example:
High-value regions can be served first.
Priority for members.
Transaction interfaces get priority.
This is crucial for e-commerce websites, as it directly affects conversion rates.
5.4 Queue Timeout Can Return Custom Pages, Enhancing User Experience
For example, NetEase CDN supports returning custom pages if a queue times out:
Show a product recommendation page.
Show an event guide page.
Display custom prompts.
This prevents users from seeing cold 500 error pages, improving their experience.
Conclusion
Ordered backhaul is basically the feature that helps websites "hold their ground." When traffic surges unexpectedly, it automatically helps distribute the load, limit traffic, and queue requests so that the origin server isn’t overwhelmed during peak times.
Many people focus on CDN’s acceleration capabilities but ignore its role in preventing website crashes at critical moments. This feature is essential, especially for those running events, news, or e-commerce sites. It’s not an optional feature, but a must-have for these scenarios.
If your website often faces unexpected traffic spikes, low cache hits, or origin server overloads during events, make sure you enable "ordered backhaul" in your CDN settings. It’s not just a bonus feature; it’s the lifesaver in critical times.