.. _extending_schemas:

============
ASDF schemas
============

ASDF schemas are YAML documents that describe validations to be performed
on tagged objects nested within the ASDF tree or on the tree itself.  Schemas
can validate the presence, datatype, and value of objects and their properties,
and can be combined in different ways to facilitate reuse.

These schemas, though expressed in YAML, are structured according to
the `JSON Schema Draft 4`_ specification.  The excellent `Understanding JSON Schema`_
book is a great place to start for users not already familiar with
JSON Schema.  Just keep in mind that the book includes coverage of later drafts
of the JSON Schema spec, so certain features (constant values, conditional
subschemas, etc) will not be available when writing schemas for ASDF.
The book makes clear which features were introduced after Draft 4.

Anatomy of a schema
===================

Here is an example of an ASDF schema that validates an object with a
numeric value and corresponding unit:

.. code-block:: yaml
    :linenos:

    %YAML 1.1
    ---
    $schema: http://stsci.edu/schemas/yaml-schema/draft-01
    id: asdf://asdf-format.org/core/schemas/quantity-2.0.0

    title: Quantity object containing numeric value and unit
    description: >-
      An object with a numeric value, which may be a scalar
      or an array, and associated unit.

    type: object
    properties:
      value:
        description: A vector of one or more values
        anyOf:
          - type: number
          - tag: tag:stsci.edu:asdf/core/ndarray-1.*
      unit:
        description: The unit corresponding to the values
        tag: tag:stsci.edu:asdf/unit/unit-1.*
      required: [value, unit]
    ...

This is similar to the quantity schema, found :ref:`here <asdf-standard:stsci.edu/asdf/unit/quantity-1.1.0>`, but
has been updated to reflect current recommendations regarding schemas.
Let's walk through this schema line by line.

.. code-block:: yaml
    :linenos:

    %YAML 1.1
    ---

These first two lines form the header of the file.  The ``%YAML 1.1``
indicates that we're following version 1.1 of the YAML spec.  The
``---`` marks the start of a new YAML document.

.. code-block:: yaml
    :lineno-start: 3

    $schema: http://stsci.edu/schemas/yaml-schema/draft-01

The ``$schema`` property contains the URI of the schema that validates
this document.  Since our document is itself a schema, the URI refers to
a *metaschema*.  ASDF comes with two built-in metaschemas:

- ``http://json-schema.org/draft-04/schema`` - The JSON Schema Draft 4 metaschema.
  Includes basic validators and combiners.

- ``http://stsci.edu/schemas/yaml-schema/draft-01`` - The YAML Schema metaschema.
  Includes everything in JSON Schema Draft 4, plus additional YAML-specific
  validators including ``tag`` and ``propertyOrder``.

Using any other metaschema is not recommended as these are generally poorly
supported by jsonschema validators.

Our schema makes use of the ``tag`` validator, so we're specifying the YAML Schema
URI here.

.. code-block:: yaml
    :lineno-start: 4

    id: asdf://asdf-format.org/core/schemas/quantity-2.0.0

The ``id`` property contains the URI that uniquely identifies our schema.  This
URI is how we'll refer to the schema when using the asdf library.

.. code-block:: yaml
    :lineno-start: 6

    title: Quantity object containing numeric value and unit
    description: >-
      An object with a numeric value, which may be a scalar
      or an array, and associated unit.

Title and description are optional (but recommended) documentation properties.
These properties can be placed multiple times at any level of the schema and do
not have an impact on the validation process.

.. code-block:: yaml
    :lineno-start: 11

    type: object

This line invokes the ``type`` validator to check the data type of the
top-level value.  We're asserting that the type must be a YAML mapping,
which in Python is represented as a `dict`.

.. code-block:: yaml
    :lineno-start: 12

    properties:

The ``properties`` validator announces that we'd like to validate certain
named properties of mapping.  If a property is listed here and is present
in the ASDF, it will be validated accordingly.

.. code-block:: yaml
    :lineno-start: 13

      value:
        description: A vector of one or more values

Here we're identifying a property named ``value`` that we'd like to
validate.  The ``description`` is used to add some additional
documentation.

.. code-block:: yaml
    :lineno-start: 15

      anyOf:

The ``anyOf`` validator is one of JSON Schema's combiners.  The ``value``
property will be validated against each of the following subschemas, and
if any validates successfully, the entire ``anyOf`` will be considered
valid.  Other available combiners are ``allOf``, which requires that all
subschemas validate successfully, ``oneOf``, which requires that one and
only one of the subschemas validates, and ``not``, which requires that
a single subschema does *not* validate.

.. code-block:: yaml
    :lineno-start: 16

        - type: number

The first subschema in the list contains a ``type`` validator that
succeeds if the entity assigned to ``value`` is a numeric literal.

.. code-block:: yaml
    :lineno-start: 17

        - tag: tag:stsci.edu:asdf/core/ndarray-1.*

The second subschema contains a ``tag`` validator, which makes an
assertion regarding the YAML tag URI of the object assigned to ``value``.
In this subschema we're requiring a ndarray-1.* tag
which is how n-dimensional arrays are represented in an ASDF tree. The
``*`` is a wildcard allowing this ``tag`` validator to succeed for any
minor or bugfix version of ndarray that has a major version of ``1``.
This means a ndarray-1.0.0 tag will succeed as will ndarray-1.1.0 but
not ndarray-2.0.0.

The net effect of the ``anyOf`` combiner and its two subschemas is:
validate successfully if the ``value`` object is either a numeric
literal or an n-dimensional array.

.. code-block:: yaml
    :lineno-start: 18

      unit:
        description: The unit corresponding to the values
        tag: tag:stsci.edu:asdf/unit/unit-1.*

The ``unit`` property has another bit of documentation and a
``tag`` validator that requires it to be any unit-1.* tagged object.

.. code-block:: yaml
    :lineno-start: 21

    required: [value, unit]

Since the ``properties`` validator does not require the presence of
its listed properties, we need another validator to do that.  The ``required``
validator defines a list of properties that need to be present if validation
is to succeed.

.. code-block:: yaml
    :lineno-start: 21

    ...

Finally, the YAML document end indicator indicates the end of the schema.

Composing schemas with references and tags
==========================================

For checking complex and/or structures it can often be useful to reference
other schemas. In the above example the ``tag`` keyword was used to check
that ``value`` has the ``ndarray`` tag (and consequently is validated against
the ``ndarray`` schema). This is often the most useful way of referencing
other schemas for a few reasons:

- The wildcard allows flexible matching allowing minor and bugfix versions
  of the referenced schema to be released without requiring an update
  of the referring schema.
- Since the ``tag`` validator only checks the tag of the object the
  schema associated with the tag is not reused during validation
  of the referring schema (more on this below). In other words use of
  ``tag`` avoids a duplicate validation of the tagged object.

In some cases schema authors may chose to using an even more flexible
wildcard allowing major version changes (for example ``ndarray-*``).
This is not recommended as a major version change of a tag signifies
a breaking change and increases the likelihood the tagged object will
no longer behave like the old version.

``tag`` does have a few downsides:

- It is a custom validator added by asdf and not part of JSON Schema. If
  the schemas are to be processed by non-asdf tools this might pose a challenge.
- It requires the tagged object have a particular tag (more on this below).

An alternative that doesn't have these downsides is to reference another
schema using a ``$ref``. This is a standard feature of JSON Schema and doesn't
consider the tag of the object. However ``$ref`` has a few downsides:

- When a tagged object is checked with a ``$ref`` the object will be validated
  against the referenced schema twice. Once due to the tag triggering
  validation against the corresponding schema and a second time due to the
  ``$ref``.
- ``$ref`` does not support wildcards and must refer to a specific (down to
  the bugfix) version of a schema. This means that any update to the
  referenced schema will require an update to the referring schema.

Checking schema syntax
======================

The `~asdf.schema.check_schema` function performs basic syntax checks on a schema and
will raise an error if it discovers a problem.  It does not currently accept URIs and
requires that the schema already be loaded into Python objects.  If the schema is already
registered with the asdf library as a resource (see :ref:`extending_resources`), it can
be loaded and checked like this:

.. code-block:: python

    from asdf.schema import load_schema, check_schema

    schema = load_schema("asdf://example.com/example-project/schemas/foo-1.0.0")
    check_schema(schema)

Otherwise, the schema can be loaded using pyyaml directly:

.. code-block:: python

    from asdf.schema import check_schema
    import yaml

    schema = yaml.safe_load(open("/path/to/foo-1.0.0.yaml").read())
    check_schema(schema)

Testing validation
==================

Getting a schema to validate as intended can be a tricky business, so it's helpful
to test validation against some example objects as you go along.  The `~asdf.schema.validate`
function will validate a Python object against a schema:

.. code-block:: python

  from asdf.schema import validate
  import yaml

  schema = yaml.safe_load(open("/path/to/foo-1.0.0.yaml").read())
  obj = {"foo": "bar"}
  validate(obj, schema=schema)

The validate function will return successfully if the object is valid, or raise
an error if not.

.. _testing_custom_schemas:

Testing custom schemas
----------------------

Packages that provide their own schemas can test them using `asdf`'s
:ref:`pytest <pytest:toc>` plugin for schema testing.
Schemas are tested for overall validity, and any examples given within the
schemas are also tested.

The schema tester plugin is automatically registered when the `asdf` package is
installed. In order to enable testing, it is necessary to add the directory
containing your schema files to the pytest section of your project's build configuration
(``pyproject.toml`` or ``setup.cfg``). If you do not already have such a file, creating
one with the following should be sufficient:

.. tab:: pyproject.toml

    .. code-block:: toml

        [tool.pytest.ini_options]
        asdf_schema_root = 'path/to/schemas another/path/to/schemas'

.. tab:: setup.cfg

    .. code-block:: ini

        [tool:pytest]
        asdf_schema_root = path/to/schemas another/path/to/schemas

The schema directory paths should be paths that are relative to the top of the
package directory **when it is installed**. If this is different from the path
in the source directory, then both paths can be used to facilitate in-place
testing (see `asdf`'s own ``pyproject.toml`` for an example of this).

.. note::

   Older versions of `asdf` (prior to 2.4.0) required the plugin to be registered
   in your project's ``conftest.py`` file. As of 2.4.0, the plugin is now
   registered automatically and so this line should be removed from your
   ``conftest.py`` file, unless you need to retain compatibility with older
   versions of `asdf`.

The ``asdf_schema_skip_names`` configuration variable can be used to skip
schema files that live within one of the ``asdf_schema_root`` directories but
should not be tested. The names should be given as simple base file names
(without directory paths or extensions). Again, see `asdf`'s own ``pyproject.toml`` file
for an example.

The schema tests do **not** run by default. In order to enable the tests by
default for your package, add ``asdf_schema_tests_enabled = 'true'`` to the
``[tool.pytest.ini_options]`` section of your ``pyproject.toml`` file (or ``[tool:pytest]`` in ``setup.cfg``).
If you do not wish to enable the schema tests by default, you can add the ``--asdf-tests`` option to
the ``pytest`` command line to enable tests on a per-run basis.

See also:
=========

- `JSON Schema Draft 4 <https://json-schema.org/specification-links.html#draft-4>`_

- `Understanding JSON Schema <https://json-schema.org/understanding-json-schema/>`_

- :ref:`Unit Schemas <asdf-standard:stsci.edu/asdf/unit/quantity-1.1.0>`
