Backing Services
================

The various backing services have to be implemented somehow.
For the purpose of this POC implementation,
we are using three services
(PostgreSQL, ElasticMQ and Minio).
These were chosen because they are convenient to run locally
(see the docker compose configurations in the GitHub repository)
They also have directly compatible cloud-hosted alternatives,
which greatly simplifies the "cloud engineering"
required to run the service in a a highly scalable
and highly available way.

.. uml::

   @startuml
   artifact Postgresql {
      database subscriptions
   }
   artifact Minio {
      database message_lake [
         message
	 lake
      ]
      database object_lake [
         object
	 lake
      ]
      database object_acl [
         object
	 ACL
      ]
   }
   artifact ElasticMQ {
      database notifications
      database bc_inbox [
         channel
	 inbox
      ]
      database ch_outbox [
         channel
	 outbox
      ]
      database obj_ret [
         object
	 retrieval
      ]
      database msg_upd [
         message
	 updates
      ]
      database delivery_outbox [
         delivery
	 outbox
      ]
      database reject [
         rejected
	 messages
      ]
      database pend [
         pending
	 deliveries
      ]
   }
   @enduml

What matters about these backing services
is that they are hidden behind abstractions.
These are shown as boundaries
in the API and worker process diagrams
(e.g. object_lake.get_body, object_lake.post_file,
object_acl.search, object_acl.allow_access).

.. uml::

   @startuml
   component c [
      component
   ]
   usecase uc [
      use-case
   ]
   boundary b [
      boundary
   ]
   database db [
      backing
      service
   ]
   c -- uc
   uc -- b
   b -- db
   @enduml

These boundaries deal only in objects
from the domain model,
i.e. the code in `intergov/domain`.
That domain code is independent
of any implementation (backing service).

This intent of this technical detail
is to simplify the implementation of backing services
using whatever technology is most appropriate
for the jurisdiction operating the IGL node.