How to fix “Service Unavailable: HTTP Error 503” issue

Problem:

I deployed a web site to my local IIS, and then I got this error when I tried to visit it:

Service Unavailable

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HTTP Error 503. The service is unavailable.

Solution:

There are many complicated steps described in the google search results. But the worked solution for me is this simple one:

1. Open the Internet Information Services (IIS) Manager
2. Select the Application Pools node under the root node in the left hand side frame
3. Right click on the DefaultAppPool. If the service is stopped, start it. If the service is running, restart it.
4. Issue resolved.

Thank Richie for providing this solution.

Discussion:

To improve the computer performance, I made the application pools not to start automatically, so this issue occurs.