maven/USE-CASES.txt

Trying to get a large set of use cases that we can use to help drive
development in the right direction. I would like to collect some here and then
post them to the wiki as examples users can template to express what their
use cases are. So my first example is something I chatted about with Aslak
this morning. So they are brief and possibly naive but a huge collection
of these would help I think.

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- Being able to write artifact handler that would operate to install an
xdoclet plugin correctly when an xdoclet plugin is stated as a dependency.
  * Maybe this pattern could be generalized so you could register arbitrary
  ArtifactInterceptors (possibly a DAG of them) that will be called to handle
  events and operations on artifacts. They could even be used to actually
  implement the operation (ie have a interceptor actually fetch the artifact
  on request). This is basically an expansion of handler system as it stands
  so you can add more context and generalize how artifact operations are
  handled. (PD)

- With Maven installed it should be a simple command that allows you to
checkout the sources for a project.
  * Maybe this could be generalized so that if you reference a remote
  descriptor it will download the project. Something like

  $ maven --project http://www.realityforge.org/daedalus-project.xml

  would result in download of project either from remote location or
  maybe from scm declarations in project descriptor.

  As part of this remote downloading though the user may be asked for
  input of particular configuration data (ie username etc). (PD)

- Unified source directory structure that is analagous to the repository
itself. This way locations of intermediary artifacts of a build would be
in a known location.  This would also help with developer setup i.e. getting
new developers up and running. They could run a maven command and have all
their source trees set up in the same way as their collegues.

- I would like to be able to execute some goals (like 'clean') even when
  not all dependencies are satisfied.
  * hell yes!!!! Maybe dependency resolution could be a plugin that
  other plugins depend upon (PD)
  * is this something that is needed at the goal level though? Maybe we need to be
    able to specify goal properties such as this (BP).

- Support for "specification" dependencies. I would like to be able to
  say that I depend on a "specification" dependency such as
  "javax.jaxp.parser" with specific version. It is possible that multiple
  artifacts exist that satisfy this specification. ie Several versions
  of Crimson and Xerces may implement the "specification" and I want maven
  to select one and just use it. I dont care which does it but preferrably
  the one with the latest version. (PD)

- Support for retrieval of compatible versions for objects. For example I may
  say that I want version 1.1 of Crimson but Maven may return 1.1.1, 1.1.4 etc
  as it knows that these are compatible. This means aadding in some versioning
  specification or different strategies for resolution. (PD)

- Support for transitive and group dependencies
  * I would like to see this dependency information either encoded directly
  into the artifacts where possible (ie META-INF/maven/dependencies.xml in
  jar files) or accessible from repository at peer level for each artifact
  much like signature data is accessible for each artifact. (PD)

- It should be easy to create a single report without a need to (call like it has place now)
  maven:site. It will be nice if report generators can produce docs in few diffrent formats
  (e.g pdf, html, rtf) and this format can be also choosen for group of reports.

- groupId
 - Let's assume that there exists an artifact type "web-component"
   which is a zip file containg css/js/gifs/jpegs etc
   Example of such artifact is for example distribution of tigris style.

   Say an user has declared the following dependency:

   <depenedency>
      <groupId>org.tigris</groupId>
      <artifactId>style</artifactId>
      <type>web-component</type>
      <version>2.0</version>
   </depenedency>

   A dependency of this type should be processed only when users assembles the web application
   (war:webapp goal in maven-old)

   In short:  user can be able to declare goal decorators and goals should be fine-grained
   * This very similar to only resolving some dependencies depending on what goals
   are running as specified above. (PD)

- Maven is Java Build Tool. It should be well defined what does it mean to "build java project".
  It means that stages of the process should be well defined.

  pre-compile (this is when tools like JAXB, XDoclet)
  compile (javac, aspectj)
  post-compile (aspectj? )
  pre-jar
  ...
  etc

  It should e.g be possible to use Castor, JAXB plugin, XDoclet plugin without writing a single line
  of build script. In the ideal case it should be just enough to plug-in XDoclet into process simply
  by "checking the checkbox in IDE".
  This is someting what Vincent Massol started in maven-caller plugin. It will be nice to explore this idea.

  -- definitely a good idea, but I don't think we should restrict Maven as a Java build tool. I'd like to be
     able to build C and .Net stuff with Maven at some point in the future, and the build process is similar enough
     to be possible (BP).

- Reorganization of repository layout - are we going to map groupId : "a.b.c.d"  to path "a/b/c/d"?
  Are there some better alternatives?
  * I am not sure that is such a good idea - how can you tell when a
  directory is an artifact container (ie jar, sar, distribution etc) and when
  it is a group container (ie a/b/c/d). (PD)

- (Michal)  I have tried to implement "platform dependend dependecies".

   <depenedency>
      <groupId>a</groupId>
      <artifactId>b</artifactId>
      <type>native</type>
      <version>2.0</version>
   </depenedency>

    is resolved as

   a/b-2.0.dll (on windows) or a/b-2.0.so (unix)


   Maybe it can/should be done differently?
   What other "platform depended" artifact type do we have?
   exe? shell-script?

   * the problem with this approach is that most things that load native libs
   expect the name of the lib to follow a very specific name which usually will
   not conform to maven naming conventions (ie probably wont have -version on
   end). No idea how to fix this. (PD)

 - how to declare a dependency on JDK and what are possible implications of such dependency
  (e.g javac/javadoc target jdk)

 - I don't know if this will add too much complexity, but being able to generate multiple
   and compound artifacts from a single project would be nice. If we want to have a
   "Ant Compatibility Layer", then this would be required.
   
   Think of a project for a web application, it will have java code as well as web resources.
   The java code is collected into a JAR file, and then that is packaged with the web
   resources into a WAR.
   
   Of course, none of this is really required, as it can all be accomplished by breaking the
   project out into multiple projects with a reactor, per the Maven 1 guidelines. But if this
   limitation was removed, it might help Marvin adoption. (PR)

- File inclusion in POM - project inheritance allows to factorize some common pom attributes, 
  however there are some cases where this mechanism is not enough. For instance we might want 
  to share common dependencies between projects whose ancestors union is empty. Providing poms
  easier to read by extracting various elements into their own xml fragments is another example.
  
  afaik the only way to obtain this behaviour today is to use xml entities. There are some really 
  bad limitations to this system : 
  
    * cannot use interpolation b/c doctype is processed before pom is interpolated
    * if using reactor, there is no uniform way to declare the entities since it 
      depends badly on the multiproject structure (nested vs. parallel projects)
    * being able to run maven from either parent project dir or subproject 
      dir is not straigthforward b/c we have to think about the directory structure
      which is a low-level concern
       
  Also i dont know what is the behaviour when running from an arbitrary folder (-d, -p options) 
  
  adding support for file inclusion in pom would provide simplification and better consistency 
  among pom base.
  
  POM could perhaps have an <inclusions> section :
  
    <inclusions>
      <inclusion>
        <file>path_to_xml_fragment</file>
        <alias>toBeIncluded</alias>
      </inclusion>
    </inclusions>
  
    then we could reference the fragment with something like :
    
    <include>toBeIncluded</include>
    
  where path_to_xml_fragment would be relative to ${basedir}. another advantage is the 
  possibility to perform some filtering on the included files, for instance dependending 
  on some (build dependent) properties.
  
  the drawback of this proposal is the complexity it adds to the POM structure, polluting it 
  with non project related elements.

  -- I think it makes more sense just to use <import uri="fragment.xml" /> at the point of inclusion, and avoid the
     declaration of entity-like aliases. The only extra usefulness in the declaration is that it could be stored in
     a parent POM, but that goes against the original use case and isn't worth the complexity -- BrettPorter.

- We should be able to allow a project to clearly specify which version of a plugin it uses and not get other versions
  interfering. This would be done by a plugin dependency (BP).

- How do we discover the set of plugins to use? It would be good to just use a set of plugin dependencies in the
  default model, and override them in subprojects if you need a newer/older version. Issues with this:
    - there needs to be a per-user default model and a installation wide default model that can be updated
    - the model needs to be updated when a new plugin is installed
    - this isn't flexible for discovering new goals that are not part of the project's build process. eg a plugin
      may depend on aspectj to build correctly, but "console" won't be a dependency because it is a "user" plugin.
      This is leads to plugin types, which I'll add later.

- Installing plugins should be a clearly defined process. When a new plugin is installed, is it installed for the user 
  only be default and for the Maven installation optionally. This ties in to the point above about discovery.

- Dependencies could support multiple versions (1.1+, for example, would get the latest release). This
  would be helpful for the plugins above. Issues with this: 
    - do we really search the repository for later releases, or just use what's local?
    - how is a newer release signified? How is this affected by branches and snapshots?

- We need plugin categories that treat certain plugins in different ways. I can think of:
    - reporting plugins
    - user plugins (eg console) - not project specific
    - project specific user plugins (eg cactus, idea, ...)
    - build plugins (clean, jaxb, java, test)
    - artifact generating plugins (jar, war, ear, plugin)
  It may not be that we need to treat these in any particular way, but rather that for each category, define particular
  hooks - like the reporting plugins do in maven1. One issue is that some of these categories overlap (eg cactus runs
  build tests but also generates a report) - so it may be that plugins just have to provide an interface (and can have
  many) rather than conform to a certain pattern.

- Maven needs to support forked codebases as part of its versioning strategy. This can integrate with CVS branches and
  the branches element in the model. While the actual versions should not conflict across branches, snapshots will and
  determining the latest snapshot should only find it from that branch, not everything.
  So if a project has <branch>1_0_BUGFIXES</branch> specified, snapshots would be maven-1_0_BUGFIXES-SNAPSHOT.jar, and
  SCM operations should refer to that branch tag.