Schema Contract
===============

**2 probes**

Validates dataset schemas against a declared YAML contract and detects
schema drift over time.  Contracts specify column names, data types,
nullable flags, and ordering.  Drift detection compares the live schema
against the last known baseline and emits ``CheckResult`` alerts for
added, removed, or type-changed columns.

Configuration
-------------

.. code-block:: yaml

   sources:
     - type: schema_contract
       dataset_path: /data/curated/orders.parquet
       contract: /etc/metapod/contracts/orders.yaml
       fail_on_drift: true

Contract file example:

.. code-block:: yaml

   # /etc/metapod/contracts/orders.yaml
   dataset: orders
   columns:
     - name: order_id
       type: int64
       nullable: false
     - name: customer_email
       type: string
       nullable: true
     - name: amount
       type: float64
       nullable: false

Probes
------

.. list-table::
   :header-rows: 1
   :widths: 40 60

   * - Probe
     - Description
   * - ``schema_contract.quality.validate``
     - Validate live schema against YAML contract (missing/extra/mistyped columns)
   * - ``schema_contract.quality.drift``
     - Detect schema drift vs. last recorded baseline

Usage
-----

.. code-block:: bash

   metapod run "schema_contract.quality.*" --config config.yaml

Output
------

.. code-block:: json

   {
     "probe": "schema_contract.quality.validate",
     "source": "schema_contract",
     "collected_at": "2026-03-31T08:00:00Z",
     "results": [
       {
         "dataset": "orders.parquet",
         "contract": "orders.yaml",
         "status": "fail",
         "violations": [
           {
             "column": "amount",
             "expected_type": "float64",
             "actual_type": "string",
             "issue": "type_mismatch"
           }
         ]
       }
     ]
   }