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86 lines
3.8 KiB
HTML
86 lines
3.8 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
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"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
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<html xmlns="http://www.w3.org/1999/xhtml">
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<head>
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<meta name="generator" content="HTML Tidy, see www.w3.org" />
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<title>Apache 2.0 Layered I/O</title>
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</head>
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<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
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<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
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vlink="#000080" alink="#FF0000">
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<h1 align="center">Apache Layered I/O</h1>
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<p>Layered I/O has been the holy grail of Apache module writers
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for years. With Apache 2.0, module writers can finally take
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advantage of layered I/O in their modules.</p>
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<p>In all previous versions of Apache, only one handler was
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allowed to modify the data stream that was sent to the client.
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With Apache 2.0, one module can modify the data and then
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specify that other modules can modify the data if they would
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like.</p>
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<h2>Taking advantage of layered I/O</h2>
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<p>In order to make a module use layered I/O, there are some
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modifications needed. A new return value has been added for
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modules, RERUN_HANDLERS. When a handler returns this value, the
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core searches through the list of handlers looking for another
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module that wants to try the request.</p>
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<p>When a module returns RERUN_HANDLERS, it must modify two
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fields of the request_rec, the handler and content_type fields.
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Most modules will set the handler field to NULL, and allow the
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core to choose the which module gets run next. If these two
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fields are not modified, then the server will loop forever
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calling the same module's handler.</p>
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<p>Most modules should not write out to the network if they
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want to take advantage of layered I/O. Two BUFF structures have
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been added to the request_rec, one for input and one for
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output. The module should read and write to these BUFFs. The
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module will also have to setup the input field for the next
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module in the list. A new function has been added,
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ap_setup_input, which all modules should call before they do
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any reading to get data to modify. This function checks to
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determine if the previous module set the input field, if so,
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that input is used, if not the file is opened and that data
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source is used. The output field is used basically the same
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way. The module must set this field before they call ap_r* in
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order to take advantage of layered I/O. If this field is not
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set, ap_r* will write directly to the client. Usually at the
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end of a handler, the input (for the next module) will be the
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read side of a pipe, and the output will be the write side of
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the same pipe.</p>
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<h3>An Example of Layered I/O.</h3>
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<p>This example is the most basic layered I/O example possible.
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It is basically CGIs generated by mod_cgi and sent to the
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network via http_core.</p>
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<p>mod_cgi executes the cgi script, and then sets
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request_rec->input to the output pipe of the CGI. It then
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NULLs out request_rec->handler, and sets
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request_rec->content_type to whatever the CGI writes out (in
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this case, text/html). Finally, mod_cgi returns
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RERUN_HANDLERS.</p>
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<p>ap_invoke_handlers() then loops back to the top of the
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handler list and searches for a handler that can deal with this
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content_type. In this case the correct module is the
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default_handler from http_core.</p>
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<p>When default handler starts, it calls ap_setup_input, which
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has found a valid request_rec->input, so that is used for
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all inputs. The output field in the request_rec is NULL, so
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when default_handler calls an output primitive it gets sent out
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over the network.</p>
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<i>Ryan Bloom, 25th March 2000</i>
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</body>
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</html>
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