The reason for HTTP Error 503 is

Now HTTP Error 503. The service is unavailable error is generally caused by the following reasons. The specific reason needs to be analyzed through weblog access log:

Site is attacked 503 error when the limit is exceeded, can be restored after the attack stops;

The site size is large, and there are too many concurrent requests. This suggestion to modify the optimizer or need to upgrade a higher type host;

The program has an error, it will cause multiple work process crashes in a short time, and the program pool will be closed due to the fast fault protection function of IIS7;

The site provides download, when the bandwidth exceeds the limit, it will report an error and need to stop. Download the feature, or upgrade the host to solve.