[xwiki-devs] Choosing the default component implementation [take 2]

Hi devs, Hmm, time flies, I realized that it's been a month and a half since I started this thread, but didn't get to counter-argument so far. Because I wanted to give more compelling reasons after I wrote the initial draft of the original proposal, I got slammed with counter arguments centering mostly on why our current implementation already allows to work around the technical difficulties that I presented, although those technical difficulties were just extra arguments that weren't actually that important. Let's start from scratch. I believe that using "default" as a component hint is in most cases wrong, since it doesn't actually say anything about the particular implementation. Let's take some examples: EntityReferenceSerializer, with its many implementations: - "compact", which creates compact representations that don't repeat the parts that are the same as the current document - "compactwiki" that only skips the wiki part when not needed - "path" which creates references usable as a path on the filesystem or in URLs - "uid" which creates non-ambiguous representations - and "default", which does a... default representation... whatever that means... So, while the meaning of the other hints can be guessed from their name, that's not true for "default". WikiModel, with its only implementation, XWikiWikiModel, labeled as "default". First, this "default" is defined in xwiki-platform instead of xwiki-rendering, like the components that actually use that component, which means that we're defining a component interface without any implementation in our "standalone rendering engine", and thus we're using "exceptions as normal decision code" which is wrong. Second, this "default" actually means "xwiki default". Why can't the hint be "xwiki" instead of "default", since that what that implementation is actually doing: "this is the model used in XWiki", and not "this is the default model that most wiki engines use". When looking up an instance of the WikiModel component, we don't request "the default wiki model", but "the wiki model currently in use, whichever that is". ConfigurationSource and its implementations: - "space" which looks into space preferences - "wiki" which looks into wiki preferences - "user" which looks at user preferences - "xwikiproperties" which looks in xwiki.properties - "all" which looks into all the above - "void" which is always empty - "memory" which stores settings explicitly set by code - and "default" which does... stuff... let me get back at you after I look into its code to check what it actually does. XHTMLLinkTypeRenderer is an example where "default" does make sense, since we have special treatment for "doc"ument, "attach"ment, "mailto", "unc", "interwiki", and then there's a "default" that handles all the others, like "url" and "path". I was writing a component called UploadedFileManager, which is supposed to parse uploaded files from a request, and the implementation for it was called CommonsFileUploadManager, since it used the Apache commons-fileupload library for the actual request parsing. I don't think that calling it "default" is appropriate, since in the 3.0 servlet specification the upload behavior is embedded in the specification, in the ServletRequest interface, and that seems more "default" than using a particular library to do the job. A more appropriate hint is "commons", since that's what it actually does: "this implementation handles file uploads by using the apache commons library that does that", and not "this implementation handles file upload in the default way, which everyone should know what it actually is". So, I strongly feel that what the component manager returns when looking up a component without a hint shouldn't be the implementation labeled "default", but one of the existing implementations, as configured somehow. I'm not yet sure how that configuration takes place, that would be the subject of another discussion, but I'd like to get a consensus on whether we need this change or not. -- Sergiu Dumitriu http://purl.org/net/sergiu/