After ISAPI is installed, the website displays service unavailable

This problem occurs because not all IIS services are loaded. For example, if IIS fails to load isapi_rewrite, it will continue to load, resulting in excessive CPU usage and service unavailable. This may be because the installation directory of isapi_rewrite does not have the access permission of everyone, you can attach the permission to the instance. Of course, it may be that loading other databases is not successful. Following this idea, it is easier to solve the problem of Service unavailable.

If the service is unavailable in other cases, there are at least two possible causes:
1. The number of connections to your website exceeds the limit of IIS. Because 2003 of the operating system prompts "Service unavailable" when there are too many IIS prompts, instead of prompting "too many connections" like 2000 ", this is because the website exceeds the system resource limit, mainly because the program occupies too many resources.
If a website program occupies too many resources or has too many errors, the system log will prompt: "The application pool 'xxx' is automatically disabled, the reason is that a series of errors occur in the process that provides services for this application pool, or the prompt is: The application pool 'xxx' exceeds its job restriction settings. When you access this website, the message "Service unavailable" is displayed. Generally, the system returns to normal in about 30 seconds, and can be accessed after several refreshes.
2. The system is under DDoS attack. Dos generally refers to the attempt by hackers to prevent normal users from using services on the network. For example, the disconnection of telephone lines in the building causes the user to be unable to make calls. However, due to network bandwidth, network devices, server hosts, and other processing capabilities, hackers cannot process excessive network packets, so that normal users cannot use the service normally. DDoS attacks are attacks by hackers using multiple machines at the same time to prevent normal users from using services. A message "Service unavailable" is often displayed on a host under DDoS attack.