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WebDAV and Dynamic Content

From: <jamsden@us.ibm.com>
Date: Tue, 16 Nov 1999 16:14:46 -0500
To: w3c-dist-auth@w3.org
Message-ID: <8525682E.006649D2.00@d54mta03.raleigh.ibm.com>

Although WebDAV is silent about dynamic content, it is clear that it will
be used to author the source of such content in web application
development. In the spirit of incremental development, content authors will
want to test and debug their dynamic resource in a tight edit, execute,
debug cycle. This implies that extra publish steps required to make
authored resources available to the server runtime, and running multiple
servers for authoring and testing are less than optimal.

Even though specific support for dynamic content is currently out of scope
for WebDAV, we should explore the issues and options to be sure there are
no protocol consequences or opportunities that should be addressed now.
This note is the beginning of a thread to do that.

Dynamic content presents two fundamental problems for a WebDAV server:
1. How does the server distinguish between access to source and access to
processed results
2. How are resources made available to server-side extensions that access
them through the file system
We'll explore each of these issues from the point of view of the client,
and both non-versioning and versioning servers.

Accessing the Source of Dynamic Resources

For many dynamic resources, the dynamic content and the source resources
are different MIME types and have different URLs. For example Java source
resources end in .java while binary executable resources end in .class.
These resources present no problem because the client can do a GET on the
source URL to get the source, and a GET on the separate executable URL to
get the processed results. The DAV:src property on the executable can be
used to get the source resource URL.

For other resources, the dynamic content and the source have the SAME URL.
For example, Java Server Pages, Active Server Pages, server-side includes,
server-side scripting, etc. This presents a problem because there's only
one method to get both, GET. The DAV:src property doesn't help because it
has the same URL as the executable. And, there's no header on GET that can
be used to distinguish source from dynamic results. We could use the Accept
header to specify what MIME type is desired, and use that to distinguish
source from dynamic results, but that might now work either as they could
have the same MIME type, and the dynamic resource might support multiple
MIME types.

The FrontPage extensions resolve this conflict by either using a method
extension to get the source, or a new header. Could someone supply the
details? They might be a useful model for how we should extend WebDAV.

The current WebDAV doesn't provide any interoperable way to resolve this
issue. However, advanced collections and binding does. Here is a list of
possible solutions (assuming advanced collections), and their pros and

1. Configure the WebDAV server to not process any dynamic content. Require
the document author to publish the modified source to another server that
is configured to process dynamic content, and not return source (might be
another WebDAV server). The publish operation could be done with a cross
server COPY. The web application developer uses two different URLs to two
different servers to distinguish the source resource from the dynamic
resource. A variant of this approach would be to use virtual hosts in a
single server, but this is more server dependent.
+ works with existing WebDAV servers, doesn't even need advanced
+ also solves the second issue, how the runtime gets access to WebDAV
resources through the file system
- creates redundant resources consuming both developer and CPU time and
disk space
- results in the potential for stale copies of runtime resources
introducing difficult to find bugs
- requires an extra publish step after every update that's easy to forget
- requires the administration of an extra Web server
- user has to remember two URLs, one for the source and one for the dynamic
results. However, they could differ only in the server name or port.

2. For every dynamic resource, create a binding from the dynamic resource
to its source, and configure the server so that the URL for the source
binding does not invoke dynamic content. For example, bind HelloWorld.jsps
to HelloWorld.jsp and configure the server so that resources ending in
.jsps are text/plain while resources ending in .jsp are processed Java
Server Pages.
+ only need one server running
+ no duplicate resources
+ no need to copy, and therefore no possibility of forgetting and getting
stale dynamic resources
+ no publish step, the BIND only has to be done once.
- have to do the BIND for each dynamic resource, but only once
- have to administer the server to not process some URLs (the ones bound to
the dynamic resources for the purpose of accessing their source)
- user has to remember two URLs

3. Extend WebDAV to include a Process-Source Boolean header. GET on a URL
with Process-Source set to 'F' would return the dynamic content while 'T'
would return the source. If the content is not dynamic, the header is
effectively ignored as both 'T' and 'F' would have the same effect.
+ only need one server
+ no duplicate resources
+ no copy and no stale dynamic resources
+ no BIND step or publish
+ no administration required to distinguish source and dynamic content URLs
+ user only has to remember one URL
- requires an extension/change to WebDAV
- introduces another header

4. Add a GETSOURCE method.
+ no new header required
+ user only has to remember one URL
- another method that usually does the same thing as GET

Allowing dynamic resource to access resources using the file system

Most dynamic resources, i.e., servlets, Java Server Pages, Active Server
Pages, server-side scripting in HTML, CGI-BIN applications, etc. that are
invoked by a Web server assume Web resources managed by the server are
stored in the file system, often at specific locations relative to their
own file system path. This is very unfortunate coupling between the
server-side extensions, and the Web server implementation, but it is
standard practice. For many WebDAV servers, and almost certainly versioning
WebDAV servers, the resources are not stored in the file system, but rather
are stored in some private repository. For the authoring environment, this
is no problem because the authoring client just does GETs and PUTs and
isn't aware of the server's implementation. However, any dynamic content
processed by the Web server is free to open files on the server as it
wishes, and if those files are Web resources authored through WebDAV, they
might not be visible to the dynamic resource and its runtime environment.
Ideally, the dynamic resource would reference other resources through a URL
so it could access them through WebDAV with versioning support and proper
access controls. But in cases where they just open files, we have to have
some other solution.

1. Require the user to publish from the WebDAV store to the server runtime.
+ it works
- introduces redundancy with associated system resource consumption and
update anomalies.
- requires a separate publish step for every update before it can be
tested. Likely to cause lots of bugs

2. Mirror the resources in the file system.
+ makes them accessible to dynamic resources
- introduces redundancy with associated system resource consumption and
update anomalies.
- what version of a versioned resource would be mirrored? Some published
- requires users to do a publish step to control when the server cache is
updated or requires servers to keep them up to date.

3. Build a file system device driver on top of WebDAV so that applications
that access the file system will transparently access WebDAV resources.
+ eliminates the redundancy
+ eliminates the publish step
- has performance implications
- introduces OS dependencies, there's no standard file system access API.
- file system APIs may not be capable of specifying enough information or
parameters to support versioning, properties, locking, etc.
Received on Friday, 19 November 1999 13:37:37 GMT

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