What if the CDN node assigned by intelligent DNS goes down?

　　The core value of intelligent DNS scheduling lies in "directing users to the optimal node." However, a deeper question arises: what happens to users if this "optimal node" suddenly goes down? This isn't unfounded—CDN nodes may become unavailable due to server crashes, network interruptions, DDoS attacks, or excessive load. Intelligent DNS, as the "brain" of the CDN, addresses this issue through a series of automated disaster recovery mechanisms in its high-availability design.

　　I. Health Checks: How to "Detect" a Node Failure?

　　Before a switchover, the system must know that a node is experiencing a problem. This relies on a continuous health monitoring mechanism.

　　The intelligent DNS system initiates periodic health checks on CDN nodes from multiple monitoring points globally. The checks include metrics such as HTTP/HTTPS response status codes, response time, packet loss rate, and TCP connection success rate.

　　The check frequency is typically set to once every 5 seconds. Only after multiple consecutive failures (e.g., 3 times) is a node deemed "unavailable," to avoid erroneous switchovers caused by occasional network fluctuations. This mechanism ensures that the system's judgment of node status is both sensitive and robust—it won't mistakenly remove a healthy node due to a single timeout, nor will it allow a truly downed node to continue providing service.

　　The design of the detection granularity is also crucial. Industry-leading CDN architectures perform fine-grained detection at the region + carrier level. For example, a node failure under the specific combination of "Beijing-Mobile" will only affect traffic scheduling in that region and will not trigger a global switchover.

　　II. Automatic Failover: Who "Directs" Traffic Transfer?

　　After confirming a node anomaly, the next step is "where to redirect traffic." There are two main architectures here:

　　1. GTM/Global Traffic Management Solution (Origin Layer Disaster Recovery)

　　This is a switchover mechanism completed at the intelligent DNS layer, primarily targeting the CDN origin pull stage. When the origin server (not the CDN edge node) fails, the Global Traffic Management (GTM) product takes over the scheduling:

　　Configure multiple address pools: Configure multiple origin server IPs or domain names for the service, serving as primary and backup address pools respectively.

　　Continuous Health Check Probing: GTM initiates health checks on each address pool from multiple probe nodes.

　　Automatic Fault Removal and Switchover: When an anomaly is detected in the primary address pool, GTM automatically removes it from the resolution results and switches traffic to the backup address pool; once the primary address pool recovers, it automatically switches back.

　　This solution is suitable for multi-active origin server or disaster recovery scenarios, and can be combined with CDN to form a "serial" architecture—CDN is responsible for accelerating distribution, and GTM is responsible for origin server health checks and fault switching.

　　2. Anycast Network Solution (Edge Node Layer Disaster Recovery)

　　This is a switching mechanism completed at the network layer, complementing the GTM solution.

　　In the Anycast architecture, multiple CDN nodes share the same IP address and broadcast the route for that IP to the Internet via the BGP protocol. When a user accesses that IP, the network layer automatically directs the request to the "nearest, reachable" node based on the BGP route.

　　When a node fails, it stops broadcasting the IP route. Routers in the network automatically reroute traffic to other healthy nodes that are still broadcasting the IP. The entire process is completed at the network layer, and the user is unaware of it.

　　The advantages of this mechanism are: extremely fast switching speed (relying on BGP convergence time, typically in the second range) and no DNS-level intervention required. In China Telecom Cloud's CDN architecture, multiple nodes use Anycast technology for mutual hot standby, ensuring that traffic can be quickly transferred to other available nodes when one node becomes unavailable.

　　III. Supplementary Disaster Recovery Measures: HTTPDNS and Multi-CDN Scheduling

　　On the client side, a self-built HTTPDNS service can achieve dual protection of "primary link + backup link". The client prioritizes initiating resolution requests to HTTPDNS (bypassing LocalDNS). When the primary link fails, it automatically falls back to the traditional LocalDNS, ensuring basic availability.

　　Furthermore, some solutions support cross-cloud service provider CDN disaster recovery. Through an intelligent scheduling layer, user requests are proportionally distributed to the CDNs of multiple cloud providers. When a CDN node from a cloud provider experiences a regional failure, the system automatically switches traffic for that region to other healthy providers, preventing a complete service interruption due to a single CDN provider failure.

　　IV. Complete Link: What Happens When a Smart DNS Node Goes Down?

　　By linking the above mechanisms together, when a scheduled node actually fails:

　　The health detection system detects the node's unavailability or severely degraded service quality within 5-30 seconds.

　　If using the GTM scheme: The DNS authority server removes the abnormal node's IP from the resolution results and returns a backup IP for subsequent new requests.

　　If using the Anycast scheme: The failed node stops BGP route broadcasting, and the network layer automatically redirects traffic to adjacent healthy nodes.

　　On the client side: If there is a dual HTTPDNS link, it can automatically switch to the backup link to ensure basic availability.

　　After the node recovers: The system automatically adds it back to the scheduling pool, completing the rollback.

　　In short: When a CDN node scheduled by intelligent DNS fails, it will not cause service interruption. The health detection system will quickly detect the anomaly and redirect user traffic to a backup node through GTM intelligent DNS switching or Anycast network layer automatic routing. The switching time of this mechanism is usually in the range of seconds to minutes. Combined with client retry and buffering strategies, users will be unaware of the problem in most cases.