reclass operations

YAML FS storage

While reclass has been built to support different storage backends through plugins, currently only the yaml_fs storage backend exists. This is a very simple, yet powerful, YAML-based backend, using flat files on the filesystem (as suggested by the _fs postfix).

yaml_fs works with two directories, one for node definitions, and another for class definitions. The two directories must not be the same, nor can one be a parent of the other.

Files in those directories are YAML-files, specifying key-value pairs. The following three keys are read by reclass:

Key Description
classes a list of parent classes
appliations a list of applications to append to the applications defined by ancestors. If an application name starts with ~, it would remove this application from the list, if it had already been added — but it does not prevent a future addition. E.g. ~firewalled

key-value pairs to set defaults in class definitions, override existing data, or provide node-specific information in node specifications. By convention, parameters corresponding to an application should be provided as subkey-value pairs, keyed by the name of the application, e.g.:

- ssh.server
    permit_root_login: no
environment only relevant for nodes, this allows to specify an “environment” into which the node definition is supposed to be place.

Classes files may reside in subdirectories, which act as namespaces. For instance, a class ssh.server will result in the class definition to be read from ssh/server.yml. Specifying just ssh will cause the class data to be read from ssh/init.yml or ssh.yml. Note, however, that only one of those two may be present.

Nodes may also be defined in subdirectories. However, node names (filename) must be unique across all subdirectories, and reclass will exit with an error if a node is defined multiple times. Subdirectories therefore really only exist for the administrator’s local data structuring. They may be used in mappings (see below) to tag additional classes onto nodes.

Data merging

reclass has two modes of operation: node information retrieval and inventory listing. The second is really just a loop of the first across all defined nodes, and needs not be further described.

When retrieving information about a node, reclass first obtains the node definition from the storage backend. Then, it iterates the list of classes defined for the node and recursively asks the storage backend for each class definition (unless already cached).

Next, reclass recursively descends each class, looking at the classes it defines, and so on, until a leaf node is reached, i.e. a class that references no other classes.

Now, the merging starts. At every step, the list of applications and the set of parameters at each level is merged into what has been accumulated so far.

Merging of parameters is done “deeply”, meaning that lists and dictionaries are extended (recursively), rather than replaced. However, a scalar value does overwrite a dictionary or list value. While the scalar could be appended to an existing list, there is no sane default assumption in the context of a dictionary, so this behaviour seems the most logical. Plus, it allows for a dictionary to be erased by overwriting it with the null value.

After all classes (and the classes they reference) have been visited, reclass finally merges the applications list and parameters defined for the node into what has been accumulated during the processing of the classes, and returns the final result.

Wildcard/Regexp mappings

Using the configuration file, it is also possible to provide a list mappings between node names and classes. For instance:

  - \* default
  - /^www\d+/ webserver
  - \*.ch hosted@switzerland another_class_to_show_that_it_can_take_lists

This will assign the default class to all nodes (make sure to escape a leading asterisk (*) to keep YAML happy), webserver to all nodes named www1 or www999, and hosted-in-switzerland to all nodes whose names end with .ch (again, note the escaped leading asterisk). Multiple classes can be assigned to each mapping by providing a space-separated list (class names cannot contain spaces anyway).


The class mappings do not really belong in the configuration file, as they are data, not configuration inmformation. Therefore, they are likely going to move elsewhere, but I have not quite figured out to where. Most likely, there will be an additional file, specified in the configuration file, which then lists the mappings.

Note that mappings are not designed to replace node definitions. Mappings can be used to pre-populate the classes of existing nodes, but you still need to define all nodes (and if only to allow them to be enumerated for the inventory).

The mapped classes can also contain backreferences when regular expressions are used, although they need to be escaped, e.g.:

  - /\.(\S+)$/ tld-\\1

Furthermore, since the outer slashes (‘/’) are used to “quote” the regular expression, any slashes within the regular expression must be escaped. For instance, the following class mapping assigns a subdir-X class to all nodes that are defined in a subdirectory (using yaml_fs):

  - /^([^\/]+)\// subdir-\\1

Parameter interpolation

Parameters may reference each other, including deep references, e.g.:

  location: Munich, Germany
    header: This node sits in ${location}
  for_demonstration: ${motd:header}
  dict_reference: ${motd}

After merging and interpolation, which happens automatically inside the storage modules, the for_demonstration parameter will have a value of “This node sits in Munich, Germany”.

Types are preserved if the value contains nothing but a reference. Hence, the value of dict_reference will actually be a dictionary.

You should now be ready to use reclass!

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