Symptoms
Alibaba Cloud CDN adds cache points of presence (POPs) to the existing network to cache resources retrieved from the origin server to the network POPs 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.
Causes
CDN cache POPs are classified into L1 POPs and L2 POPs. L1 POPs are distributed across provinces and cities in the Chinese mainland, and L2 POPs are distributed in several regions. The following figure shows the architecture of the POPs.
CDN POPs cache resources based on the following strategy:
- When a client sends a request to a domain name, it first requests the IP address of the domain name 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 acquired, Alibaba Cloud CDN allocates the optimal POPs for this request based on the intelligent scheduling system.
- The client then sends a connection request to the CDN POP. If the requested resource has already been cached on the L1 POP, the L1 POP directly returns the resource to the client. If the requested resource is not cached on the L1 POP, the L1 POP requests the resource from the L2 POP. If the resource is cached on the L2 POP, the L2 POP returns the resource to the L1 POP, which then returns the resource to the client. If the resource is not cached on the L2 POP, the L2 POP requests the resource from the origin server and caches the resource based on the configured cache policy.