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From eric <spamsin...@gmail.com>
Subject [users@httpd] Server Drops Client Intermittently
Date Tue, 19 Aug 2008 13:44:32 GMT
Hello,

My webserver intermittently drops the client connection, how can I determine
the root cause of this problem?

The server is Apache 2.2 on Suse 10.2.  I have implemented a server application
which is a C++ binary running under mod_fcgid (1.10).  The client is a C++
command line application which may be compiled under either Windows or Linux.
Both the client and the server applications are statically linked to gSOAP
2.7.10 which I have configured for FastCGI and zlib.  The client sends
a query to
the server which performs a calculation and sends a response.  The client
application is deployed to five sites.

The query from the client may vary in size.  When the query is small, the
application always succeeds.  When the query is large (i.e. when its compressed
size exceeds approximately 10MB), the application fails intermittently.  At
four of the five client sites the success rate with the larger query varies
between 5% and 50%.  At the fifth client site the application always succeeds
regardless of query size, I'm unable to determine what differentiates this
sites from the others.  The client application is also installed on the
webserver itself, for testing, and that build always succeeds.

On failure, mod_fcgid writes this message to the error log: "can't get data
from http client".  This message originates from function bridge_request() in
mod_fcgid following a failed call to ap_get_brigade().  I'm unable to determine
the root cause of the failure.

I run tcpdump on the server port.  A successful query looks like this:

1a) client->server: query - compressed XML containing the request
1b) server->client: ACK
1c) server->client: HTTP 200 OK + compressed XML containing the reponse

A failed query looks like this:

2a) client->server: query - compressed XML containing the request
2b) server->client: HTTP 500 Internal Server Error

Note that in the case of failure, the request from the client (2a) is received
in full on the server port.  mod_fcgid fails without invoking the server
application.

How do I determine the root cause of this problem?

Thanks,
Eric

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