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107 lines
4.6 KiB
Plaintext
107 lines
4.6 KiB
Plaintext
Custom business logic
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=====================
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.. contents:: Table of contents
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:backlinks: none
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:depth: 1
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:local:
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For the backends of most apps, you may have to implement custom business logic to complement the CRUD and
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real-time API provided by GraphQL Engine. Depending on the nature of the use case and its position vis-a-vis
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GraphQL Engine/Postgres, different avenues are recommended for introducing such business logic in your app's backend:
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- **Pre-CRUD**: :ref:`remote-schemas`
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- **Post-CRUD**: :ref:`event-triggers`
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- :ref:`derived-data`
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.. thumbnail:: ../../../img/graphql/manual/business-logic/custom-business-logic.png
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.. _remote-schemas:
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Custom resolvers in remote schemas
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----------------------------------
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Merging remote schemas is ideal for adding "pre-CRUD" business logic (*logic to be run before you invoke
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GraphQL Engine's GraphQL API to insert/modify data in Postgres*) or custom business logic that is not part of
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your GraphQL Engine schema. Here are some use-cases where remote schemas are ideal:
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- Customizing mutations (e.g. running validations before inserts)
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- Supporting features like payments, etc. and providing a consistent interface to access them i.e. behind the
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GraphQL Engine’s API
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- Fetching disparate data from other sources (e.g. from a weather API or another database)
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To support these kinds of business logic, a custom GraphQL schema with resolvers that implement said business
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logic is needed (*see link below for boilerplates*). This remote schema can then be merged with GraphQL Engine's
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schema using the console. Here's a reference architecture diagram for such a setup:
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.. thumbnail:: ../../../img/graphql/manual/schema/schema-stitching-v1-arch-diagram.png
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For more details, links to boilerplates for custom GraphQL servers, etc. please head to :doc:`../remote-schemas/index`.
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.. _event-triggers:
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Asynchronous business logic / Events triggers
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---------------------------------------------
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"post-CRUD" business logic (*follow up logic to be run after GraphQL Engine's GraphQL API has been used to insert
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or modify data in Postgres*) typically tends to be asynchronous, stateless and is triggered on changes to data
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relevant to each use case. E.g. for every new user in your database, you may want to send out a notification. This
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business logic is triggered for every new row in your ``users`` table.
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GraphQL Engine comes with built-in events triggers on tables in the Postgres database. These triggers capture events
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on specified tables and then invoke configured webhooks, which contain your business logic.
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If your business logic is stateful, it can even store its state back in the Postgres instance configured to work
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with GraphQL Engine, allowing your frontend app to offer a reactive user experience, where the app uses GraphQL
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subscriptions to listen to updates from your webhook via Postgres.
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.. thumbnail:: ../../../img/graphql/manual/event-triggers/database-event-triggers.png
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Event triggers are ideal for use cases such as the following:
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- Notifications: Trigger push notifications and emails based on database events
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- ETL: Transform and load data into external data-stores.
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- E.g. transform data from Postgres and populate an Algolia index when a product is inserted, updated or deleted.
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- Long-running business logic:
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- Provision some infrastructure
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- Process multimedia files
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- Background jobs
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- Cache/CDN purge: invalidate/update entries in your cache/CDN when the underlying data in Postgres changes.
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For more information on event triggers and how to set them up, please see :doc:`../event-triggers/index`.
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.. _derived-data:
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Derived data / Data transformations
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-----------------------------------
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For some use cases, you may want to transform your data in Postgres or run some predetermined function on it to
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derive another dataset (*that will be queried using GraphQL Engine*). E.g. let's say you store each user's location
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data in the database as a ``point`` type. You are interested in calculating the distance (*say the haversine distance*)
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between each set of two users i.e. you want this derived dataset:
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.. list-table::
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:header-rows: 1
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* - user_id_1
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- user_id_2
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- distance between users
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* - 12
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- 23
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- 10.50
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* - 12
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- 47
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- 76.00
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The easiest way to handle these kinds of use cases is to create a view, which encapsulates your business logic
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(*in our example, calculating the distance between any two users*), and query your derived/transformed data as you
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would a table using GraphQL Engine (*with permissions defined explicitly for your view if needed*).
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For more information on how to do this, please see :doc:`../queries/derived-data`.
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