trove-specs/specs/liberty/guestagent-configuration-manager.rst

Guestagent Configuration Manager

Launchpad Blueprint: https://blueprints.launchpad.net/trove/+spec/guestagent-configuration-manager

Problem Description

Each datastore currently has to implement a strategy for maintaining its configuration files including functionality for parsing, updating properties, saving and managing overrides. With increasing number of supported datastores this approach leads to a significant duplication in the production and testing codebase.

Proposed Change

Facilitate code reuse by implementing a manager class that could be used by all guestagents to manage their configuration files and overrides.

The patch set will consist of four main features.

1. Codecs for serialization and deserialization of configuration files.

   These would be responsible for parsing a Python structure (dict) into an appropriate serialized form (see the simple example below).

   An INI-style file (serialized form):

   [section_1]
   key1 = value1
   key2 = value2
   ...
   
   [section_2]
   key1 = value1
   key2 = value2
   ...

   Dict representation of the above contents (deserialized form):

   {'section_1': {'key1': 'value1', 'key2': 'value2', ...},
    'section_2': {'key1': 'value1', 'key2': 'value2', ...}
    ...
   }

   Codecs for common configuration formats currently present in Trove will be implemented.

   These include:

   • INI-style format: see the example above

   • YAML format: see http://pyyaml.org/wiki/PyYAMLDocumentation for examples of Python representation.

   • Properties format:

     Serialized form:

     key1 k1arg1 k1arg2 ... k1argN
     key2 k2arg1 k2arg2 ... k2argN
     key3 k3arg1 k3arg2 ...
     key3 k3arg3 k3arg4 ...

     Dict representation of the above contents (deserialized form):

     {'key1': [k1arg1, k1arg2 ... k1argN],
      'key2': [k2arg1, k2arg2 ... k2argN],
      'key3': [[k3arg1, k3arg2, ...], [k3arg3, k3arg4, ...]]}

2. Functions for reading and writing using a given codec.

   A 'write' function saves a serialized form of a given dict into a file. A 'read' function applied on the file produces the same dict structure that was written in the previous step. The write function will write a file with superuser privileges when provided an optional 'as_root' keyword. This will be implemented by writing a temporary file first moving it to place using the existing operating system calls.

3. The configuration manager.

   ConfigurationManager will be responsible for management of datastore configuration files. Its base functionality will include reading and writing configuration files and updating or retrieving current values. It will be also responsible for validating user inputs and requests. When supplied an override strategy (below) it allows the user to manage configuration overrides as well.

   The configuration manager will be responsible for enforcing the limit on the number of applied overrides (there can currently be only one override applied to an instance at any given time).

4. Common override strategies.

   An override strategy object responsible for management of overrides in the configuration manager (when provided).

   It will implement functions to:

   • apply updated values on the current revision of the configuration file.
   • remove the last applied overrides and effectively restoring the previous version of the configuration file.

   The strategies provided with this patch set will be:

   • Rolling Override Strategy:

     A strategy suitable for applications that do not support includes in their configuration files. It applies updates to the configuration file while maintaining a backup of the previous version that can be restored when the overrides get removed.

     Apply procedure:

     • Save a backup copy of the current configuration file to a known configurable location.
     • Load and parse the current configuration file into a dict (using an appropriate codec).
     • Update the dict with the overridden values.
     • Overwrite the configuration file with the serialized form of the dict (using the same codec).

     Remove procedure:

     • Move the appropriate backup revision over the current configuration file.

   • Import Override Strategy:

     A strategy useful for datastores that support imports in their configuration files (like MySQL). The overrides are stored in a known configurable directory which is then imported by the base configuration file which itself remains intact.

     Apply procedure:

     • Store a serialized form (using an appropriate codec) of the overrides dict in an imported location.

     Remove procedure:

     • Remove the appropriate file from the imported location.

Configuration

None

Database

None

Public API

None

Public API Security

None

Python API

None

CLI (python-troveclient)

None

Internal API

None

Guest Agent

The affected files would include:

• guestagent/common/operating_system.py: I/O functions
• common/stream_codecs.py: Implementations of codecs.
• common/configurations.py: Reuse the codecs in configuration parsers.
• guestagent/common/configuration.py: Implementations of the configuration manager and override strategies.

Alternatives

None

Implementation

Assignee(s)

Petr Malik <pmalik@tesora.com>

Milestones

Liberty-1

Work Items

• Implement codecs for serialization and deserialization of common configuration files.
• Implement functions for reading and writing files using a given codec.
• Implement the configuration manager.
• Implement common override strategies.
• Add unit tests for the above functionality.

Upgrade Implications

None

Dependencies

The current implementation works around limitations of the ConfigParser in Python 2.6. OpenStack no longer supports this version of Python and Trove gate tests run against Python 2.7 We can therefore remove the compatibility requirement and make full use of the ConfigParser in Python 2.7 in the existing code base.

Testing

Unit tests will be added to cover non-trivial code paths.

Documentation Impact

None

References

None