Table of Contents
Section 6, “Repositories” introduces the concept of a “Repository”, which is an object that describes a collection of packages. The System Repository is a database that implements a repository of packages installed by LPMtool. This chapter describes how the System Repository database is implemented.
The
FileStore
object represents an arbitrary collection of files.
Files are not accessed directly, but through the
FileStore
object.
These files are expected to be
comparatively small. The contents of each file are requested individually.
The
FileStore
maintains an internal cache.
The first time a file is read, it is saved in the cache.
Subsequent requests for the same file will use the cached copy of the
file's contents.
In other words:
FileStore
functions as a caching layer between LPMtool, and the actual files.
The
FileStore
object defines the following methods:
Return the contents of the file, as a string.
If the file's contents are already cached, used the cached copy.
If not, invoke the
fget() function.
fget must be defined by
FileStore's
subclass.
fget opens
the file and reads its contents (or return an empty string if the file
does not exist).
These methods update the contents of a file, or remove it.
These methods do not actually make any changes, they only update
FileStore's
internal cache to reflect these changes.
Subsequent calls to
get
and
enumerate reflect these changes.
Enumerate all files in the
FileStore.
This method invokes the
fenumerate method, which must be defined by the subclass
to return the list of files in the collection.
The list of files returned by
fenumerate is modified according to the contents of
FileStore's internal cache.
That is, files returned by fenumerate which have
been removed are suppressed; and new files added via
update are also included in the list returned by
enumerate.
enumerate's output reflects the underlying collection
of files, with all changes specified by prior
updates and
removes applied.
A
FileStore
is just a caching layer that also reflects changes to the collection of files
it represents.
The job of updating the actual files is left to the subclass.
The
FileStore
object implements a virtual caching layer that keeps track of the changes to
collection of files it represents.