Provider Selection Algorithms

Provider Selection Algorithms

Provider selection algorithms allow administrators to define complex provider selection for each pool. This article is about how to write a custom algorithm.

Chaining

Since the administrator can enable multiple strategies, these strategies should be written in a chainable manner. To allow chaining of the strategies you just have to include the following modules in your strategy class:

<code class="ruby">
include ProviderSelection::ChainableStrategy::InstanceMethods
extend ProviderSelection::ChainableStrategy::ClassMethods
</code>

The first module handles the chaining of the strategies. The second module adds some common methods to the class, e.g. properties which may contain the edit path for the strategy.

Next, you should define the calculate method:

<code class="ruby">
def calculate
  rank = @strategies.calculate

  # Modify the rank object

  rank
end
</code>

This code will first make a call to the strategy chain before the current strategy to calculate the rank. To alter the ranking you should modify the rank object and it should be returned from method.

The Rank

The rank contains all the priority groups. The priority groups are ordered by their score from low to high. It also has special priority group: the default_priority_group which contains all the valid matches. Your algorithm should use the matches from this group to avoid the duplication of the filtering logic. Also worth noting that the default_priority_group has a score of 10000, way much higher than the user defined priority groups whose score can be from the [–100, 100] interval. For now, a match has a provider_account, hardware_profile, instance_hwp and a score attributes. In the future it can be extended with provider realm, provider image and more.

Files

You can put your strategy class in vendor/provider_selection/strategies/strategy_name/strategy_name.rb and it will be automatically loaded.

I18n

To define the translated name for the strategy just create a new entry in the config/locales/strategies directory:

<code class="yaml">
en:
  strategies:
    strategy_name:
      name: 'Strategy Name'
</code>

http://www.aeolusproject.org/docs/presentations/2012-nov-conference/ProviderSelection.pdf