create decision record for regionally resilient infra (#20)

.adr-dir

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+docs/architecture/decisions

docs/architecture/decisions/0001-record-architecture-decisions.md

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+# 1. Record architecture decisions
+
+Date: 2022-08-05
+
+## Status
+
+Accepted
+
+## Context
+
+We need to record the architectural decisions made on this project.
+
+## Decision
+
+We will use Architecture Decision Records, as [described by Michael Nygard](http://thinkrelevance.com/blog/2011/11/15/documenting-architecture-decisions).
+
+## Consequences
+
+See Michael Nygard's article, linked above. For a lightweight ADR toolset, see Nat Pryce's [adr-tools](https://github.com/npryce/adr-tools).

docs/architecture/decisions/0002-regionally-resiliant-infrastructure.md

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+# 2. Regionally resiliant infrastructure
+
+Date: 2022-08-05
+
+## Status
+
+Accepted
+
+## Context
+
+The issue motivating this decision, and any context that influences or constrains the decision.
+
+Disaster recovery and global high availability are difficult to introduce to infrastructure later, and slow to achieve when not considered first.
+
+We know already that the Modernising LPA service will need to meet short RTO and RPO objectives.
+
+## Decision
+
+We will design infrastructure in a way that enables disaster recovery and global high availability strategies.
+
+Infrastructure will be organsised in terraform as regional and global at a per-account and per-environment level.
+
+Modules will be used to define a region Resources within a region will be defined as sub-modules also.
+
+## Consequences
+
+Infrastructure will be easier to implement in a way that enables disaster recovery planning and high availability with RTO and RPO times counted in minutes
+
+Infrastructure will be easy to replcicate across regions, with shared global resources between each region.
+
+For example the account terraform configuration will have a structure like this
+
+```shell
+.
+├── region
+│   ├── modules
+│   │   └── certificates
+│   │       ├── main.tf
+│   │       └── terraform.tf
+│   ├── certificates.tf
+│   ├── network.tf
+│   ├── terraform.tf
+│   └── variables.tf
+├── README.md
+├── regions.tf
+├── kms.tf
+├── terraform.tf
+```
+
+Regions.tf will instatiate the /region module for each AWS region required.
+
+Rsources inside /region will be grouped as modules also, allowing for parts of a region to be replicated as and when needed.
+
+This will allow us to deploy the service in a way that is globally resiliant, and highly available.
+
+Global resources such as IAM roles, or Route53 records will be created in a global region.
+
+Where a resource supports regional replication such as DynamoDB tables, of KMS keys, they will exist in the global layer.