========================
GraphQL Schema Stitching
========================

Schema stitching is the process of creating a single GraphQL schema from multiple underlying GraphQL APIs.

If you need to add custom business logic or customize your GraphQL schema then we recommend using schema stitching.

Here are 2 common use cases:

- Fetch data from sources that are not in the database (eg: a weather API)
- Customize mutations (eg: running validations before inserts)

.. note::

  If you are looking for ``graphql-bindings``, please check out `this git repository
  <https://github.com/hasura/generate-graphql-bindings>`_.

Schema stitching allows you to have one unified API that allows the client to query multiple GraphQL Schemas at the
same time, including relations between the schemas.

.. image:: ../../../img/graphql/manual/schema/graphql-schema-stitching.png
    :scale: 50%

In the above architecture, we see that there are multiple decoupled graphql services running somewhere and a central
server acts as a GraphQL Proxy server and it combines the different schemas into a unified API that the client can
query on.

Let's go through a simple example to understand why schema stitching might be required in certain use cases and how
it can be leveraged to unify all your APIs to a single GraphQL API.

Assume the following database schema in PostgreSQL:

+----------------------------------------+----------------------------------------+
|Table                                   |Columns                                 |
+========================================+========================================+
|person                                  |id, name, city                          |
+----------------------------------------+----------------------------------------+

We have a simple ``person`` table with columns ``id``, ``name`` and ``city``. For this example, the above table has
anonymous select permission.

The GraphQL query in Hasura Data API for the above table would look like:

.. code-block:: graphql

    query fetch_person {
      person {
        id
        name
        city
      }
    }

This is a simple select on table person.

On the other hand, we have a GraphQL server for fetching weather information that connects to ``Meta Weather API``.

The GraphQL schema for this weather API looks like:

.. code-block:: graphql

    type CityWeather {
      temp: String
      min_temp: String
      max_temp: String
      city_name: String!
      applicable_date: String!
    }

.. code-block:: graphql

    type Query {
      cityWeather(city_name: String! applicable_date: String): CityWeather
    }

The GraphQL query to fetch this weather information would look like:

.. code-block:: graphql

    query {
      cityWeather (city_name: "Bangalore") {
        city_name
        temp
        min_temp
        max_temp
        applicable_date
      }
    }

Explore this API on `Apollo LaunchPad <https://launchpad.graphql.com/nxw8w0z9q7>`_.

Note the usage of ``city_name`` as an argument for the ``cityWeather`` query. Using this we can extend our original
Postgres's ``person`` schema to include weather information based on the ``city`` column of the person table.

.. code-block:: graphql

    extend type person {
      city_weather: CityWeather,
    }

We have extended the type person to have one more field called ``city_weather``. This will resolve to the weather
schema defined above and the respective resolver will return appropriate data.

The source code for the custom resolver can be found on GitHub - `graphql-schema-stitching-demo
<https://github.com/hasura/graphql-schema-stitching-demo>`_. Note the usage of ``mergeSchemas``, a
``graphql-tools`` utility that enables schema stitching.

Now the merged schema can be queried as:

.. code-block:: graphql

    query {
      person {
        id
        name
        city
        city_weather {
          city_name
          temp
          min_temp
          max_temp
          applicable_date
        }
      }
    }

This is a neat abstraction for the client making the GraphQL API, as all the merging of different schemas are
done by the server and exposed as a single API.

Read the official docs on `schema stitching <https://www.apollographql.com/docs/graphql-tools/schema-stitching.html>`_
by Apollo for detailed guides.