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ApacheToday: Filtering I/O in Apache 2.0

“One of the holy grails of the Apache developers has always been
filtered or layered I/O, the ability for one module to modify the
data that was generated by an earlier module. This ability was
originally slated for inclusion in Apache 2.0, but when work began
in earnest on 2.0, this feature was pushed aside, and marked for
inclusion in 2.1 or 3.0. Two months ago however, the Apache
developers had a small meeting, and designed filtered I/O for
Apache 2.0. The work has been started, and there have been some
filters written. Over the next few months, I will explain how this
feature works, and how your modules can take advantage of it.”

“The general premise of the filtered I/O design in Apache 2.0 is
that all data served by a web server can be broken into chunks.
Each chunk of data comes from the same place either a file, a CGI
program, or it is generated by a module. We also knew that all of
the data could always be represented as a string of characters,
although that string may not be human-readable. Armed with that
knowledge, we sat down to design the filtering system. One of our
overriding goals, was that the filtering logic needed to be
performance aware. It didn’t matter if filters chose to ignore
performance issues, but it did matter if the design hindered
filters from knowing about performance issues. This meant that we
needed to know more about that data than just what the data was, we
also needed to know where the data comes from, and what it’s
lifetime is.”

“This meta-data is important when actually writing the response
to the network. For example, if we have a very simple request that
is just a page from disk, then we want to use sendfile (sendfile is
provided by APR, and is available on all platforms, it the platform
doesn’t have a native sendfile, then APR loops reading the file and
writing to the network.) If we take this example a step further,
and make the whole response an SSI page, where one element is a
file from disk, and the rest is generated, such as date strings,
then we want to use a single sendfile call if possible. APR’s
sendfile provides an opportunity to include both header and trailer
information with the file, which are sent using writev. In this
example, we can send the HTTP headers, the full file, and the date
string with one APR call (The number of system calls will differ
depending on platform). Keeping the meta-data accessable is
obviously a good idea.”


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