152 lines
4.4 KiB
ReStructuredText
152 lines
4.4 KiB
ReStructuredText
..
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Copyright 2017 AT&T Intellectual Property.
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All Rights Reserved.
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Licensed under the Apache License, Version 2.0 (the "License"); you may
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not use this file except in compliance with the License. You may obtain
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a copy of the License at
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http://www.apache.org/licenses/LICENSE-2.0
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Unless required by applicable law or agreed to in writing, software
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distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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License for the specific language governing permissions and limitations
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under the License.
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.. _layering:
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Document Layering
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=================
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Layering provides a restricted data inheritance model intended to help reduce
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duplication in configuration. Documents with different ``schema``'s are never
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layered together (see the :ref:`substitution` section if you need to combine data
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from multiple types of documents).
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Layering is controlled in two places:
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1. The ``LayeringPolicy`` control document (described below), which defines the
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valid layers and their order of precedence.
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2. In the ``metadata.layeringDefinition`` section of normal
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(``metadata.schema=metadata/Document/v1``) documents.
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When rendering a particular document, you resolve the chain of parents upward
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through the layers, and begin working back down each layer rendering at each
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document in the chain.
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When rendering each layer, the parent document is used as the starting point,
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so the entire contents of the parent are brought forward. Then the list of
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`actions` will be applied in order. Supported actions are:
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* ``merge`` - "deep" merge child data at the specified path into the existing data
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* ``replace`` - overwrite existing data with child data at the specified path
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* ``delete`` - remove the existing data at the specified path
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After actions are applied for a given layer, substitutions are applied (see
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the Substitution section for details).
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Selection of document parents is controlled by the ``parentSelector`` field and
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works as follows. A given document, ``C``, that specifies a ``parentSelector``
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will have exactly one parent, ``P``. Document ``P`` will be the highest
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precedence (i.e. part of the lowest layer defined in the ``layerOrder`` list
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from the ``LayeringPolicy``) document that has the labels indicated by the
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``parentSelector`` (and possibly additional labels) from the set of all
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documents of the same ``schema`` as ``C`` that are in layers above the layer ``C``
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is in. For example, consider the following sample documents:
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.. code-block:: yaml
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---
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schema: deckhand/LayeringPolicy/v1
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metadata:
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schema: metadata/Control/v1
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name: layering-policy
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data:
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layerOrder:
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- global
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- region
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- site
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---
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schema: example/Kind/v1
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metadata:
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schema: metadata/Document/v1
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name: global-1234
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labels:
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key1: value1
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layeringDefinition:
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abstract: true
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layer: global
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data:
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a:
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x: 1
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y: 2
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---
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schema: example/Kind/v1
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metadata:
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schema: metadata/Document/v1
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name: region-1234
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labels:
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key1: value1
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layeringDefinition:
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abstract: true
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layer: region
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parentSelector:
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key1: value1
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actions:
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- method: replace
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path: .a
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data:
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a:
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z: 3
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---
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schema: example/Kind/v1
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metadata:
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schema: metadata/Document/v1
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name: site-1234
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layeringDefinition:
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layer: site
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parentSelector:
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key1: value1
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actions:
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- method: merge
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path: .
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data:
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b: 4
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When rendering, the parent chosen for ``site-1234`` will be ``region-1234``,
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since it is the highest precedence document that matches the label selector
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defined by ``parentSelector``, and the parent chosen for ``region-1234`` will be
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``global-1234`` for the same reason. The rendered result for ``site-1234`` would
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be:
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.. code-block:: yaml
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---
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schema: example/Kind/v1
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metadata:
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name: site-1234
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data:
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a:
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z: 3
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b: 4
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If ``region-1234`` were later removed, then the parent chosen for `site-1234`
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would become ``global-1234``, and the rendered result would become:
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.. code-block:: yaml
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---
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schema: example/Kind/v1
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metadata:
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name: site-1234
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data:
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a:
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x: 1
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y: 2
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b: 4
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.. TODO: Add figures for this example, with region present, have site point
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.. with dotted line at global and indicate in caption (or something) that it's
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.. selected for but ignored, because there's a higher-precedence layer to select
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