Apache Overview How-to

Apache Overview How-to
Daniel Lopez Ridruejo, [email protected]
v0.4, 11 September 2000

The purpose of this document is to give an overview of the Apache web�
server and related projects. It provides pointers for further informa�
tion and implementation details.
______________________________________________________________________

Table of Contents

1. Introduction

2. Apache

3. Apache Software Foundation

4. Developing web applications with Apache

4.1 Static
4.2 Dynamic content
4.3 CGI scripts
4.4 Site generators
4.5 Out of process servers
4.6 Fast CGI
4.7 Java servlets
4.8 Embeded interpreters

5. Performance and bandwidth management

6. Load balancing

7. Secure transactions

8. SNMP

9. Authentication modules

10. GUIs for Apache

11. Writing Apache modules

12. Apache books

13. Java projects

13.1 Ant
13.2 ORO and Regexp
13.3 Slide
13.4 Struts
13.5 Taglibs
13.6 Tomcat
13.7 Velocity
13.8 Watchdog
13.9 JServ
13.10 JSSI
13.11 Apache JMeter
13.12 Server Pages Foundation Classes
13.13 mod_java
13.14 Element Construction Set
13.15 Avalon
13.16 JAMES (Java Apache Mail Enterprise Server)
13.17 PicoServer
13.18 Jetspeed
13.19 Turbine
13.20 Jyve
13.21 Alexandria

14. XML projects

14.1 Introduction to XML
14.2 Xerces
14.3 Xalan
14.4 FOP
14.5 Cocoon
14.6 Xang
14.7 SOAP
14.8 Other XML projects

15. Perl

15.1 Embperl
15.2 Mason
15.3 Mod_Perl

16. PHP

17. Python

18. Tcl

19. Modules for other languages

20. Apache 2.0

21. Migrating from Netscape (iPlanet) web servers

22. Migrating from Microsoft IIS

23. Links

23.1 Websites
23.2 Java application servers

24. Contacting the author

24.1 Translations

______________________________________________________________________

1. Introduction

The purpose of this document is to give an overview of the Apache web
server and related projects. Apache is the most popular server on the
Internet <http://www.netcraft.com/survey/>. New Apache users,
specially those coming from a Windows background, are often unaware of
the possibilities of Apache, useful addons and how everything works
together. This document aims to show a general picture of such
possibilities with a brief description of each one and pointers for
further information. The information has been gathered from many
sources, including projects' web pages, conference talks, mailing
lists, Apache websites and my own hands-on experience. Full credit is
given to these authors. Without them and their work this document
would not have been possible or necessary.

Disclaimer: I work for Covalent <http://www.covalent.net>. We provide
products and support services for the Apache webserver, and I mention
some of them here, as I do for our competitors and similar open source
projects.

2. Apache

Apache is the leading internet web server, with over 60% market share,
according to the Netcraft survey <http://www.netcraft.com/survey>.
Several key factors have contributed to Apache's success:

� The Apache license <http://www.apache.org/LICENSE.txt>. It is an
open source, BSD-like license that allows for both commercial and
non-commercial usage of Apache.

� Talented community of developers
<http://www.apache.org/contributors/index.html> with a variety of
backgrounds and an open development process based on technical
merits.

� Modular architecture.

� Portable: Apache runs on nearly all flavors of Unix (and Linux),
Windows, BeOs, mainframes...

Many commercial vendors have adopted Apache based solutions for
their products, including Oracle <http://www.oracle.com>, Red Hat
<http://www.redhat.com> and IBM <http://www.ibm.com>. In addition,
Covalent <http://www.covalent.net> provides add-on modules and 24x7
support for Apache.

The following websites use Apache or derivatives. Chances are that if
Apache is good enough for them, it is also good enough for you :)

� Amazon.com <http://www.amazon.com>

� Yahoo! <http://www.yahoo.com>

� W3 Consortium <http://www.w3c.org>

� Financial Times <http://www.ft.com>

� Network solutions <http://www.networksolutions.com>

� MP3.com <http://www.mp3.com>

� Stanford <http://www.stanford.edu>

>From the Apache website <http://www.apache.org>:

The Apache Project is a collaborative software development effort
aimed at creating a robust, commercial-grade, featureful, and freely-
available source code implementation of an HTTP (Web) server.

The Apache project has grown beyond building just a web server into
other critical server side technologies like Java or XML. The Apache
Software Foundation, described in the next section serves as an
umbrella for these projects.

3. Apache Software Foundation

The Apache Software Foundation exists to provide organizational,
legal, and financial support for the Apache open-source software
projects. Formerly known as the Apache Group, the Foundation has been
incorporated as a membership-based, not-for-profit corporation in
order to ensure that the Apache projects continue to exist beyond the
participation of individual volunteers, to enable contributions of
intellectual property and funds on a sound basis, and to provide a
vehicle for limiting legal exposure while participating in open-source
software projects.

Or, as Roy T. Fielding, the chairman of the ASF describes it: The
mission of the Apache Software Foundation is to facilitate and support
collaborative software development projects that use the Apache
methods of collaboration over the Internet to create, maintain, and
extend the infrastructure of the Web and enforce the standards that
define it.

You can learn more about the foundation here
<http://www.apache.org/foundation/>.

4. Developing web applications with Apache

There are several ways of providing content with Apache.

4.1. Static

Apache can serve static content, like HTML files, images, etc. If
this is all you need, Apache is probably right for you. A low end
Pentium running Linux and Apache can easily saturate a 10Mbps line
serving static content. If that is your primary use of Apache, make
sure you also check the ``performance'' section.

4.2. Dynamic content

For many websites, the information changes constantly and pages need
to be updated periodically or generated on the fly. This is what
server side programming is all about: programming languages, tools and
frameworks that help developers query and modify information from
different sources (databases, directory services, customer records,
other websites) and deliver the content to the user.

4.3. CGI scripts

CGI stands for common gateway interface. CGI scripts are external
programs that are called when a user requests a certain page. The CGI
receives information from the web server (forms variable values, type
of browser, IP address of the client, etc) and uses that information
to output a web page for the client.

Pros: Since it is an external program, it can be coded any language
and the same script will also be portable among different web servers.
The CGI protocol is simple, and the return result consists of writing
the response to the standard output. It is a mature technology, and
there are plenty of online and book references and examples.

Cons: Spawning and initializing a process takes time. Since a CGI is
external to the server and an instance has to be launched/destroyed
for every request there is a performance hit. If the process has to
load external libraries or perform a connection to an external
database the delay can be important. Same thing if the number of hits
per second is high. CGIs are stateless and session management has to
be achieved by external means.

Since CGI usually involves heavy text manipulation, scripting
languages are the natural choice. Part of Perl <http://www.perl.com/>
popularity stems from being the CGI programming language of choice.
This is due to its extensive support for string handling and text
processing. There are plenty of freely available CGI scripts and
libraries. A good starting point is: the Open Directory CGI section
<http://dmoz.org/Computers/Programming/Internet/CGI/>

4.4. Site generators

If your site is high volume, you may run into performance problems
when generating content dynamically. Offline content generators are an
alternative. These solutions separate content from presentation. The
HTML generator reads both sources and outputs the static files that
build the website. The generator can be run periodically or triggered
by content changes.

Future versions of ``Cocoon'' plan on having a batch mode to
accomplish this. Another option is the Web site meta language
<http://www.engelschall.com/sw/wml/>.

4.5. Out of process servers

The web server can pass dynamic requests to another program. This
program sits idle until a request comes. The request is processed and
returned to the webserver which in turn returns it to the client. This
eliminates the overhead associated with CGI scripts. Examples of this
approach are ``Fast CGI'', ``Java servlets'', etc.

4.6. Fast CGI

This standard was developed to address some shortcomings of the CGI
protocol. The main improvement is that a single spawned process can
process more than one request. There is an Apache module that
implements the Fast CGI protocol and libraries for Tcl, Perl etc. More
information at

4.7. Java servlets

An external Java virtual machine processes requests. The JVM can
reside in the same computer or in a different one. This is how a lot
of application servers work. Usually standard libraries are included
for server side processing. You want to check ``JServ'' and
``Tomcat''. Related Java application server projects can be found
``here''

4.8. Embeded interpreters

An alternative to out-of-process webservers is to embed the
interpreter in the server itself. There are roughly two categories in
this kind of modules: Modules that answer or modify requests directly
and modules aimed to process commands embeded in HTML pages before
serving it to the client. The most representative approaches are
``mod_perl'' and ``PHP''

5. Performance and bandwidth management

Raw performance is only one of the factors to consider in a web server
(flexibility and stability come usually first).

Having said that, there are solutions to improve performance on heavy
loaded webservers serving static content. If you are in the hosting
business Apache also provides ways in which you can measure and
control bandwidth usage. Throttling in this context usually means
slowing down the delivery of content based on the file requested, a
specific client IP address, etc. This is done to prevent abuse.

� mod_mmap: Included in current Apache releases, it maps to memory a
statically configured list of frequently requested but not changed
files.

� Mod_bandwidth <http://www.cohprog.com/mod_bandwidth.html>: Enables
the setting of server-wide or per connection bandwidth limits,
based on the specific directory, size of files and remote
IP/domain.

� Bandwidth share module
<http://www.topology.org/src/bwshare/README.html>: provides
bandwidth throttling and balancing by client IP address. It is
actively maintained.

� Mod_throttle
<http://www.snert.com/Software/Throttle/index.shtml>:Throttle
bandwidth per virtual host or user.

� Mod_throttle_access <http://www.fremen.org/apache/>: useful if you
are slashdotted <http://everything2.com/index.pl?node_id=13464>.
Allows throttling based on resources (file, directory, etc.)

6. Load balancing

Apache has several modules that allow distribution of requests among
servers, for redundancy, increased availability, etc.

� Reverse proxying + mod_rewrite: There is nothing in Apache that you
can not do with mod_rewrite
<http://www.apache.org/docs/mod/mod_rewrite.html> ... :) This
technique consists of having an Apache front-end server acting as a
proxy for the backend servers. You can find more information here
<http://www.apache.org/docs/misc/rewriteguide.html>

� Mod_redundacy: Takeover web and ip in case of failure. You can find
more information here <http://www.ask-the-guru.com>.

� Mod_backhand: Allows seamless redirection of HTTP requests from one
web server to another. This redirection can be used to target
machines with under-utilized resources, thus providing fine-
grained, per-request load balancing of web requests. More
information at .

7. Secure transactions

There are several solutions that provide secure transactions for
Apache servers. This enables Apache servers to be used for ecommerce
or other scenarios where sensitive information is exchanged (like
credit card numbers).

� Mod_ssl <http://www.modssl.org> and Apache-SSL <http://www.apache-
ssl.org> are open source implementations. They are European based,
unencumbered by RSA patents.

� Red Hat <http://www.redhat.com> offers a secure server derived from
Apache. Red Hat adquired C2Net, makers of StrongHold, another
Secure server derived from Apache.

� Covalent <http://www.covalent.net> sells secure versions of Apache
as well as the RavenSSL module that plugs on existing Apache
installations.

Credit card transactions

Apache specific solutions exist for credit card transactions:

� Cypay <http://www.cypay.com/> credit card module for Apache.
Template based, tax calculations.

� Covalent credator <http://www.covalent.net>, multiple
clearinghouses support, failover operation, PHP, Perl, Java
support.

8. SNMP

SNMP stands for Simple Network Management Protocol. It allows
monitoring and management of network servers, equipment, etc. SNMP
modules for Apache help manage large deployments of web servers,
measure the quality of service offered and integration of Apache in
existing management frameworks.

� Open source Mod SNMP
<http://www.simpleweb.org/software/packages/mod-snmp/> for Apache
1.3.

� Raven SNMP <http://www.covalent.net> provides a commercial SNMP
module, support for the latest SNMPv3 standard, integration with
HP-Openview, Tivoli, etc.

9. Authentication modules

In many situations (subscription services, sensitive information,
private areas), user authentication is required. Apache includes basic
authentication support. Additional authentication modules exist that
connect Apache to existing security frameworks or databases,
including: NT Domain controller, Oracle, mySQL, PostgresSQL, etc.

The LDAP modules are specially interesting, as they allow integration
with company and enterprise wide existing directory services.

You can find these modules at .

10. GUIs for Apache

Apache is configured thru text configuration files. This has
advantages and disadvantages. Management can be done from any computer
that has internet access via ssh <http://www.openssh.com>. Editing a
configuration file by hand implies a learning curve. There are open
source graphical tools that make this task easier:

� Comanche <http://www.comanche.org>: It is crossplatform and runs on
Unix/Linux, Windows and Mac. Check the website for screenshots and
in-depth information. Disclaimer: I am the main author of Comanche,
so remember, there are no bugs, only undocumented features :)

� gui.apache.org <http://gui.apache.org>: GUI interfaces for Apache
project. Programs with various degrees of development.

� Webmin <http://www.webmin.com/webmin/>: It is a nice web based
interface.

11. Writing Apache modules

Apache, like many other successful open source projects has a modular
architecture. This means that to add or modify functionality you do
not need to know the whole code base. Source code access for Apache
means that you can custom build the server with only the modules that
you need and include your owns.

Extending Apache can be done in C or in a variety of other languages
using appropriate modules. These modules expose Apache's internal
functionality to different programming languages like Perl or Tcl.

Writing modules in C: Apache is written in C and so they are the
modules distributed with Apache. The best way to get started writing
Apache modules is to read Doug MacEachern and Lincoln Stein Writing
Apache modules with Perl and C <http://www.modperl.com>. It is a well-
written, easy to read book by two Apache and Perl gurus. The above
link will lead you to the book website, which has some of its chapters
online. If you have not the money to buy the book or cannot borrow it
from a friend, there are other ways. You can read some of the online
tutorials on writing Apache modules: Ken Coar, an Apache Group member,
has a nice tutorial and slides online
<http://web.golux.com/coar/slides/>. An overview of the Apache
architecture can be found here
<http://www.grad.math.uwaterloo.ca/~oadragoi/CS746G/a1/apache_conceptual_arch.html>.
The Apache website has some API notes
<http://www.apache.org/docs/misc/API.html> that can help you get
started. You are also encouraged to browse the source code of the
modules included with Apache. Apache includes a simple one
(mod_example.c) for that purpose.

Writing Apache modules in other languages: There is a variety of
Apache modules that enable third party languages to access the
internal Apache API. The most popular is ``mod_perl''.

If you have any questions about the development of an Apache module
you should join the Apache modules mailing list at . Remember to do
your homework first, research past messages and check all the
documentation previously described. Chances are somebody had the same
problem that you are experiencing and he got an useful response.

If you are interested in the development of core Apache itself, you
should checkout the Apache development site <http://dev.apache.org>.

12. Apache books

A comprehensive list of Apache books can be found
athttp://www.apache.org/info/apache_books.html <here>.

A couple of books that I personally recommend are:

� Writing Apache Modules with Perl and C <http://www.modperl.com> if
you are interested in Apache internals.

� Apache server for dummies <http://apache-server.com/> if you want
to get started with Apache. Do not get fooled by the name. This is
a comprehensive book packed with useful information.

13. Java projects

For historical reasons, Java projects can be found both under the
java.apache.org and jakarta.apache.org umbrellas. The final goal is
that over time all Java pojects will move under the Jakarta umbrella.

The goal of the Jakarta Project is to provide commercial-quality
server solutions based on the Java Platform that are developed in an
open and cooperative fashion.

The Java on Apache community is a very dynamic and active one, as
shows the quantity and quality of its subprojects, which are described
now.

13.1. Ant

You can think of Ant as the Java equivalent of make. It is a big
success with Java related projects. Developers can write Java instead
of shell commands. This means increased portability and extensibility.
Instead of Makefiles Ant has XML files. You can learn more about ANT
here <http://jakarta.apache.org/ant/index.html>.

13.2. ORO and Regexp

ORO is a complete package that provides regular experession support
for Java. It includes Perl5 regular expression support, glob
expressions, etc. All under the Apache license. You can learn more
about ORO here <http://jakarta.apache.org/oro/index.html>. You can
find another lightweight regular expression package, Regexp
<http://jakarta.apache.org/regexp/>.

13.3. Slide

Slide is a high-level content management framework. Conceptually, it
provides a hierarchical organization of binary content which can be
stored into arbitrary, heterogenous, distributed data stores. In
addition, Slide integrates security, locking and versioning services.

If you are familiar with WedDAV <http://www.webdav.org>, Slide uses it
extensively. In simple words, what Slides provides is an unified,
simple way to access resources and information. These resources can be
stored in a database, the filesystem, etc. and accessed either thru a
WebDAV interface or Slide own API.

You can learn more at the Slide home page
<http://jakarta.apache.org/slide/index.html>.

13.4. Struts

Struts is an Apache project that tries to bring the Model-View-
Controller (MVC) design paradigm to web development. It builds on
Servlet <http://java.sun.com/products/servlet> and JavaServer Pages
<http://java.sun.com/products/jsp> technologies. The model part are
the Java server objects, which represent the internal estate of the
application. Enterprise Java Beans are commonly used here. The view
part is constructed via JavaServer Pages (JSP) which are a combination
of static HTML/XML and Java. JSPs also allow the developer to define
its own tags. The controller part are servlets, which take requests
(GET/POST) from the client, perform actions on the model and update
the view by providing the appropriate JSP. You can learn more at the
Struts project pages <http://jakarta.apache.org/struts/index.html>.

13.5. Taglibs

The JavaServer pages technology allows developers to provide
functionality by adding custom tags. The Taglibs project intends to be
a common repository for these extensions. It includes tags for common
utilities (for, date), SQL database access, etc.

You can learn about TagLibs here
<http://jakarta.apache.org/taglibs/index.html>. More documentation is
included in the package.

13.6. Tomcat

Tomcat is the flagship product of the Jakarta project. It is the
official reference implementation for the Java Servlet 2.2 and
JavaServer Pages 1.1 technologies.

You can learn more in the Tomcat homepage
<http://jakarta.apache.org/tomcat/index.html>. The Tomcat project was
started with a code donation from Sun Microsystems.

13.7. Velocity

Velocity is a Java based template engine. It can be used as a stand-
alone utility for generating source code, HTML, reports, or it can be
combined with other systems to provide template services. Velocity
has a Model View Controller paradigm that enforces separation of Java
code and the HTML template.

You can learn more about Velocity here
<http://jakarta.apache.org/velocity/index.html>. Velocity is part of
other projects like ``Turbine''
13.8. Watchdog

The Watchdog project provides the validation tests for the Servlet and
JavaServer Pages specifications. You can find more information here
<http://jakarta.apache.org/watchdog/index.html>

13.9. JServ

Apache JServ is a 100% pure Java servlet engine fully compliant with
the JavaSoft java Servlet APIs 2,0 specification.(...)The result is a
pure servlet engine that works on any "version 1.1" Java Virtual
Machine.

JServ is one of the original Java Apache projects. ``Tomcat'' will be
the successor of JServ once it is finished. You can learn more at the
JServ home page <http://java.apache.org/jserv/index.html>.

13.10. JSSI

JSSI is an implementation of server side included in the Java
language. Server side includes are tags includes in files that get
processed before the page is served to the client (for example to
include the current date) You can find more information here
<http://java.apache.org/jservssi/index.html>.

13.11. Apache JMeter

The Apache JMeter is a 100% pure Java desktop application designed to
load test functional behavior and measure performance. It was
originally designed for testing Web Applications but has since
expanded to other test functions.

It can be used to test static and dynamic resources and get inmediate
visual feedback.

You can see some screenshots and learn more here
<http://java.apache.org/jmeter/index.html>.

13.12. Server Pages Foundation Classes

Is a set of libraries to help solve common problems in server side
application development. They focus on two of them:

� Mixing HTML and Java: Provides a library of classes that takes care
of the HTML generation and that can be integrated with the rest of
the Java code.

� HTTP is a stateless protocol: SPFC provides session support, so
applications can keep track of users as they navigate the website.
The application developer does not need to worry about the specific
details of page generation. He can think in more general
traditional application terms. You can learn more about SPFC here
<http://java.apache.org/spfc/index.html>

13.13. mod_java

Is the Java equivalent of mod_perl. Allows access to the Apache
internals from inside a JVM. This allows for increased flexibility and
the possibility of writing Apache Modules directly in Java.
Unfortunately, no code seems to be present at the moment. You can find
more information here <http://java.apache.org/mod_java/index.html>.

13.14. Element Construction Set

Element Construction Set (ECS) is a JAVA API or generating elements
for various markup languages it directly supports HTML 4.0 and XML,
but can easily be extended to create tags for any markup language.

It allows the generation of mark up tags using Java function calls,
leading to a much cleaner solution that mixing HTML and Java code.
You can learn more at the ECS project page
<http://java.apache.org/ecs/index.html>.

13.15. Avalon

If you are familiar with Perl or BSD systems, Avalon is roughly the
equivalent of CPAN <http://www.cpan.org> or the Ports collection for
Java Apache technologies. It does not only provide guidelines for a
common repository of code, it goes one step further: is an effort to
create, design, develop and maintain a common framework for server
applications written using the Java language. It provides the means so
server side Java projects can be easily integrated and build on each
other.

13.16. JAMES (Java Apache Mail Enterprise Server)

Complementary to the other Apache server side technologies, JAMES
provides a 100% pure Java server designed to be a complete and
portable enterprise mail engine solution based on currently available
open protocols (SMTP, POP3, IMAP, HTTP)

More information can be found here
<http://java.apache.org/james/index.html>.

13.17. PicoServer

A lightweight HTTP/1.0 server in pure Java. The project seems to be
stalled and no code is available. The website can be found here
<http://java.apache.org/picoserver/index.html>.

13.18. Jetspeed

Jetspeed <http://java.apache.org/jetspeed/site/overview.html> is a web
based portal written in Java. It has a modular API that allows
aggregation of differnt data sources (XML, SMTP, iCalendar)

13.19. Turbine

Turbine is a servlet based framework that allows experienced Java
developers to quickly build secure web applications. Turbine brings
together a platform for running Java code and reusable components,
everything under the Apache license. Some of it features

� Integration with template systems

� MVC style development

� Access Control Lists

� Localization support

� etc.

If you are interested, you can visit the Turbine web site
<http://java.apache.org/turbine/features.html>.

13.20. Jyve

The Jyve project <http://java.apache.org/jyve/index.html> is built on
top of the Turbine framework. It is an application that provides a web
based FAQ system

13.21. Alexandria

Alexandria is an integrated documentation management system. It brings
together technologies common to many open source projects like CVS and
JavaDoc. The goal is to integrate source code and documentation to
encourage code documentation and sharing. More information here
<http://java.apache.org/alexandria/index.html>

14. XML projects

Directly from the Apache XML project website, its goals are:

� To provide commercial-quality standards-based XML solutions that
are developed in an open and cooperative fashion.

� To provide feedback to standards bodies (such as IETF and W3C) from
an implementation perspective.

� To be a focus for XML-related activities within Apache projects

The project homepage is located at . It is an umbrella for a variety
of subprojects.

14.1. Introduction to XML

This is a quick introduction to XML. To know more about XML, a good
starting point is . XML is a markup language (think HTML) for
describing structured content using tags and attributes. Once content
is separated from presentation, you can choose how to display
(cellphone, html, text) or exchange it. The XML standard only
describes how the tags and attributes can be arranged, not its names
of what they mean. Apache provides the tools described in the
following sections.

14.2. Xerces

The Xerces project provides XML parsers for a variety of languages,
including Java, C++ and Perl. The Perl bindings are based on the C++
sources. There are Tcl bindings for Xerces in the 2.0 version of
TclXML <http://www.zveno.com/>, by Steve Ball. This 2.0 version is
only available at the moment thru Ajuba CVS repository
<http://dev.ajubasolutions.com/software/tcltk/netcvs.html>. A XML
parser is a tool used for programatic access to XML documents. This
is a description of the standards supported by Xerces:

� DOM <http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/level-one-
core.html>: DOM stands for Document Object Model. XML documents are
hierarchical by nature (nested tags). XML documents can be accessed
thru a tree like interface. The process is as follow:

� Parse document

� Build tree

� add/delete/modify nodes

� Serialize tree

� SAX <http://www.megginson.com/SAX/index.html>:Simple API for XML.
This is a stream based API. This means that we will receive
callbacks as elements are encountered. These callbacks can be used
to construct a DOM tree for example.

� XML Namespaces <http://www.w3.org/TR/REC-xml-names/>

� XML Schema: The XML standard provides the syntax for writing
documents. XML Schema provides the tools for defining the contents
of the XML document (semantics). It allows to define that a certain
element in the document must be an integer between 10 and 20, etc.

The Xerces XML project initial code base was donated by IBM. You
can find more information in the Xerces Java
<http://xml.apache.org/xerces-j/index.html>, Xerces C
<http://xml.apache.org/xerces-c/index.html> and Xerces Perl
<http://xml.apache.org/xerces-p/index.html> homepages.

14.3. Xalan

Xalan is an XSLT processor available for Java and C++. XSL is a style
sheet language for XML. The T is for Transformation. XML is good at
storing structured data (information). We sometimes need to display
this data to the user or apply some other transformation. Xalan takes
the original XML document, reads transformation configuration
(stylesheet) and outputs HTML, plain text or another XML document.
You can learn more about Xalan at the Xalan Java
<http://xml.apache.org/xalan/index.html> and Xalan C
<http://xml.apache.org/xalan-c/index.html> project homepages.

14.4. FOP

From the website FOP is a Java application that reads a formatting
object tree and then turns it into a PDF document. So FOP takes an XML
document and outputs PDF, in a similar way that Xalan does with HTML
or text. You can learn more about FOP here
<http://xml.apache.org/fop>.

14.5. Cocoon

Cocoon leverages other Apache XML technologies like Xerces, Xalan and
FOP to provide a comprehensive publishing framework. Cocoon is based
around XML and XSL and targeted to sites of medium - high complexity.
It separates content, logic and presentation as described in the
website:

� XML creation: the XML file is created by the content owners. They
do not require specific knowledge on how the XML content is further
processed rather than the particular chosen DTD/namespace. This
layer is always performed by humans directly through normal text
editors or XML-aware tools/editors.

� XML process generators: the logic is separated from the content
file.

� XSL rendering: The created document is then rendered by applying an
XSL stylesheet to it and formatting it to the specified resource
type (HTML, PDF, XML, WML, XHTML)

You can learn more about Cocoon at the project homepage
<http://xml.apache.org/cocoon/index.html>

14.6. Xang

The goal of the Xang project is make it easy for developers to build
commercial quality XML aware applications for the Web. The application
logic is defined in a hierarchical XML file which can be scripted via
JavaScript. This file defines how to access the data (which can be
other XML files, Java plug-ins, etc.). The Xang engine takes care of
mapping HTTP requests to the appropriate handlers. You can learn more
about Xang at the project homepage
<http://xml.apache.org/xang/samples.html>.

14.7. SOAP

Apache SOAP ("Simple Object Access Protocol") is an implementation of
the SOAP submission <http://www.w3.org/TR/SOAP> to W3C. It is based
on, and supersedes, the IBM SOAP4J implementation.

From the draft W3C specification: SOAP is a lightweight protocol for
exchange of information in a decentralized, distributed environment.
It is an XML based protocol that consists of three parts:

� An envelope that defines a framework for describing what is in a
message and how to process it,

� a set of encoding rules for expressing instances of application-
defined datatypes,

� and a convention for representing remote procedure calls and
responses.

Think of SOAP as an XML based remote procedure call or CORBA
system. It is based on HTTP and XML. In one hand this means it is
verbose and slow compared to other systems. On the other hand it
eases interoperatibility, debugging and development of clients and
servers for a variety of languages (C, Java, , Perl, Python, Tcl,
etc.) since most modern languages have HTTP and XML modules. You
can learn more at the Apache SOAP homepage
<http://xml.apache.org/soap/>

14.8. Other XML projects

There are other projects based on Apache and XML that do not live
under the Apache XML umbrella

� mod_xslt <http://modxslt.userworld.com/>. It is a C based module
for delivering XML/XSL based content. It has a GPL license.

� AxKit <http://axkit.org> is an XML based Application Server for
mod_perl and Apache. It allows separation of content and
presentation.

15. Perl

Perl and Apache are a powerful and popular combination. There are
several projects that use these two technologies.

15.1. Embperl

Allows embedding of Perl in HTML pages. These pages are processed in
the server before they are delivered to the client. It is similar to
``PHP''. You can learn more here
<http://perl.apache.org/embperl/index.html>.

15.2. Mason

The Mason project <http://www.masonhq.com/> embeds Perl in HTML with a
reusable component model approach. It allows caching, templating, etc.

15.3. Mod_Perl

Mod_perl is one of the most veteran and successful Apache projects. It
embeds a Perl interpreter in Apache and allows access to the web
server internals from Perl. This allows for entire modules to be
written in Perl or a mixture of Perl and C code. In the 1.3 Apache
versions, one interpreter has to be embedded in each child, since the
server is multiprocess based. In heavy traffic dynamic sites, the
increased size could make a difference. Apache 2.0 is multithreaded,
as recent versions of Perl are. The next generation of mod_perl takes
advantage of this and allows for sharing of code, data and session
state among interpreters. This results in a faster, leaner solution.

Make sure you check also ``Axkit''

16. PHP

From the PHP website <http://www.php.net> website: PHP is a server-
side, cross-platform, HTML embedded scripting language. PHP is a
scripting language like Perl, Python or Tcl. It is the most popular
module for Apache
<http://www.securityspace.com/s_survey/data/man.200008/apachemods.html>
and this is due to a variety of reasons:

� Learning curve is quite low

� Great documentation

� Extensive database support

� Modularity

PHP has a modular design. There are modules that provide support
for:

� Database connetivity for Oracle, ODBC, mySQL, mSQL, PostgreSQL, MS-
SQL server... and many more, check the PHP website
<http://www.php.net>.

� XML support

� File transfer: FTP

� HTTP

� Directory support: LDAP

� Mail support: IMAP, POP3, NNTP

� PDF document generation

� CORBA

and many more. You only need to compile/use the modules you need.

PHP can be used with Apache, as an external CGI or with other
webservers. It is crossplatform and it runs on most varieties of Unix
and Windows.

If you come from a Windows background, you probably have used Internet
Information Server with Active Server Pages and MS-SQL Server. A
common replacement in the Unix world for this trio is Apache with PHP
and mySQL. Since PHP works:

� with Apache and with Microsoft IIS

� with mySQL and with MS-SQL server

� on Unix an on Windows

you have a nice migration path from a Microsoft-centric solution to
more secure, stable, high performance Unix based solutions (like
FreeBSD <http://www.freebsd.org>, Solaris <http://www.sun.com>,
Linux <http://www.linux.com> or OpenBSD <http://www.openbsd.com>)

17. Python

Python is an scripting language similar to Perl or Tcl. Several
modules embed Python in the Apache web server:

� Mod Python <http://www.modpython.org>

� Mod Snake <http://modsnake.sourceforge.net>: runs both in Apache
1.3.x and the upcoming 2.0

Both modules would be useful if you plan on writing Apache modules
in Python or run existing Python CGIs faster. Mod Snake allows to
embed Python in HTML , much like ``PHP'' does.

18. Tcl

The Tcl Apache project <http://tcl.apache.org> integrates Tcl with the
Apache webserver, like Mod_dtcl. Mod_Dtcl allows for embedding Tcl on
HTML pages like ``PHP'' does. Tcl is a lightweight, extensible
scripting language. You can learn more about Tcl here
<http://dev.ajubasolutions.com/>. Other Tcl based Apache solutions
are Neo Web Script <http://www.neosoft.com/neowebscript/> and WebSH
<http://websh.com/>

19. Modules for other languages

This document have described modules for popular server side languages
such as Perl, Python, PHP. You can find additional language modules
(JavaScript, Haskell, etc.) at the Apache modules directory
<http://modules.apache.org>.

20. Apache 2.0

The current version of Apache (the 1.3 series) is process based. That
means that the server forks itself a number of times to answer
simultaneous requests. The children are isolated from each other.
This is reliable: if a module misbehaves, the parent process kills
that child and it only affects the request being served, not the
server as a whole. Threads are similar to lightweight processes.
Threads can share common data. If a thread misbehaves it can corrupt
other threads and the server as a whole can go down. On the other
hand, the thread model allows for faster, leaner webservers. Apache
2.0 brings the best of both worlds, allowing the user to define number
of processes and number of threads per process. Apache 2.0 introduces
APR, the Apache Portable Runtime, which increases even more Apache's
portability. Finally, layered I/O brings a new level of modularity to
Apache development.

21. Migrating from Netscape (iPlanet) web servers

The bulk of the work may reside in converting custom modules from
NSAPI to the Apache API. Nearly all the other server side
technologies (Java, Perl, CGIs) should be portable with little or no
change. Netscape is tightly integrated with LDAP servers. You may be
also interested in LDAP modules in . Netscape includes server side
JavaScript support, you can check the Apache equivalent,
mod_javascript
<http://www.geocities.com/TimesSquare/Fortress/9743/binjs.html>.

22. Migrating from Microsoft IIS

Common reasons why people migrate from IIS to Apache (and not the
other way around) include stability, performance and security. This is
partly because most people running Apache do it on an Unix variant
(like Solaris, FreeBSD or Linux). Fortunately, Apache is multiplatform
and runs on both Unix and Windows, offering a sensible migration path.

Common Windows based web development environments like Coldfusion or
Active Server Pages have Unix ports or compatible environments (some
are commmercial, some are freely available):

� Coldfusion for Linux <http://www.allaire.com/Products/coldfusion/>

� Perl ASP module <http://www.apache-asp.org/>

� Halcyon ASP <http://www.halcyonsoft.com/>

� OpenASP <http://www.activescripting.org/>

Apache for Windows supports also the ISAPI interface.

If you want to go for a complete open source solution and you come
from a Windows background ( IIS + ASP + MS-SQL server) the roughly
equivalent (and highly popular) combination is Apache + PHP + MySQl
<http://www.mysql.com> or PostgresSQL <http://www.postgresql.org>.
You can learn more about PHP ``here''

Support for Windows is greatly improved in the new 2.0 Apache version,
still in alpha stage at the time of this writing.

23. Links

Additional Apache related resources

23.1. Websites

� Apache <http://www.apache.org>

� Apache modules directory <http://modules.apache.org>

� Apache today <http://www.apachetoday.com>

� Slashdot Apache section
<http://slashdot.org/index.pl?section=apache>

23.2. Java application servers

These are open source application servers that build on or are known
to play well with Apache.

� Resin <http://www.caucho.com/>: Servlets, JSP, XSL

� Enhydra <http://www.enhydra.com>: Java/XML application server.

� Locomotive <http://www.locomotive.or>: Servlets, load balancing,
failover.

24. Contacting the author

You can contact me at . I welcome suggestions and corrections, but
please, please, do not send me messages asking me to troubleshoot your
Apache installation. I just do not have the bandwidth and your mail
will be most likely ignored. If you need support, consider:

� Check the error logs, read the docs, specially the FAQ
<http://www.apache.org/docs/misc/FAQ.html>.

� If you still do not find the solution, go for a walk. Afterwards
read the docs, again.

� Try comp.infosystems.www.servers.unix at . Search for a similar
problem.

� If you are still stuck. Provide as much information as you can,
relevant error_log entries and steps you have taken so far and post
to that newsgroup. This will increase the chances someone will
answer your question

If you want commercial support, consider contacting Covalent
<http://www.covalent.net>, which provides expert support for Apache
(at a fee, of course). If you are using Apache on Linux, your Linux
vendor may have support plans that include Apache too.

24.1. Translations

I encourage translations of this document. You probably should use the
SGML source. Check for info. Please drop me a note so I can make
sure you get the most recent version