Web Commander

Web Commander Solves the Lost Update Problem

The Lost Update Problem has been present in most distributed authoring sessions using rudimentary HTTP/1.0 features. the Web Commander can now demonstrate how to avoid this problem based on HTTP/1.1 features including persistent cache, strong etags, if-match header fields and HEAD requests.

A couple of notes:

• Web Commander uses the normal libwww Request API without modifications
• As the requests contain the TE header field indicating that the client understands deflate, the server sends back deflated and chunked responses
• Web Commander only detects conflicts - it doesn't try to resolve them

Saving a Document for the First Time

If the user wants to save a document that we don't know anything about then we have to issue a HEAD request before doing the save using a PUT request. The HEAD request is necessary as we don't know whether we are speaking to an HTTP/1.0 or an HTTP/1.1 server - only the latter understands etags. Note, that the PUT request uses the "If-None-Match: *" header field to ensure that no race condition occurs between the two requests. There are two situations that have to be handled:

The document doesn't already exist

The first trace shows a situation where the document doesn't exist and the PUT request is executed normally

The document already exists

The second trace shows a situation where the document already exists. The user is then given two options in a GUI dialog box: to download the existing version or to override it. In the trace the user overrides the existing version which is done by removing any preconditions.

Saving a Document for the 2nd or more Time

When a new document has been created on the server, it sends back an etag in the 201 (Created) response. This etag is stored in our persistent HTTP/1.1 cache. Once the etag is known, it is used on all subsequent PUT requests in a if-match header field. If the document hasn't changed, the etags will match and the PUT can proceed. However, if it has changed, then the etags will not match and the PUT can not proceed. Instead the user will be given the same two options as above, that is, either to get the new version or to override it.

The document has not been changed

This trace shows a PUT request with a if-match header field. In this situation the etag matches and the PUT request is executed as normal.

The document has been changed

The last trace shows a situation where someone else updated the document and so the etags don't match anymore. THe PUT therefore fails and the user is then presented with the option of downloading the new version of overriding the existing one. In this case, the existing version is simply overridden.

In both cases, the new etag sent back is saved in the persistent cache and the old etag deleted.