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   1  Some quick notes on the file/repository architecture.
   2  
   3  Functionality is, as always, driven by data model.
   4  
   5  * The repository object stores configuration information about a file storage
   6    method.
   7  
   8  * The file object is a process-local cache of information about a particular
   9    file.
  10  
  11  Thus the file object is the primary public entry point for obtaining information
  12  about files, since access via the file object can be cached, whereas access via
  13  the repository should not be cached.
  14  
  15  Functions which can act on any file specified in their parameters typically find
  16  their place either in the repository object, where reference to
  17  repository-specific configuration is needed, or in static members of File or
  18  FileRepo, where no such configuration is needed.
  19  
  20  File objects are generated by a factory function from the repository. The
  21  repository thus has full control over the behavior of its subsidiary file
  22  class, since it can subclass the file class and override functionality at its
  23  whim. Thus there is no need for the File subclass to query its parent repository
  24  for information about repository-class-dependent behavior -- the file subclass
  25  is generally fully aware of the static preferences of its repository. Limited
  26  exceptions can be made to this rule to permit sharing of functions, or perhaps
  27  even entire classes, between repositories.
  28  
  29  These rules alone still do lead to some ambiguity -- it may not be clear whether
  30  to implement some functionality in a repository function with a filename
  31  parameter, or in the file object itself.
  32  
  33  So we introduce the following rule: the file subclass is smarter than the
  34  repository subclass. The repository should in general provide a minimal API
  35  needed to access the storage backend efficiently.
  36  
  37  In particular, note that I have not implemented any database access in
  38  LocalRepo.php. LocalRepo provides only file access, and LocalFile provides
  39  database access and higher-level functions such as cache management.
  40  
  41  Tim Starling, June 2007