Design Considerations

This section will discuss several aspects of the design of Bcfg2, and the particular use cases that motivated them. Initially, this will consist of a discussion of the system metadata, and the intended usage model for package indices as well.

System Metadata

Bcfg2 system metadata describes the underlying patterns in system configurations. It describes commonalities and differences between these specifications in a rigorous way. The groups used by Bcfg2’s metadata are responsible for differentiating clients from one another, and building collections of allocatable configuration.

The Bcfg2 metadata system has been designed with several high-level goals in mind. Flexibility and precision are paramount concerns; no configuration should be undescribable using the constructs present in the Bcfg2 repository. We have found (generally the hard way) that any assumptions about the inherent simplicity of configuration patterns tend to be wrong, so obscenely complex configurations must be representable, even if these requirements seem illogical during the implementation.

In particular, we wanted to streamline several operations that commonly occurred in our environment.

  • Copying one node’s profile to another node.

    In many environments, many nodes are instances of a common configuration specification. They all have similar roles and software. In our environment, desktop machines were the best example of this. Other than strictly per-host configuration like SSH keys, all desktop machines use a common configuration specification. This trivializes the process of creating a new desktop machine.

  • Creating a specialized version of an existing profile.

    In environments with highly varied configurations, departmental infrastructure being a good example, “another machine like X but with extra software” is a common requirement. For this reason, it must be trivially possible to inherit most of a configuration specification from some more generic source, while being able to describe overriding aspects in a convenient fashion.

  • Compose several pre-existing configuration aspects to create a new profile.

    The ability to compose configuration aspects allows the easy creation of new profiles based on a series of predefined set of configuration specification fragments. The end result is more agility in environments where change is the norm.

    In order for a classing system to be comprehensive, it must be usable in complex ways. The Bcfg2 metadata system has constructs that map cleanly to first-order logic. This implies that any complex configuration pattern can be represented (at all) by the metadata, as first-order logic is provably comprehensive. (There is a discussion later in the document describing the metadata system in detail, and showing how it corresponds to first-order logic)

These use cases motivate several of the design decisions that we made. There must be a many to one correspondence between clients and groups. Membership in a given profile group must imbue a client with all of its configuration properties.

Package Management

The interface provided in the Bcfg2 repository for package specification was designed with automation in mind. The goal was to support an append only interface to the repository, so that users do not need to continuously re-write already existing bits of specification.

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