Appendix A. Extension Architecture Interface
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# Appendix A. Extension Architecture Interface

The Cedar Backup Extension Architecture Interface is the application programming interface used by third-party developers to write Cedar Backup extensions. This appendix briefly specifies the interface in enough detail for someone to succesfully implement an extension.

You will recall that Cedar Backup extensions are third-party pieces of code which extend Cedar Backup's functionality. Extensions can be invoked from the Cedar Backup command line and are allowed to place their configuration in Cedar Backup's configuration file.

There is a one-to-one mapping between a command-line extended action and an extension function. The mapping is configured in the Cedar Backup configuration file using a section something like this:

    <extensions>
       <action>
          <name>database</name>
          <module>foo</module>
          <function>bar</function>
          <index>101</index>
       </action>
    </extensions>

In this case, the action “database” has been mapped to the extension function `foo.bar()`.

Extension functions can take any actions they would like to once they have been invoked, but must abide by these rules:

1. Extensions may not write to `stdout` or `stderr` using functions such as `print` or `sys.write`.

2. All logging must take place using the Python logging facility. Flow-of-control logging should happen on the `CedarBackup2.log` topic. Authors can assume that ERROR will always go to the terminal, that INFO and WARN will always be logged, and that DEBUG will be ignored unless debugging is enabled.

3. Any time an extension invokes a command-line utility, it must be done through the `CedarBackup2.util.executeCommand` function. This will help keep Cedar Backup safer from format-string attacks, and will make it easier to consistently log command-line process output.

4. Extensions may not return any value.

5. Extensions must throw a Python exception containing a descriptive message if processing fails. Extension authors can use their judgement as to what constitutes failure; however, any problems during execution should result in either a thrown exception or a logged message.

6. Extensions may rely only on Cedar Backup functionality that is advertised as being part of the public interface. This means that extensions cannot directly make use of methods, functions or values starting with with the `_` character. Furthermore, extensions should only rely on parts of the public interface that are documented in the online Epydoc documentation.

7. Extension authors are encouraged to extend the Cedar Backup public interface through normal methods of inheritence. However, no extension is allowed to directly change Cedar Backup code in a way that would affect how Cedar Backup itself executes when the extension has not been invoked. For instance, extensions would not be allowed to add new command-line options or new writer types.

8. Extensions must be written to assume an empty locale set (no `$LC_*` settings) and `$LANG=C`. For the typical open-source software project, this would imply writing output-parsing code against the English localization (if any). The `executeCommand` function does sanitize the environment to enforce this configuration.

Extension functions take three arguments: the path to configuration on disk, a `CedarBackup2.cli.Options` object representing the command-line options in effect, and a `CedarBackup2.config.Config` object representing parsed standard configuration.

    def function(configPath, options, config):
       """Sample extension function."""
       pass

This interface is structured so that simple extensions can use standard configuration without having to parse it for themselves, but more complicated extensions can get at the configuration file on disk and parse it again as needed.

The interface to the `CedarBackup2.cli.Options` and `CedarBackup2.config.Config` classes has been thoroughly documented using Epydoc, and the documentation is available on the Cedar Backup website. The interface is guaranteed to change only in backwards-compatible ways unless the Cedar Backup major version number is bumped (i.e. from 2 to 3).

If an extension needs to add its own configuration information to the Cedar Backup configuration file, this extra configuration must be added in a new configuration section using a name that does not conflict with standard configuration or other known extensions.

For instance, our hypothetical database extension might require configuration indicating the path to some repositories to back up. This information might go into a section something like this:

    <database>
       <repository>/path/to/repo1</repository>
       <repository>/path/to/repo2</repository>
    </database>

In order to read this new configuration, the extension code can either inherit from the `Config` object and create a subclass that knows how to parse the new `database` config section, or can write its own code to parse whatever it needs out of the file. Either way, the resulting code is completely independent of the standard Cedar Backup functionality.

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