Cause analysis of Error 503 Service unavailable

This occurs because your Web site exceeds the IIS limit caused by
Because 2003 of the operating system in the hint of IIS is not like the 2000 system prompts "too many links", but prompted "Service unavailable", this situation occurs because the site is more than the system resource constraints caused by the main program to occupy too much resources.
For example, the same 100-person online forum, the Forum's resources are the PW forum accounted for more than 10 times times the resources; In addition, some dead loop program, or not optimized program will occupy too much system resources, and system resources are obviously limited. However, the WINDOWS2003 operating system, the Web site is running in a separate process, will not affect each other.
If a Web site's programs account for too much or too many errors, the system log prompts: "Application pool ' xxx ' is automatically disabled because of a series of errors in the process of servicing the application pool?" or Hint: application pool ' xxx ' has exceeded its job limit setting. At this point, visit this site will be prompted: Service unavailable. The general system will return to normal in about 30 seconds, more than a few times can be normal access to refresh.

In addition, if your site currently has too many visitors, more than the system's IIS connection limit, there will be a service unavailable (Win2K host too many connections will be prompted: Too many connections, please try again later While the Win2003 host just directly prompts: Service unavailable)

Two, do not restrict IIS connection, or encounter service unavailable
See more sites using an Access database
Users who are generally using Windows 2003 IIS 6 may have a normal system for this problem, and suddenly a website can't be opened.
Tip: Service unavailable But this site does not limit the number of IIS connections. Then immediately affect other sites, less than a while, other sites are all turned into Service unavailable
What's the reason?
We can know after analysis, or MS's old problem. The access engine was. With the server doctor's file Doctor repair, when you view the fix results, you will find some files that caused the access engine to "catastrophic failure" and "not set object references to instances of objects." The system will not return to normal until the file doctor fixes it.

Third, when browsing a Windows SharePoint Services Web site, prompt: Service unavailable

This problem may occur when the application pool for the virtual server is not configured correctly in 1.Microsoft Internet information Services (IIS) 6.0.
Solution
To resolve this issue, follow these steps: 1. Verify that the application pool has been configured for the virtual server. The default application pool is MSSharePointPortalAppPool.
Follow these steps to determine which application pool the virtual server is using.
A. Click Start, point to Administrative Tools, and then click Internet Information Services (IIS) Manager.
B. Expand ServerName, expand Web Sites, right-click the virtual server, and then click Properties.
C. Click the Home Directory tab. The application pool configured for the virtual server is listed in the Application pool box.
D. Click OK.

2.
Verify that the password used by the application pool account is correct. IIS does not automatically poll for password changes in the Active Directory directory service. If the application pool account is a domain account and its password has expired, you may receive the error message that is described in the "Symptoms" section of this article after you reassign a new password for this account.

3.
Verify that the application pool account is a member of the IIS_WPG and STS_WPG groups on the server.

4.
Restart IIS to recycle the application pool