Add ADR for removing forms-api app

Author: lfdebrux

ADR/ADRXXX-remove-forms-api.md

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+# ADRxxx: Remove forms-api
+
+Date: 2025-08-19
+
+## Status
+
+> Accepted or Rejected
+>
+> Valid ADR statuses are: "Proposed", "Accepted", "Rejected", "Deprecated", "Superseded by ADRzzz", but since we use the GitHub Pull request mechanism to approve ADRs, "Proposed" isn't applicable.
+
+## Context
+
+In [ADR004: Store form schemas in an API] we decided to implement a separate API app so that we could scale the services required for form submissions separately to the services required for building forms.
+
+However, maintaining multiple apps has resulted in increased toil. Having multiple services has also resulted in challenges around designing and maintaining the interface between forms-admin and forms-api.
+
+As the cost of compute is low at the moment, the cost of the additional developer time outweighs any savings due to efficient capacity management.
+
+We are currently refactoring forms-admin and forms-api to work with the v2 API described in [ADR034: API v2 for Forms]. We have an opportunity now as part of this work to make a large architectural change, namely using the forms-admin app to serve the v2 API endpoints needed for forms-runner, and removing the need for forms-api.
+
+[ADR004: Store form schemas in an API]: ADR004-store-form-schemas-in-an-api.md
+[ADR034: API v2 for Forms]: ADR034-api-v2-for-forms.md
+
+## Decision
+
+We will add endpoints to forms-admin that can be used to retrieve v2 API form documents in JSON format.
+
+We will change forms-runner to read form documents from these new endpoints in forms-admin.
+
+We will keep the new endpoints in forms-admin accessible from the private VPC only (keep them not accessible from the public internet), and not require authentication for their use.
+
+We will retire the forms-api app and remove it, its database, and associated systems, networks and configuration files.
+
+## Consequences
+
+Positives:
+- We will have fewer separate apps to maintain
+
+  For each app we have to maintain we have additional maintenance workload,
+  including regular security patches, keeping files common to all repositories
+  in sync, and other causes of toil. Fewer apps means less toil for developers.
+
+Negatives:
+- forms-admin will be in the critical path for submitting a form
+
+  The system will be less resilient (all other things being equal) because an
+  issue in forms-admin could cause form submissions to fail. We could look at
+  ways to mitigate this loss of resilience if it is thought to be necessary.
+
+- We won't have an external API for retrieving form documents
+
+  This will make it harder for developers to debug and/or investigate the
+  production system, but we have found that with the current API it is not
+  common for developers to make API requests directly to forms-api. If we find
+  that there is a need we can consider adding an external API with access
+  control.
+
+### Consequences for costs and resource usage
+
+This change is likely to be resource-neutral, in the best case it may use
+slightly less resources, but it is more likely it will increase our resource
+usage. Currently we have fewer forms-admin instances than forms-api instances
+running in production, but each forms-api instance uses more compute. It is
+difficult to predict what the right number of instances for the forms-admin app
+including API endpoints would be, we will probably over-provision initially and
+optimise resource usage later.
+
+In terms of costs we are currently on a [compute savings plan] that has pay a
+fixed annual cost for a pre-agreed amount of compute, so we may not see an
+increase of cost.
+
+In terms of CO2 usage our AWS systems are currently nominally carbon-neutral,
+as Amazon has "purchased renewable energy that compensated all or part of the
+emissions associated with [our] usage", and the change in this ADR is unlikely
+to increase our emissions beyond the point of being carbon-neutral.
+
+If we do find that we need to improve the scaling properties of the system, we
+could consider cache layers between forms-admin and forms-runner rather than a
+full API.
+
+[compute savings plan]: https://aws.amazon.com/savingsplans/compute-pricing/