examples: Added some commentary.

[ashd.git] / README

		      ashd -- A Sane HTTP Daemon

Ashd  is a  HTTP  server  that follows  standard  Unix philosophy  for
modularity.  Instead  of  being  a monolithic  program  with  loadable
modules,  as most  other HTTP  servers seem  to be,  Ashd is  simply a
collection  of much simpler  programs, passing  HTTP requests  to each
other using a simple protocol.

Among  the  nice  properties  brought  about by  such  a  design,  the
following might be said to stand out:

 * Sanity of design -- The  clean delineation of functions allows each
   program  to be  very  small and  simple  – currently,  each of  the
   programs  in the collection  (including even  the core  HTTP parser
   program,  htparser,  as long  as  one  does  not count  its,  quite
   optional,  SSL implementation)  is implemented  in less  than 1,000
   lines  of C  code (and  most are  considerably smaller  than that),
   allowing them to be easily studied and understood.

 * Security -- Since each program runs in a process of its own, it can
   be  assigned  proper  permissions.   Most noteworthy  of  all,  the
   userplex program ensures that serving of user home directories only
   happens by code that is actually logged in as the user in question;
   and  the htparser  program,  being the  only  program which  speaks
   directly with  the clients,  can run perfectly  well as  a non-user
   (like nobody) and be chroot'ed into an empty directory.

 * Persistence -- Though Ashd is a multi-process program, it is not in
   the same  sense as e.g.  Apache. Each request handler  continues to
   run  indefinitely and  does not  spawn multiple  copies  of itself,
   meaning that all process  state persists between requests – session
   data can be kept in memory, connections to back-end services can be
   kept open, and so on.

 * Clean modularity -- With only  a rather basic understanding of HTTP
   and  the internal  Ashd protocol,  it is  quite easy  to  write new
   request handlers to extend  the server's functionality; and one can
   do that even without needing root privileges on the system.

			Architecture Overview

Though the  server as  a whole  is called `Ashd',  there is  no actual
program by  that name.   The `htparser' program  of Ashd  implements a
minimal HTTP server. It speaks HTTP (1.0 and 1.1) with clients, but it
does  not  know  anything  about  actually handling  the  requests  it
receives. Rather, having started a handler program as specified on the
command-line,  it packages  the requests  up and  passes them  to that
handler program. That handler program  may choose to only look at part
of the URL and pass the  request on to other handler programs based on
what  it sees.  In that  way, the  handler programs  form  a tree-like
structure, corresponding roughly  to the URL space of  the server.  In
order to  do that,  the packaged request  which is passed  between the
handler  programs contains the  part of  the URL  which remains  to be
parsed, referred to as the  `rest string' or the `point' (in deference
to Emacs parlance).

For  a technical  description  of the  architecture,  see the  ashd(7)
manpage, available in the `doc' directory of this source tree.

			       The Cast

As an introduction to the  various programs that compose Ashd, here is
a listing of  the more important programs. All  of them have manpages,
so please see those for further details.

 * htparser -- The `actual' HTTP  server. htparser is the program that
   listens to TCP connections and speaks HTTP with the clients.

 * dirplex  -- dirplex  is the  program  used for  serving files  from
   actual directories, in a manner  akin to how most other HTTP server
   work. In order to do that, dirplex maps URLs into existing physical
   files, and then performs  various kinds of pattern-matching against
   the names of those physical  files to determine the handler to call
   to actually serve them.

 * patplex  --  Performs  pattern  matching  against  logical  request
   parameters  such as  the rest  string,  URL or  various headers  to
   determine a program to pass the request to. As such, patplex can be
   used  to implement such  things as  virtual directories  or virtual
   hosts.

 * sendfile  -- A  simple  handler program  for  sending literal  file
   contents, normally called by dirplex for serving ordinary files. It
   handles  caching using  the Last-Modified  and related  headers. It
   also handles  MIME-type detection if  a specific MIME-type  was not
   specified.

 * callcgi -- Translates  an Ashd request into a  CGI environment, and
   runs  either the requested  file directly  as a  CGI script,  or an
   external CGI handler.  Thus, it can be used  to serve, for example,
   PHP pages.

 * userplex -- Handles `user directories', to use Apache parlance; you
   may know them otherwise as /~user/ URLs. When a request is made for
   the directory  of a specific user,  it makes sure  that the request
   handler  runs  as the  user  in  question.