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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta name="generator" content="HTML Tidy, see www.w3.org" />
<title>The Apache EBCDIC Port</title>
</head>
<!-- Background white, links blue (unvisited), navy (visited), red (active) -->
<body bgcolor="#FFFFFF" text="#000000" link="#0000FF"
vlink="#000080" alink="#FF0000">
<!--#include virtual="header.html" -->
<blockquote>
<strong>Warning:</strong> This document has not been updated
to take into account changes made in the 2.0 version of the
Apache HTTP Server. Some of the information may still be
relevant, but please use it with care.
</blockquote>
<h1 align="CENTER">Overview of the Apache EBCDIC Port</h1>
<p>Version 1.3 of the Apache HTTP Server is the first version
which includes a port to a (non-ASCII) mainframe machine which
uses the EBCDIC character set as its native codeset.<br />
(It is the SIEMENS family of mainframes running the <a
href="http://www.siemens.de/servers/bs2osd/osdbc_us.htm">BS2000/OSD
operating system</a>. This mainframe OS nowadays features a
SVR4-derived POSIX subsystem).</p>
<p>The port was started initially to</p>
<ul>
<li>prove the feasibility of porting <a
href="http://dev.apache.org/">the Apache HTTP server</a> to
this platform</li>
<li>find a "worthy and capable" successor for the venerable
<a href="http://www.w3.org/Daemon/">CERN-3.0</a> daemon
(which was ported a couple of years ago), and to</li>
<li>prove that Apache's preforking process model can on this
platform easily outperform the accept-fork-serve model used
by CERN by a factor of 5 or more.</li>
</ul>
<br />
<br />
<p>This document serves as a rationale to describe some of the
design decisions of the port to this machine.</p>
<h2 align="CENTER">Design Goals</h2>
<p>One objective of the EBCDIC port was to maintain enough
backwards compatibility with the (EBCDIC) CERN server to make
the transition to the new server attractive and easy. This
required the addition of a configurable method to define
whether a HTML document was stored in ASCII (the only format
accepted by the old server) or in EBCDIC (the native document
format in the POSIX subsystem, and therefore the only realistic
format in which the other POSIX tools like grep or sed could
operate on the documents). The current solution to this is a
"pseudo-MIME-format" which is intercepted and interpreted by
the Apache server (see below). Future versions might solve the
problem by defining an "ebcdic-handler" for all documents which
must be converted.</p>
<h2 align="CENTER">Technical Solution</h2>
<p>Since all Apache input and output is based upon the BUFF
data type and its methods, the easiest solution was to add the
conversion to the BUFF handling routines. The conversion must
be settable at any time, so a BUFF flag was added which defines
whether a BUFF object has currently enabled conversion or not.
This flag is modified at several points in the HTTP
protocol:</p>
<ul>
<li><strong>set</strong> before a request is received
(because the request and the request header lines are always
in ASCII format)</li>
<li><strong>set/unset</strong> when the request body is
received - depending on the content type of the request body
(because the request body may contain ASCII text or a binary
file)</li>
<li><strong>set</strong> before a reply header is sent
(because the response header lines are always in ASCII
format)</li>
<li><strong>set/unset</strong> when the response body is sent
- depending on the content type of the response body (because
the response body may contain text or a binary file)</li>
</ul>
<br />
<br />
<h2 align="CENTER">Porting Notes</h2>
<ol>
<li>
The relevant changes in the source are #ifdef'ed into two
categories:
<dl>
<dt><code><strong>#ifdef
CHARSET_EBCDIC</strong></code></dt>
<dd>Code which is needed for any EBCDIC based machine.
This includes character translations, differences in
contiguity of the two character sets, flags which
indicate which part of the HTTP protocol has to be
converted and which part doesn't <em>etc.</em></dd>
<dt><code><strong>#ifdef _OSD_POSIX</strong></code></dt>
<dd>Code which is needed for the SIEMENS BS2000/OSD
mainframe platform only. This deals with include file
differences and socket implementation topics which are
only required on the BS2000/OSD platform.</dd>
</dl>
</li>
<li style="list-style: none"><br />
</li>
<li>The possibility to translate between ASCII and EBCDIC at
the socket level (on BS2000 POSIX, there is a socket option
which supports this) was intentionally <em>not</em> chosen,
because the byte stream at the HTTP protocol level consists
of a mixture of protocol related strings and non-protocol
related raw file data. HTTP protocol strings are always
encoded in ASCII (the GET request, any Header: lines, the
chunking information <em>etc.</em>) whereas the file transfer
parts (<em>i.e.</em>, GIF images, CGI output <em>etc.</em>)
should usually be just "passed through" by the server. This
separation between "protocol string" and "raw data" is
reflected in the server code by functions like bgets() or
rvputs() for strings, and functions like bwrite() for binary
data. A global translation of everything would therefore be
inadequate.<br />
(In the case of text files of course, provisions must be
made so that EBCDIC documents are always served in
ASCII)</li>
<li style="list-style: none"><br />
</li>
<li>This port therefore features a built-in protocol level
conversion for the server-internal strings (which the
compiler translated to EBCDIC strings) and thus for all
server-generated documents. The hard coded ASCII escapes \012
and \015 which are ubiquitous in the server code are an
exception: they are already the binary encoding of the ASCII
\n and \r and must not be converted to ASCII a second time.
This exception is only relevant for server-generated strings;
and <em>external</em> EBCDIC documents are not expected to
contain ASCII newline characters.</li>
<li style="list-style: none"><br />
</li>
<li>By examining the call hierarchy for the BUFF management
routines, I added an "ebcdic/ascii conversion layer" which
would be crossed on every puts/write/get/gets, and a
conversion flag which allowed enabling/disabling the
conversions on-the-fly. Usually, a document crosses this
layer twice from its origin source (a file or CGI output) to
its destination (the requesting client): <samp>file ->
Apache</samp>, and <samp>Apache -> client</samp>.<br />
The server can now read the header lines of a CGI-script
output in EBCDIC format, and then find out that the remainder
of the script's output is in ASCII (like in the case of the
output of a WWW Counter program: the document body contains a
GIF image). All header processing is done in the native
EBCDIC format; the server then determines, based on the type
of document being served, whether the document body (except
for the chunking information, of course) is in ASCII already
or must be converted from EBCDIC.</li>
<li style="list-style: none"><br />
</li>
<li>
For Text documents (MIME types text/plain, text/html
<em>etc.</em>), an implicit translation to ASCII can be
used, or (if the users prefer to store some documents in
raw ASCII form for faster serving, or because the files
reside on a NFS-mounted directory tree) can be served
without conversion.<br />
<strong>Example:</strong>
<blockquote>
to serve files with the suffix .ahtml as a raw ASCII
text/html document without implicit conversion (and
suffix .ascii as ASCII text/plain), use the directives:
<pre>
AddType text/x-ascii-html .ahtml
AddType text/x-ascii-plain .ascii
</pre>
</blockquote>
Similarly, any text/foo MIME type can be served as "raw
ASCII" by configuring a MIME type "text/x-ascii-foo" for it
using AddType.
</li>
<li style="list-style: none"><br />
</li>
<li>Non-text documents are always served "binary" without
conversion. This seems to be the most sensible choice for,
.<em>e.g.</em>, GIF/ZIP/AU file types. This of course
requires the user to copy them to the mainframe host using
the "rcp -b" binary switch.</li>
<li style="list-style: none"><br />
</li>
<li>Server parsed files are always assumed to be in native
(<em>i.e.</em>, EBCDIC) format as used on the machine, and
are converted after processing.</li>
<li style="list-style: none"><br />
</li>
<li>For CGI output, the CGI script determines whether a
conversion is needed or not: by setting the appropriate
Content-Type, text files can be converted, or GIF output can
be passed through unmodified. An example for the latter case
is the wwwcount program which we ported as well.</li>
<li style="list-style: none"><br />
</li>
</ol>
<br />
<br />
<h2 align="CENTER">Document Storage Notes</h2>
<h3 align="CENTER">Binary Files</h3>
<p>All files with a <samp>Content-Type:</samp> which does not
start with <samp>text/</samp> are regarded as <em>binary
files</em> by the server and are not subject to any conversion.
Examples for binary files are GIF images, gzip-compressed files
and the like.</p>
<p>When exchanging binary files between the mainframe host and
a Unix machine or Windows PC, be sure to use the ftp "binary"
(<samp>TYPE I</samp>) command, or use the
<samp>rcp -b</samp> command from the mainframe host (the
-b switch is not supported in unix rcp's).</p>
<h3 align="CENTER">Text Documents</h3>
<p>The default assumption of the server is that Text Files
(<em>i.e.</em>, all files whose <samp>Content-Type:</samp>
starts with <samp>text/</samp>) are stored in the native
character set of the host, EBCDIC.</p>
<h3 align="CENTER">Server Side Included Documents</h3>
<p>SSI documents must currently be stored in EBCDIC only. No
provision is made to convert it from ASCII before
processing.</p>
<h2 align="CENTER">Apache Modules' Status</h2>
<table border="1" align="middle">
<tr>
<th>Module</th>
<th>Status</th>
<th>Notes</th>
</tr>
<tr>
<td align="LEFT">http_core</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_access</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_actions</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_alias</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_asis</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_auth</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_auth_anon</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_auth_db</td>
<td align="CENTER">?</td>
<td>with own libdb.a</td>
</tr>
<tr>
<td align="LEFT">mod_auth_dbm</td>
<td align="CENTER">?</td>
<td>with own libdb.a</td>
</tr>
<tr>
<td align="LEFT">mod_autoindex</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_cern_meta</td>
<td align="CENTER">?</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_cgi</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_digest</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_dir</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_so</td>
<td align="CENTER">-</td>
<td>no shared libs</td>
</tr>
<tr>
<td align="LEFT">mod_env</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_example</td>
<td align="CENTER">-</td>
<td>(test bed only)</td>
</tr>
<tr>
<td align="LEFT">mod_expires</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_headers</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_imap</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_include</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_info</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_log_agent</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_log_config</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_log_referer</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_mime</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_mime_magic</td>
<td align="CENTER">?</td>
<td>not ported yet</td>
</tr>
<tr>
<td align="LEFT">mod_negotiation</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_proxy</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_rewrite</td>
<td align="CENTER">+</td>
<td>untested</td>
</tr>
<tr>
<td align="LEFT">mod_setenvif</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_speling</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_status</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_unique_id</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_userdir</td>
<td align="CENTER">+</td>
<td>
</td>
</tr>
<tr>
<td align="LEFT">mod_usertrack</td>
<td align="CENTER">?</td>
<td>untested</td>
</tr>
</table>
<h2 align="CENTER">Third Party Modules' Status</h2>
<table border="1" align="middle">
<tr>
<th>Module</th>
<th>Status</th>
<th>Notes</th>
</tr>
<tr>
<td align="LEFT"><a
href="http://java.apache.org/">mod_jserv</a> </td>
<td align="CENTER">-</td>
<td>JAVA still being ported.</td>
</tr>
<tr>
<td align="LEFT"><a href="http://www.php.net/">mod_php3</a>
</td>
<td align="CENTER">+</td>
<td>mod_php3 runs fine, with LDAP and GD and FreeType
libraries</td>
</tr>
<tr>
<td align="LEFT"><a
href="http://hpwww.ec-lyon.fr/~vincent/apache/mod_put.html">
mod_put</a> </td>
<td align="CENTER">?</td>
<td>untested</td>
</tr>
<tr>
<td align="LEFT"><a
href="ftp://hachiman.vidya.com/pub/apache/">mod_session</a>
</td>
<td align="CENTER">-</td>
<td>untested</td>
</tr>
</table>
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</body>
</html>
|
