Symptoms

CDN adds cache nodes to the existing network to cache resources retrieved from the origin server to the network nodes closest to the requesters. When the resources are requested again, CDN returns the resources directly from the cache instead of retrieving them from the origin server. This helps to improve the access speed of the website. How are CDN cache nodes divided?

Causes

CDN cache nodes are divided into L1 nodes and L2 nodes. L1 nodes are distributed across provinces and cities in mainland China, and L2 nodes are distributed in several regions. The following figure shows the simple architecture.CDN cache nodes
CDN nodes cache resources as follows:
  1. When a client sends an access request to a domain, it first requests the IP address of the domain from the local DNS server. If no record is found on the local DNS server, the local DNS server queries the IP address from the authoritative DNS server. After the IP address is queried, Alibaba Cloud CDN will allocate the optimal nodes for this request based on the intelligent scheduling system.
  2. The client then sends a connection request to the CDN node. If the requested resource is found in the cache of the L1 node, the L1 node directly returns the resource to the client. If the requested resource is not found in the cache of the L1 node, the L1 node requests the resource from the L2 node. If the resource is found in the cache of the L2 node, the L2 node returns the resource to the L1 node, which then returns the resource to the client. If the resource is not found in the cache of the L2 node, the L2 node requests the resource from the origin server and caches the resource based on the configured caching policy.