Escaping of References and Inventory Queries
--------------------------------------------

Reference and inventory queries can be escaped to produce literal strings, for example:

.. code-block:: yaml

  parameters:
    colour: Blue
    unescaped: The colour is ${colour}
    escaped: The colour is \${colour}
    double_escaped: The colour is \\${colour}


This would produce:

.. code-block:: yaml

  parameters:
    colour: Blue
    unescaped: The colour is Blue
    escaped: The colour is ${colour}
    double_escaped: The colour is \Blue


Ignore class not found
----------------------

At some cases (bootstrapping, development) it can be convenient to ignore some missing classes.
To control the feature there are two options available:

.. code-block:: yaml

  ignore_class_notfound: False
  ignore_class_regexp: ['.*']

If you set regexp pattern to ``service.*`` all missing classes starting 'service.' will be logged with warning, but will not
fail to return rendered reclass. Assuming all parameter interpolation passes.


Merging Referenced Lists and Dictionaries
-----------------------------------------

Referenced lists or dicts can now be merged:

.. code-block:: yaml

  # nodes/test.yml
  classes:
    - test1
    - test2
  parameters:
    one:
      a: 1
      b: 2
    two:
      c: 3
      d: 4
    three:
      e: 5

  # classes/test1.yml
  parameters:
    three: ${one}

  # classes/test2.yml
  parameters:
    three: ${two}

``running reclass.py --nodeinfo node1`` then gives:

.. code-block:: yaml

  parameters:
    one:
      a: 1
      b: 2
    three:
      a: 1
      b: 2
      c: 3
      d: 4
      e: 5
    two:
      c: 3
      d: 4

This first sets the parameter three to the value of parameter one (class test1) then merges parameter two into
parameter three (class test2) and finally merges the parameter three definition given in the node definition into
the final value.


Allow override list and dicts by empty entity,None instead of merge
-------------------------------------------------------------------

With settings:

.. code-block:: yaml

  allow_none_override: True       # default True

  # note dict,list over None is allowed and not configurable

Referenced lists or dicts can now be overriden by None or empty type of dict, list:

.. code-block:: yaml

  # nodes/test.yml
  parameters:
    one:
      a: 1
      b: 2
    two: {}
    three: None

  # classes/test1.yml
  parameters:
    one: ${two}

  # classes/test2.yml
  parameters:
    three: ${one}


Nested References
-----------------

References can now be nested, for example:

.. code-block:: yaml

  # nodes/node1.yml
  parameters:
    alpha:
      one: ${beta:${alpha:two}}
      two: a
    beta:
      a: 99

``reclass.py --nodeinfo node1`` then gives:

.. code-block:: yaml

  parameters:
    alpha:
      one: 99
      two: a
    beta:
      a: 99

The ``${beta:${alpha:two}}`` construct first resolves the ``${alpha:two}`` reference to the value 'a', then resolves
the reference ``${beta:a}`` to the value 99.


Ignore overwritten missing references
-------------------------

Given the following classes:

.. code-block:: yaml
  # node1.yml
  classes:
  - class1
  - class2
  - class3

  # class1.yml
  parameters:
    a: ${x}

  # class2.yml
  parameters:
    a: ${y}

  # class3.yml
  parameters:
    y: 1

  
The parameter ``a`` only depends on the parameter ``y`` through the reference set in class2. The fact that the parameter ``x`` referenced
in class1 is not defined does not affect the final value of the parameter ``a``. For such overwritten missing references by default a warning is
printed but no error is raised, providing the final value of the parameter being evaluated is a scalar. If the final value is a dictionary or list
an error will always be raised in the case of a missing reference.

Default value is True to keep backward compatible behavior.

.. code-block:: yaml

  ignore_overwritten_missing_reference: True


Print summary of missed references
----------------------------------

Instead of failing on the first undefinded reference error all missing reference errors are printed at once.

.. code-block:: yaml
  reclass --nodeinfo mynode
  -> dontpanic
     Cannot resolve ${_param:kkk}, at mkkek3:tree:to:fail, in yaml_fs:///test/classes/third.yml
     Cannot resolve ${_param:kkk}, at mkkek3:tree:another:xxxx, in yaml_fs:///test/classes/third.yml
     Cannot resolve ${_param:kkk}, at mykey2:tree:to:fail, in yaml_fs:///test/classes/third.yml

.. code-block:: yaml

  group_errors: True


Use references in class names
-----------------------------

Allows to use references in the class names.

References pointed to in class names cannot themselves reference another key, they should be simple strings.

To avoid pitfalls do not over-engineer your class references. They should be used only for core conditions and only for them.
A short example: `- system.wrodpress.db.${_class:database_backend}`.

Best practices:
- use references in class names always load your global class specification prior the reference is used.
- structure your class references under parameters under one key (for example `_class`).
- use class references as a kind of "context" or "global" available options you always know what they are set.

Class referencing for existing reclass users. Frequently when constructing your models you had to load or not load some
classes based on your setup. In most cases this lead to fork of a model or introducing kind of template generator (like cookiecutter) to
create a model based on the base "context" or "global" variables. Class referencing is a simple way how to avoid
"pre-processors" like this and if/else conditions around class section.


Assuming following class setup:

* node is loading `third.yml` class only


Classes:

.. code-block:: yaml
  #/etc/reclass/classes/global.yml
  parameters:
    _class:
      env:
        override: 'env.dev'
    lab:
      name: default

  #/etc/reclass/classes/lab/env/dev.yml
  parameters:
    lab:
      name: dev

  #/etc/reclass/classes/second.yml
  classes:
    - global
    - lab.${_class:env:override}

  #/etc/reclass/classes/third.yml
  classes:
    - global
    - second


Reclass --nodeinfo then returns:

.. code-block:: yaml

  ...
  ...
  applications: []
  environment: base
  exports: {}
  classes:
  - global
  - lab.${_class:env:override}
  - second
  parameters:
    _class:
      env:
        override: env.dev
    lab:
      name: dev
    ...
    ...


Inventory Queries
-----------------

Inventory querying works using a new key type - exports to hold values which other node definitions can read using a $[] query, for example with:

.. code-block:: yaml

  # nodes/node1.yml
  exports:
    test_zero: 0
    test_one:
      name: ${name}
      value: 6
    test_two: ${dict}

  parameters:
    name: node1
    dict:
      a: 1
      b: 2
    exp_value_test: $[ exports:test_two ]
    exp_if_test0: $[ if exports:test_zero == 0 ]
    exp_if_test1: $[ exports:test_one if exports:test_one:value == 7 ]
    exp_if_test2: $[ exports:test_one if exports:test_one:name == self:name ]

  # nodes/node2.yml
  exports:
    test_zero: 0
    test_one:
      name: ${name}
      value: 7
    test_two: ${dict}

  parameters:
    name: node2
    dict:
      a: 11
      b: 22


``running reclass.py --nodeinfo node1``  gives (listing only the exports and parameters):

.. code-block:: yaml

  exports:
    test_one:
      name: node1
      value: 6
    test_two:
      a: 1
      b: 2
  parameters:
    dict:
      a: 1
      b: 2
    exp_if_test0:
      - node1
      - node2
    exp_if_test1:
      node2:
        name: node2
        value: 7
    exp_if_test2:
      node1:
        name: node1
        value: 6
    exp_value_test:
      node1:
        a: 1
        b: 2
      node2:
        a: 11
        b: 22
    name: node1


Exports defined for a node can be a simple value or a reference to a parameter in the node definition.
The ``$[]`` inventory queries are calculated for simple value expressions, ``$[ exports:key ]``, by returning
a dictionary with an element (``{ node_name: key value }``) for each node which defines 'key' in the exports
section. For tests with a preceeding value, ``$[ exports:key if exports:test_key == test_value ]``, the
element (``{ node_name: key value }``) is only added to the returned dictionary if the test_key defined in
the node exports section equals the test value. For tests without a preceeding value,
``$[ if exports:test_key == test_value ]``, a list of nodes which pass the test is returned. For either test
form the test value can either be a simple value or a node parameter. And as well as an equality test
a not equals test (``!=``) can also be used.


**Inventory query options**

By default inventory queries only look at nodes in the same environment as the querying node. This can be
overriden using the +AllEnvs option:

.. code-block:: yaml

  $[ +AllEnvs exports:test ]

Any errors in rendering the export parameters for a node will give an error for the inventory query as a whole.
This can be overriden using the ``+IgnoreErrors`` option:

.. code-block:: yaml

  $[ +IgnoreErrors exports:test ]

With the ``+IgnoreErrors`` option nodes which generate an error evaluating ``exports:test`` will be ignored.

Inventory query options can be combined:

.. code-block:: yaml

  $[ +AllEnvs +IgnoreErrors exports:test ]

**Logical operators and/or**

The logical operators and/or can be used in inventory queries:

.. code-block:: yaml

  $[ exports:test_value if exports:test_zero == 0 and exports:test_one == self:value ]

The individual elements of the if statement are evaluated and combined with the logical operators starting from the
left and working to the right.


**Inventory query example**

Defining a cluster of machines using an inventory query, for example to open access to a database server to a
group of nodes. Given exports/parameters for nodes of the form:

.. code-block:: yaml

  # for all nodes requiring access to the database server
    exports:
      host:
        ip_address: aaa.bbb.ccc.ddd
      cluster: _some_cluster_name_

.. code-block:: yaml

  # for the database server
  parameters:
    cluster_name: production-cluster
    postgresql:
      server:
        clients: $[ exports:host:ip_address if exports:cluster == self:cluster_name ]

This will generate a dictionary with an entry for node where the ``export:cluster`` key for a node is equal to the
``parameter:cluster_name`` key of the node on which the inventory query is run on. Each entry in the generated dictionary
will contain the value of the ``exports:host:ip_address`` key. The output dictionary (depending on node definitions)
would look like:

.. code-block:: yaml

  node1:
    ip_address: aaa.bbb.ccc.ddd
  node2:
    ip_address: www.xxx.yyy.zzz

For nodes where exports:cluster key is not defined or where the key is not equal to self:cluster_name no entry is made
in the output dictionary.

In practise the exports:cluster key can be set using a parameter reference:

.. code-block:: yaml

  exports:
    cluster: ${cluster_name}
  parameters:
    cluster_name: production-cluster

The above exports and parameter definitions could be put into a separate class and then included by nodes which require
access to the database and included by the database server as well.