Re: [xwiki-dev] [ArchitectureV2] Model proposal

Hi Vincent, hi all, I tried to explain below which advantages I see in reusing JCR concepts as much as possible. Vincent Massol wrote: I agree we need in any case an expressive model that will capture the specific features of XWiki. However, since a significant part of XWiki features are already quite well expressed by the JCR API, we could take into account the well thought and well accepted JCR content manipulation concepts and APIs such as Node, NodeDefinition, NodeType, Property, PropertyDefinition, PropertyType, Workspace, Session, for adopting the following methodology: we could consider the current version of the JCR as a set of services to be supplemented with services that we're missing for building XWiki features, introducing specific concepts only when necessary. In that sense, I agree with Luis for saying that the JCR is a possible "metamodel" for our specific XWiki model. As Ludovic also pointed out earlier in this thread, we should not reduce the JCR to a storage API, since it is is a general API for representing operations on content, and is, as such, an API that models well part of our application behaviour. Note about Session: Session is not part of the data model indeed, but I think it's a handy class to be present at the model level, avoiding to pass the context to each method of the data model classes, as opposed to the current XWiki API. However, the JCR has drawbacks as well, and it is obviously missing important features needed for XWiki: - the JCR is too monolithic: all the services live in the same API, and new services cannot easily be added. Moreover, the API provides several identical functionalities from different places (redundant API example: a node can be moved from different classes: Session and Workspace). - the JCR is lacking a service for advanced management of properties: for instance, it's not possible with the JCR API to get all the back-references to a Node. - the JCR is obviously missing other services that we need for XWiki. Following a discussion with Mikhail, my proposal would be the following: Step 1: we split the JCR into the following services(to be possibly submitted as a JCR evolution proposal later on): - StorageService for atomic types including binaries - ContentMonitoringService - LockService - IndexingService - RepositoryAccessControlService - QueryService - XMLExportService - VersioningService - TransactionService - TypedPropertyService - DirectReferenceService - [...] probably others Step 2: implement those services using an existing JCR implementation, and also possibly using our own implementation, reusing the XWikiHibernateStore and the surrounding classes. Step 3: supplement these content services with the property management services we're missing for dealing with our XClass functionalities, but not introducing any specific XWiki concept at this stage. The advantage of having those services as extensions to the JCR instead of having them at the XWiki level is that the services would remain as generic as possible while providing easy interoperability with the JCR. Complementing the JCR with services providing advanced management of node properties is a topic that is currently of high interest to several communities. Participating in that effort may lead to a sound and standard API, implemented by several vendors, XWiki among others, and may make our community a strategic one among the emerging semantic web content repository initiatives. The needs for these supplementary services are well captured by the idea of a "Semantic Web Content Repository" in the slides below by Max Völkel [1],[2] and are mentioned in a recent email to the jackrabbit-dev list [3]. [1] http://swecr.googlecode.com/files/2007-03-12-SWECR-2.pdf [2] SWECR project: http://code.google.com/p/swecr/ [3] http://www.mail-archive.com/dev@jackrabbit.apache.org/msg05524.html The services we're speaking about are typically the following: - ReferenceService (for manipulating object properties) - SparqlService (if we go for RDF for representing our classes and instances) - IdentityService (ID/URI mapping) - InferenceService They should in particular cover the use cases described by Sergiu in a previous post, which are of great relevance indeed [4]. Cognium Systems has expressed interest for co-developing such services with us, in an Apache License-like. [4] http://www.nabble.com/Re%3A--ArchitectureV2--Model-proposal-p10865097.html). Step 4: specify the specific XWiki services for dealing with structured and unstructured data as complementary services to the SWECR, and design an expressive model describing XWiki specific artifacts on top of the SWECR. Step 5: implement these services on top of an SWCR implementation Step 6: design and implement the other XWiki services (i.e. all those that do not directly relate to content manipulation). As a summary, here are two schematic stacks of models and services for comparing the two approaches we're discussing, as I understand them. Approach 1 stack ================ XWiki services --------------------------- XWiki type model: XClass, XProperty --------------------------- XWiki document model: Document, Space, Attachment, Wiki, Farm --------------------------- XWiki persistence service: Data Access Objects --------------------------- JCR impl.|Relational impl.|... Advantages: - we can quite easily implement several storages (JCR, relational storage) using Data Access Objects - the model is quite close to the existing one - we don't modify the JCR Cons: - the management of types and properties is not generic - people used to the JCR concepts have a complete new model to learn, without clear correspondance between XWiki concepts and JCR concepts - we have no obvious reason to participate in content management standardization efforts Approach 2 stack ================ XWiki services -------------------------- XWiki document model --------------------------- SWCR services: - Type/Property services - JCR services splitted into serveral ones ---------------------------- JCR document model, JCR type model --------------------------- JCR impl.|Relational storage impl|... Pros: - genericity of the property/types services - we embrace the JCR movement, and take part in its evolution - we take part in the emerging area of semantic web content repositories and the design of standard Semantic Web APIs complementing the JCR Cons: - larger effort - probably harder to implement a compatibility layer with XWiki 1.0 API - bigger performance risks Stéphane -- Stéphane Laurière slauriere(a)xwiki.com XWiki Research http://www.xwiki.com http://concerto.xwiki.com http://nepomuk.semanticdesktop.org