Undeploy an Alibaba Cloud CDN POP for maintenance -

Scenarios

Alibaba Cloud CDN provides more than 3,200 points of presence (POPs) worldwide, which cover more than 70 countries and regions. To ensure that POPs operate and provide acceleration services as expected, Alibaba Cloud CDN engineers need to perform maintenance on POPs on an irregular basis, such as hardware upgrades, software updates, network cutover, and line switching. In most cases, POPs are undeployed during maintenance. Alibaba Cloud CDN uses various methods to reduce the impact of POP undeployment on users. However, due to local DNS caches of Internet service providers (ISPs) and clients, some clients may still send requests to undeployed POPs. This results in access failures.

Impact and Alibaba Cloud-side optimization methods

Alibaba Cloud CDN provides more than 3,200 POPs worldwide. Routine undeployment of POPs is a normal maintenance operation. To minimize the impact of POP maintenance on user access, Alibaba Cloud CDN engineers use optimization methods, including but not limited to:

Limiting the number of POPs that are maintained at the same time: The number of POPs that are maintained at the same time is minimized to reduce the potential impact of POP undeployment on users.
Limiting the POP maintenance time: Most POP maintenance operations are performed in the early morning to avoid peak hours, which start from 19:00 to 23:00.
POP traffic routing: POP maintenance operations are performed based on a standard processing procedure. Before engineers undeploy a POP that requires maintenance, the routing system of Alibaba Cloud CDN redirects traffic to nearby POPs that run as expected.
Exception monitoring: In some cases, POPs may be undeployed due to exceptions, such as ISP link failures and data center failures. The routing system of Alibaba Cloud CDN monitors all POPs. If a POP is unexpectedly undeployed, traffic destined for the POP is redirected to nearby POPs that run as expected in real time.

User-side optimization methods

Alibaba Cloud CDN uses various methods to reduce the impact of POP undeployment on users. However, due to local DNS caches of ISPs and clients, some clients may still send requests to undeployed POPs. This results in access failures. To resolve this issue, you can use the following methods to further reduce the impact.

Optimization method	Description
Clear the local DNS cache of ISPs	Alibaba Cloud DNS and China Telecom jointly launched the DNS Cache Refresh for China Telecom service. The service can immediately clear the local DNS cache of China Telecom and obtain the latest resolution results. This way, the resolution results immediately take effect on the China Telecom side. For more information, see DNS Cache Refresh for China Telecom.
Clear the DNS cache of clients	Windows: Type `cmd` in the search box, open Command Prompt, enter the `ipconfig /flushdns` command, and then press the Enter key. macOS: Click Launchpad, open Terminal, and then enter the `sudo dscacheutil -flushcache` and `sudo killall -HUP mDNSResponder` commands. The commands may vary based on the version of macOS.
Use HTTPDNS	You can integrate HTTPDNS into the client to bypass the local DNS of the ISP and use the HTTP protocol to access the routing system of Alibaba Cloud CDN. This way, requests are redirected to the optimal POP. The HTTPDNS technology prevents the POP switching delay that is caused by the local DNS cache of ISPs.