Hi guys.A customer of ours have an internal Apache 2.2 server under Linux Debian. Static pages of their applications are located on their own server. Some URLs are proxied by their server to our own Apache 2.2 server (Linux Debian), through the Internet, by means of a line like
ProxyPass /getthumb http://our-server.company.com and the URLs are like /getthumb?id=(thumb-id)These URLs obtain thumbnails from our server, corresponding to documents. Each thumbnail is generated on the fly and is about 10 KB in size. It takes our server on average 150 ms to process such a URL and return the thumbnail to their server. The number of thumbnails to serve for one customer application page is variable, between just a few and sometimes several hundred, with an average of 20-30 per page. In the vast majority of cases, each thumbnail request is served in around 150 ms, and we serve thousands such requests per day.
Yet sometimes, for no apparent reason that we can link to the process of generating and returning the thumbnail, nor to any load of our own Apache server or theirs, returning one arbitrary thumbnail in the series takes about 30 seconds. It is always either about 150 ms, or about 30000 ms. It is never in-between, like 10 or 20 seconds or so.
The delay does not always affect the same thumbnails either. For the same requested page with the same series of thumbnails, sometimes the 30 seconds delay will happen at the first and second thumbnail, sometimes 4 of them in the middle, and so on. Sometimes the whole page-worth of thumbnails loads in exactly (number of thumbnails X 150 ms), which is what it should always be.
If I watch our server's processes with "top", and simultaneously the end of the access log or the screen of one of their workstations, I can see nothing untoward when the browser suddenly "hangs" for 30 seconds waiting fo the next thumbnail. The Apache processes on our side seem mostly idle at that time, and there are plenty of Apache processes available to serve any request. It looks as if the browser on the customer side just hangs waiting for the thumbnail, and our server is just idle.
These times are per our own server's access log, and per their Firefox browser, to which we added the HttpFox add-on, to see what happens from their side. Their browsers do issue the requests (almost all at once), and the symptoms are the same whether they use IE or Firefox.
My first question is : does there exist any tool in Apache itself, that would show me what is happening during such a 30-second pause ? The access log does not, as it just adds a line at the end of the 30 seconds, when the request is terminated, which is too late. There are no errors in our error log. mod_log_io and mod_forensic, from their documentation, do not seem to provide much more relevant info.
For example, since their server proxies these calls to ours, and we see nothing in our logs, is there a way whereby I can set the logging of the proxy calls (and only those, preferably) to some "debug" setting ?
(I have full access to their and our Apache servers configurations)My second question would be : does anyone have an idea of what could explain sudden random 30 second pauses in such a schema ?
I am trying to avoid having to go through a TCP protocol analyser, because I am unfamiliar with them, at least at the Linux level.
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