martin 98/03/26 07:48:50
Added: htdocs/manual ebcdic.html
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Add a preliminary EBCDIC porting paper (derived from README.EBCDIC)
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1.1 apache-1.3/htdocs/manual/ebcdic.html
Index: ebcdic.html
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<H1 ALIGN="CENTER">Overview of 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 NIXDORF family of mainframes running the
<A HREF="http://www.sni.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
<UL>
<LI> prove the feasibility of porting
<A HREF="http://dev.apache.org/";>the Apache HTTP server</A>
to this platform
<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> 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.
</UL>
</P>
<P>
This document serves as a rationale to describe some of the design
decisions of the port to this machine.
</P>
<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>
<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:
<UL>
<LI><STRONG>set</STRONG> before a request is received (because the
request and the request header lines are always in ASCII
format)
<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><STRONG>set</STRONG> before a reply header is sent (because the
response header lines are always in ASCII format)
<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)
</UL>
</P>
<H1 ALIGN="CENTER">Porting Notes</H1>
<P>
<OL>
<LI>
The relevant changes in the source are #ifdef'ed into two
categories:
<DL>
<DT><CODE><STRONG>#ifdef CHARSET_EBCDIC</STRONG></CODE>
<DD>Code which is needed for any EBCDIC based machine. This
includes character translations, differences in 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 etc.
<DT><CODE><STRONG>#ifdef _OSD_POSIX</STRONG></CODE>
<DD>Code which is needed for the BS2000 SIEMENS NIXDORF
mainframe platform only. This deals with include file
differences and socket implementations topics which are
only required on the BS2000/OSD platform.
</DL>
</LI><BR>
<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 etc.) whereas the file transfer parts (i.e., GIF
images, CGI output etc.) should usually be just "passed thru"
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 the documents are always served in ASCII format)
</LI><BR>
<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 always go through a bijective EBCDIC <-> ASCII
translation table.
</LI><BR>
<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. It is now possible to 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). Likewise, the server
always generates its header lines in EBCDIC (and with ASCII
conversion enabled) and 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
is converted from EBCDIC.
</LI><BR>
<LI>
For Text documents (MIME types text/plain, text/html etc.),
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/XXXX MIME type can be served as "raw ASCII" by
configuring a MIME type "text/x-ascii-XXXX" for it using AddType.
</LI><BR>
<LI>
Non-text documents are always served "binary" without conversion.
This seems to be the most sensible choice for, .e.g., 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><BR>
<LI>
Server parsed files are always assumed to be in native (i.e.,
EBCDIC) format as used on the machine, and are converted after
processing.
</LI><BR>
<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><BR>
</OL>
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