NeQo wrote:I don't agree with that and why is this error occuring?
You only host txt and pictures here, that is not very intensive...
Nonetheless, Johan's description is accurate.
The message reflects throttling at the level of the Apache web server, caused by a large number of expensive requests arriving too quickly together.
When it occurs, our admin panels always show an unusually large number of Guest logins from what appear to be robots of one sort or another.
The standard indexing robots are not so bad. Although they make lots of requests, the requests that they make are apparently fairly inexpensive and do not cause problems. For example just before I started to type this reply, the admin panel showed that in the preceding few minutes there had been 3 registered users and 163 guest logins, most of those from Google. The forum was running fine at that time.
In contrast, last night I logged on to find that the forum was hitting resource limits, and when I checked the status, I found that some unknown robot was doing nothing except trying to check user Profiles. This is a not an effective thing for a Guest robot to do, because nobody except logged-in users are permitted to look at profiles. In response to the request, the forum presents a login screen. I have no idea where the conversation with the robot goes from there, other than apparently consuming quite a few resources for no value. I blocked the offending IP address at the level of .htaccess, and the problem went away.
That was an unusual circumstance. More typically we find a robot apparently clicking buttons at random. (Guest logins trying to Post are good indicators.) Some of them may be ill-designed indexing robots; others we think are corporate firewalls that are proactively looking for malware on sites visited by users who live behind the firewall.
In any case, these instances of unusually high demand from robots end up acting as sort of unintended denial-of-service attacks. It doesn't take much load to do that because the forum is only hosted on an inexpensive shared server account.
Yes, we could probably solve the problem by just moving the forum to a higher class server. That would not be hugely difficult, or hugely expensive, but to date the annoyance has not been worth the trouble and expense to do it. That may change in the future, of course.