How bootstrap-vz works


At its core bootstrap-vz is based on tasks that perform units of work. By keeping those tasks small and with a solid structure built around them a high degree of flexibility can be achieved. To ensure that tasks are executed in the right order, each task is placed in a dependency graph where directed edges dictate precedence. Each task is a simple class that defines its predecessor tasks and successor tasks via attributes. Here is an example:

class MapPartitions(Task):
    description = 'Mapping volume partitions'
    phase = phases.volume_preparation
    predecessors = [PartitionVolume]
    successors = [filesystem.Format]

    def run(cls, info):

In this case the attributes define that the task at hand should run after the PartitionVolume task — i.e. after volume has been partitioned (predecessors) — but before formatting each partition (successors). It is also placed in the volume_preparation phase. Phases are ordered and group tasks together. All tasks in a phase are run before proceeding with the tasks in the next phase. They are a way of avoiding the need to list 50 different tasks as predecessors and successors.

The final task list that will be executed is computed by enumerating all tasks in the package, placing them in the graph and sorting them topologically. Subsequently the list returned is filtered to contain only the tasks the provider and the plugins added to the taskset.

System abstractions

There are several abstractions in bootstrap-vz that make it possible to generalize things like volume creation, partitioning, mounting and package installation. As a rule these abstractions are located in the base/ folder, where the manifest parsing and task ordering algorithm are placed as well.