The Reconstruction Extension

The MBFS open() call in the kernel, category, and filename matching interfaces optionally support a reconstruction parameter defined by the following structure:

typedef struct {
    char *reconstruction_rule;
    envp *environment[];
    int   num_dependencies;
    char *dependencies[];
    int   data_freshness;
    Bool  reconstruct_immediately;
} mbfs_reconstruction;

reconstruction_rule:

A string containing the command-line that the system should execute to reconstruct the file.

environment:

An array of environment variables and values to be set in the shell before invocation of this file's reconstruction rule. If NULL, the current environment in assumed.

data_freshness:

Specifies how ``up-to-date'' the contents should be with respect to it's dependencies:

• LATEST_DATA - This corresponds to a guarantee that the file will always contain the newest possible data. The file should be reconstructed if any of its dependencies have changed. This feature can be used to implement intentional files.
• VERSION_DATA - The system guarantees that some version of the file data exists, not necessarily the latest. Unlike LATEST_DATA, the file's contents are not regenerated when a dependency changes. Only if the data is lost will the data be regenerated. If a dependency is deleted, the reconstruction rule cannot be executed, so the system immediately archives the data to an DA level to ensure its persistence.
• OLD_DATA - This guarantees that the file's contents will be based on versions of the dependencies as they existed at the time the file was created. Before the system will allow a dependency to be changed or deleted, the system will ensure the data reaches an DA level because reconstruction might produce different data.

reconstruct_immediately:

A boolean specifying when the system should execute the file's reconstruction rule.

• TRUE - The system will invoke the reconstruction rule as soon as lost file data is detected or, if LATEST_DATA is selected, as soon as a dependency is changed.
• FALSE - The system may postpone the invocation of the reconstruction rule for any amount of time up until the file is referenced. This is the default.

num_dependencies:

The number of entries in the dependency array.

dependencies:

An array of entries corresponding to the MBFS files on which the given file is dependent.

With the inclusion of dependencies and the LATEST_DATA freshness quality, the reconstruction extension naturally provides an automatic form of the Unix ``make'' utility and provides a form of intensional files. While an automatic ``make'' is useful in some circumstances, users may still invoke the standard make utility manually.