What is the reason that HTTP error 503 appears on the cloud server?

Now HTTP Error 503. The reason for the service is unavailable error is generally as follows: specific reasons need to be analyzed through the weblog access log.

Site was attacked, in excess of limit times 503 error, the attack can be resumed after the stop;

The site is large and there are too many concurrent requests, this recommendation modifies the optimizer or requires upgrading to a higher type of host;

Program has errors, in a short period of time to produce a number of work process crashes, will be due to IIS7 fast fault protection function and shut down the program pool;

Site to provide downloads, when the bandwidth exceeds the limit will be an error, you need to stop the download function, or upgrade the host solution.