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- Some quick notes on the file/repository architecture.
- Functionality is, as always, driven by data model.
- * The repository object stores configuration information about a file storage
- method.
- * The file object is a process-local cache of information about a particular
- file.
- Thus the file object is the primary public entry point for obtaining information
- about files, since access via the file object can be cached, whereas access via
- the repository should not be cached.
- Functions which can act on any file specified in their parameters typically find
- their place either in the repository object, where reference to
- repository-specific configuration is needed, or in static members of File or
- FileRepo, where no such configuration is needed.
- File objects are generated by a factory function from the repository. The
- repository thus has full control over the behaviour of its subsidiary file
- class, since it can subclass the file class and override functionality at its
- whim. Thus there is no need for the File subclass to query its parent repository
- for information about repository-class-dependent behaviour -- the file subclass
- is generally fully aware of the static preferences of its repository. Limited
- exceptions can be made to this rule to permit sharing of functions, or perhaps
- even entire classes, between repositories.
- These rules alone still do lead to some ambiguity -- it may not be clear whether
- to implement some functionality in a repository function with a filename
- parameter, or in the file object itself.
- So we introduce the following rule: the file subclass is smarter than the
- repository subclass. The repository should in general provide a minimal API
- needed to access the storage backend efficiently.
- In particular, note that I have not implemented any database access in
- LocalRepo.php. LocalRepo provides only file access, and LocalFile provides
- database access and higher-level functions such as cache management.
- Tim Starling, June 2007
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