docs: add ADR-6 Polling

docs/decisions/0006-polling.rst

@@ -0,0 +1,84 @@
+6. Polling
+==========
+
+Status
+------
+
+Draft
+
+.. After acceptance, becomes, for example:
+
+    Accepted (2023-02-01)
+
+
+Context
+-------
+
+We want frontend-app-payment to listen for an event.
+
+For example, our payment provider Stripe offers `webhooks`_. Stripe webhooks informs our backend when a payment is paid. When our server receives a webhook from Stripe that the payment is paid, we would like frontend-app-payment to know that has happened and forward a learner onwards in their user journey.
+
+.. _webhooks: https://stripe.com/docs/webhooks/stripe-events 
+
+
+Decision
+--------
+
+We will poll the backend for updates for an event.
+
+Consequences
+------------
+
+Polling taxes the backend because it constantly hits the same API.
+
+Rejected Alternatives
+---------------------
+
+Websockets
+~~~~~~~~~~
+
+We do not know of any use of websockets in MFEs at this time.
+
+Synchronous HTTP Requests
+~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Third-Party Services
+~~~~~~~~~~~~~~~~~~~~
+
+We could use a third-party service like `Braze`_, `Amazon Simple Queuing Service (SQS)`_, or `Amazon Simple Notification Service (SNS)`_.
+
+However:
+
+Braze focuses on more narrow use cases like push notifications or in-app messages. It does not provide generic event queuing.
+
+TODO:
+
+SQS does not...
+
+`Braze`_: https://www.braze.com/docs/developer_guide/platform_integration_guides/web
+`Amazon Simple Queuing Service (SQS)`_: https://docs.aws.amazon.com/sdk-for-javascript/v2/developer-guide/sqs-examples-send-receive-messages.html 
+`Amazon Simple Notification Service (SNS)`_:
+
+1. Polling
+2. Websockets
+3. Make Create Order Synchronous
+4. Amazon SQS/SNS
+5. Braze
+
+
+Disadvantage of Websockets and Make Create Order Synchronous is that we must hold one connection open per user who wishes to Retrieve Order Details.
+
+Go with Polling because it is simpler to create from the client-side. We will have to be careful to monitor potentials for race conditions.
+
+To mitigate this, we may want to create a cache in Redis to reject API requests that are still in flight.
+
+Need to make sure everything still works without the cache.
+
+Open question: What happens when there is a state mismatch between the cache and the backend? How will we know to resolve this?
+
+References
+----------
+* `Stripe Custom Actions`_
+* `Stripe PCI Compliance`_
+
+.. _Stripe PCI Compliance: https://stripe.com/docs/security/guide