a Grid Computing System - Utopia

3. The Basic Building Block - ONTA (Object Network Transfer Architecture)
when a file is downloaded by ONTA from the network. These two situations are different in the
sense that when storing an archive after using the object writer, a local temporary file is already
created, at it should just be renamed to a unique file name. When downloading a file, first a unique
filename should be obtained and than this file name should be used to store the downloaded file.
For handling this cases the API consists of two calls:
public String put (File f)
public String getFileName(AliceURL appl)
All the files are stored starting at a root directory that is selected by using a string parameter
passed to the constructor of the file manager when the class is instantiated. All the files are stored
starting at this directory, which is specified by the user through the use of the GUI. There is an issue
of creating too many files inside the same directory and the ONTA file manager uses a hierarchical
approach, so there are never more than a maximum number of files inside a directory. If that
number is reached, a new subdirectory is created and all the other files with the same root directory
start will be stored in that subdirectory. We do this in order to minimize the file system overhead
induce by a larger number of files being located in the same directory, thus additional level of
indirection being needed.
The put (File f) method stores a local temporary file to a unique filename inside the ONTA root
directory. This actually just gets a new file name, creates a new directory if needed (if the current
one is full - that is the maximum number of files has been reached) and moves the file there.
The second method, getFileName(AliceURL appl) should be used by all the client protocols
to store a file when it is downloaded. Since all the references contain the application that they
are related to inside the object reference taken from the space, the file manager has the ability
to store all the files downloaded on behalf of an application in the same directory. This greatly
improves the ability to have a good file structure organization and to be able to clean up the unused
files much easier. So, given an application ID (in form of an AliceURL), this call will return a
unique filename in a directory unique for that application (if this directory does not exists, it will
be created). Storing files for different applications in different directories also resolves the issue of
files with the same filename being downloaded in the same directory.
To clean up files that are not used anymore, the file manager offers two calls to handle this:
public void markNotUsed(String name)
public void deleteUnused()
The first method adds a file with a given name (the name returned by one of the two calls
presented above should be used) to a list of files that are not longer needed. When deleteUnused()
is called, all the files in the list are physically deleted from the disk.
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