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From Jeremy Boynes <jboy...@apache.org>
Subject Web container integration strategies
Date Thu, 02 Mar 2006 07:58:27 GMT
Rick asked me on IRC what I meant about different approaches to web
container integration, especially when it came to exposing entry points
as web services. I'd said I'd post it to the list.

There are two ways we can run as part of a web application:
1) Using pure J2EE APIs and running as a portable web application
2) Running as SCA module within an SCA aware web container

== Portable Web Application ==
In this mode we want to run as a self-contained application in a stock
web container - all we can rely on are the J2EE APIs and that we have
been granted sufficient privileges to run.

In this mode all classes need to be bundled with or made available to
the web application - all JAR files will be included in WEB-INF/lib or
WEB-INF/classes, made available as references to JARs in a containing
EAR file, or added to the application classpath using some
container-specific mechanism.

Application isolation will make it difficult/impossible for different
web applications to share information so each web application will be a
standlone unit, either a subsystem with all module components present in
the application or a single module component.

The runtime can be booted on application startup from
ServletContextListener. This listener will boot the runtime, load the
system definition, and load the application modules. It can bind the
system or module definitions as attributes in the ServletContext.

Each request needs to be associated with the module context for the
invoked servlet (in the subsystem case different servlets could map to
different module components). This can be done by a Filter that binds
the appropriate ModuleContext to the Thread for use by
CurrentModuleContext.getContext().

Each web-service entry point must be bound so a web-service endpoint,
which if we are using the Axis transport, done by configuring the Axis
servlet. The configuration of the Axis engine can be built during the
bootstrap above and passed to the servlet as a ServletContext attribute.

Finally, session expiration needs to be detected and a notification sent
to the runtime. This can be done using a HttpSessionListener and may be
combinable with the ServletContextListener above.

All of this must be configured by the user in their web.xml file - a
little configuration here being the price for portability across
container. Specifically, the following entries must be added:
* A <listener> configured to boot the runtime
* A <filter> configured for the request filter
* A <filter-mapping> for each path associated with a module
* A <servlet> for the axis servlet
* A <servlet-mapping> for the axis servlet
* A <listener> for session expiration (if not combined)

In this configuration, RuntimeContext is a non-priviledged API (it runs
with the permissions of application) but although it is exposed for use
by these infrastructure artifacts there is still no need for the
application to use it. We can't stop application code from doing so but
this is not harmful as everything is running with application-level
permissions.



== SCA Aware Container ==
To avoid the need for users to explicitly configure their web
applications to enable SCA, we may be able to integrate the runtime
directly with the web container. In this mode, the runtime would run as
a privileged extension to the container and would automatically
configure the SCA environment for a deployed web application. We have
started to do this with Apache Tomcat.

All classes for the runtime would be placed in the appropriate location
for the container (e.g. Tomcat's server/lib) and would not be exposed to
the application. The only access the application would have would be via
the standard SCA APIs.

The runtime would bootstrap with the container, before any applications
were deployed. How this is configured is container-specific - with the
current Tomcat integration we have a specialized implementation of Host
that is activated by specifying the appropriate class name in server.xml

When an application is deployed, the container extension detects this,
inspects the application to see if it requires SCA and if so configures
the container as necessary. The user only place the sca.module file in
the application for this to happen - there are no SCA-specific entires
in web.xml

The container will need to configure the same sort of entry-points as
described above but may do so in container-specific ways. For example,
the Tomcat integration adds a Valve to the processing pipeline instead
of a Filter as a Valve has access to internals that a Filter does not
(for example, it is able to attach notes to the request).

For web-services, the extension code needs to examine the SCA module and
make sure all the entry-points are bound. One way it could do this is to
configure a single Axis servlet with a engine configuration that maps
each request to the appropriate entry-point; another is to define a
separate servlet (or internal equivalent e.g. in Tomcat's case a custom
Wrapper) for each entry-point that is attached directly to the entry
point's context in the runtime.

In this type of configuration, the RuntimeContext is a privileged API
which should not be (and does not need to be) exposed to the application.



== Where we are ==
Up until now we have had a hybrid solution where all implementation
classes run with container priviledge but are fully exposed to and
shared by every application. This has some serious issues.

Supporting the first mode (portable web app) requires documentation for
the application assembler and the implementation of a couple of
listeners that are very close to the current implementations. These are
not Tomcat specific (by definition) and can be placed either in core or
in a new webapp module.

We have the bones of a Tomcat container extension that covers SCA
modules but which does not support the auto-configuration of
web-services. This should be quick to implement as part of the Axis2
work. It also does not support sessions yet (see TUSCANY-51).

--
Jeremy

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