Plugin Variants

Issue

Some plugins are generic in the sense that they cannot fully define their contract statically. (Note: if they can decide it statically, you should prefer compilation variants.) Instead their contract is based on their configuration. We will call every combination of plugins+configuration where we get a different contract plugin variant.

The current issue is that there is no way to enumerate possible plugin variants as needed to list all functionality of Elektra.

Constraints

• User Customizability:
  • Users must be able to add new plugin variants
  • Users should be able to suppress plugin variants
• Modularity:
  • The plugins themselves yield a list of plugin variants
  • The code defining variants is part of the plugin's code
  • To get a full list of all variants of all plugins, one simply has to iterate over all plugins and ask the plugins for possible variants.
• libtool should provide a convenience layer for easy access of all plugin variants. In this case there should also be a performance optimization, that plugins do not need to be reopened required multiple times.

Assumptions

• The number of variants needs to be bounded: plugin variants need to be fully enumerable

Considered Alternatives

A specification of the plugin's configuration and a tool that can enumerate all possible essential configurations. Issues here are:

• Plugins might need to self-reference (a validation plugin might have plugin variants, too)
• Elektra's specification language was not complete nor consistent at point of writing
• Does not fit with the checkconf (see here) approach.

Decision

1. Provide a function int genconf (KeySet * ks, Key * errorKey) where ks is filled with a list of all variants with the essential configuration (subkeys config) and the changed parts of the contract (subkeys infos).
2. Keys defined in system/elektra/plugins/variants have the same content, but take precedence. If a variant with the same name is defined, only config or infos from genconf are considered if they are not mentioned in system/elektra/plugins/variants. If the keys config or infos are present, it will be overwritten (deleted), if only subkeys thereof are present, it will be enhanced.
3. Empty config and infos mean:
   • config: The "default variant" (without any parameter) should be also available (has useful functionality)
   • infos: It is not possible to determine the changes of the contract, the user need to instantiate the plugin and enquiry the contract.

Example

genconf for augeas yields:

access
access/config
access/config/lens = Access.lns
access/infos
access/infos/format = access
aliases
aliases/config
aliases/config/lens = Aliases.lns
aliases/config/path = /etc/aliases
aliases/infos
aliases/infos/format = aliases

genconf for python yields:

configparser/config
configparser/config/script = python_configparser.py

the user specifies:

system/elektra/plugins/variants/access
system/elektra/plugins/variants/aliases/infos/status = 10000
system/elektra/plugins/variants/configparser
system/elektra/plugins/variants/configparser/config
system/elektra/plugins/variants/configparser/config/script = mybetter_configparser.py

then the plugin variant:

1. access is not available (system/elektra/plugins/variants/access overrides genconf)
2. aliases as defined from genconf, but with changes in contract (infos/status)
3. configparser is completely redefined (result from genconf will not be considered)

Argument

• The genconf API was chosen to be consistent with checkconf. The pluginhandle might be missing for some scenarios. If it is needed, we should change both APIs for consistency reasons.
• The very verbose (deeply nested) subkeys (and names instead of arrays) provide better customizability and extensibility.

Implications

• genconf needs to be implemented for the plugins:
  • augeas
  • jni
  • lua
  • python
  • python2
• PluginDatabase needs an extension to list all plugin variants
• kdb list should be able to list all variants, e.g. like: augeas lens=Access.lns augeas format=access so that a user can copy and paste this for the kdb mount command.

Related decisions

• Global Plugins

Notes

Discussions took place here.