The Bcfg2 Server

The Bcfg2 server is responsible for taking a network description and turning it into a series of configuration specifications for particular clients. It also manages probed data and tracks statistics for clients.

The Bcfg2 server takes information from two sources when generating client configuration specifications. The first is a pool of metadata that describes clients as members of an aspect-based classing system. That is, clients are defined in terms of aspects of their behavior. The other is a file system repository that contains mappings from metadata to literal configuration. These are combined to form the literal configuration specifications for clients.

The Configuration Specification Construction Process

As we described in the previous section, the client connects to the server to request a configuration specification. The server uses the client’s metadata and the file system repository to build a specification that is tailored for the client. This process consists of the following steps:

  • Metadata Lookup

    The server uses the client’s IP address to initiate the metadata lookup. This initial metadata consists of a (profile, image) tuple. If the client already has metadata registered, then it is used. If not, then default values are used and stored for future use. This metadata tuple is expanded using some profile and class definitions also included in the metadata. The end result of this process is metadata consisting of hostname, profile, image, a list of classes, a list of attributes and a list of bundles.

  • Abstract Configuration Construction

    Once the server has the client metadata, it is used to create an abstract configuration. An abstract configuration contains all of the configuration elements that will exist in the final specification without any specifics. All entries will be typed (i.e. the tagname will be one of Package, Path, Action, etc) and will include a name. These configuration entries are grouped into bundles, which document installation time interdependencies.

    Here is an example of an abstract configuration entry:

    <Package name='openssh-server'/>
  • Configuration Binding

    The abstract configuration determines the structure of the client configuration, however, it doesn’t yet contain literal configuration information. After the abstract configuration is created, each configuration entry must be bound to a client-specific value. The Bcfg2 server uses plugins to provide these client-specific bindings. The Bcfg2 server core contains a dispatch table that describes which plugins can handle requests of a particular type. The responsible plugin is located for each entry. It is called, passing in the configuration entry and the client’s metadata. The behavior of plugins is explicitly undefined, so as to allow maximum flexibility. The behaviours of the stock plugins are documented elsewhere in this manual. Once this binding process is completed, the server has a literal, client-specific configuration specification. This specification is complete and comprehensive; the client doesn’t need to process it at all in order to use it. It also represents the totality of the configuration specified for the client.

    Here is the entry from above once it has been bound to its literal specification (In this case, using the Packages plugin).

    <Package name="openssh-server" version="auto" type="deb"/>

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