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From Graham Leggett <minf...@sharp.fm>
Subject Re: svn commit: r1215525 - in /httpd/httpd/trunk: ./ docs/manual/mod/ docs/manual/programs/ modules/debugging/ support/
Date Tue, 28 Feb 2012 23:47:11 GMT
On 29 Feb 2012, at 12:21 AM, William A. Rowe Jr. wrote:

> After two months, firehose still didn't obtain another +1, so the vote to
> incorporate firehose into trunk stands at 3 +1's, 1 -1, and therefore
> failed the vote for inclusion in trunk.

I count 4 +1s on the dev@httpd list:

minfrin, issac, sctemme, jim

and igalic on the pmc list. poirier and niq expressed support, but didn't vote. By my reckoning,
it passes.

> There are 4 +1's for a firehose subproject at httpd.  If you wish to continue
> this effort you can keep this simple by svn mv'ing those respective files across
> from httpd/httpd/trunk to httpd/mod_firehose/trunk, where it can build a larger
> community within httpd project, and things like the data representation format
> can be evaluated and enhanced by the community.

So far, you are the only one who has questioned the data representation format, claiming that
wireshark/pcap could be used instead.

When I invented the firehose I wanted an ASCII based protocol, so that I could look at a firehose
being recorded and by inspection see the boundaries between the buckets. When you're debugging
httpd or an application server behind httpd the bucket boundaries are important. The protocol
used is simply chunked encoding, with additional fields behind the chunked value allowing
you to tie the buckets back together, and it is very clear which bits are firehose and which
part is the bucket by inspection, on purpose.

In addition, there was a clear need to be able to capture both individual requests, and whole
connection streams, and have the ability to choose which. With the individual requests we
were able to isolate some tricky protocol edge cases in mod_cache, and with the connections
we were able to identify issues with HTTP/1.0 clients and keepalive in a highly loaded service
oriented environment.

Further crucial information is dropped buckets, which is either a sign of congestion over
the pipe to which these buckets are written, or a sign that the child process crashed unexpectedly.

When you brought up pcap as a possibility, I went and did a whole bunch of research into it.
What I discovered is that pcap itself gives you a binary encapsulation format, meaning that
you lose the ability to see bucket boundaries, and the ability to debug by inspection. Then
I looked for existing client support that might help, and discovered there is none - clients
expect the pcap packets to contain clearly identified packets you would expect to find at
layer 2, not free form binary that you can expect to find in an HTTP body. I considered hacking
up fake TCP packets, but then ran into the problem that I now needed to care whether they
were IPV4 or IPV6 packets, and trying to hack up fake packets when you captured individual
requests on a long lived connection is a non trivial exercise. At the end of the day you want
confidence in your debugging data, and a hacked together reconstruction of TCP is the exact
opposite of that.

The HTTP protocol dump is called a "firehose" for a reason - at one point we were recording
and analysing hundreds of gigabytes of request data, and firehose had to decode and process
that in situ on a production box that had been taken out of the pool without the installation
of any scripting language stack (which would have been too slow anyway), or the moving around
of the data. Both mod_firehose and firehose are battle tested in that real world scenario.

I believe the pcap based solution you proposed is inferior to what we have now, and I believe
there is no compelling reason at this point to attempt any alternative solution.

Regards,
Graham
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