> ## Documentation Index
> Fetch the complete documentation index at: https://docs.esakrissa.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Introduction

> Architecture design for a high-scale, Cognito-authenticated User Service

## Overview

This document presents the architecture design for a **User Service** built on the AWS Serverless Framework. The service handles CRUD operations on related **User** and **Email** entities, using **DynamoDB** with a single-table design pattern and **AWS Cognito** for authentication.

<CardGroup cols={2}>
  <Card title="Architecture" icon="sitemap" href="/architecture">
    System components and how they interact
  </Card>

  <Card title="Data Model" icon="database" href="/data-model">
    DynamoDB single-table design and access patterns
  </Card>

  <Card title="Authentication" icon="lock" href="/authentication">
    Cognito integration and JWT token flow
  </Card>

  <Card title="API Design" icon="code" href="/api-design">
    Endpoints, validation, and edge cases
  </Card>
</CardGroup>

## Key Requirements

| Requirement    | Solution                         |
| -------------- | -------------------------------- |
| Authentication | AWS Cognito with JWT tokens      |
| Data Storage   | DynamoDB single-table design     |
| API Layer      | API Gateway + Lambda             |
| Inter-Service  | EventBridge pub/sub              |
| Infrastructure | Serverless Framework + Terraform |

## Design Principles

<AccordionGroup>
  <Accordion title="Type Safety First">
    Every function, handler, and data model is strongly typed. No `any` types. Generics for reusable patterns. Type inference for cleaner consumer code.
  </Accordion>

  <Accordion title="Single-Table DynamoDB">
    User and Email entities in one table. Designed for access patterns, not normalized relations. Atomic transactions where needed.
  </Accordion>

  <Accordion title="Event-Driven Communication">
    Loose coupling via EventBridge. Services react to events, not direct calls. Dead Letter Queues for failure handling.
  </Accordion>

  <Accordion title="Defense in Depth">
    JWT validation at API Gateway. User ID from token claims only. Rate limiting on sensitive operations.
  </Accordion>
</AccordionGroup>

## Document Structure

| Section                           | What You'll Learn                                   |
| --------------------------------- | --------------------------------------------------- |
| [Architecture](/architecture)     | High-level system design and component interactions |
| [Data Model](/data-model)         | DynamoDB table schema, GSIs, and access patterns    |
| [Authentication](/authentication) | Cognito setup, token flow, and authorization        |
| [API Design](/api-design)         | Endpoints, request/response schemas, edge cases     |
| [Inter-Service](/inter-service)   | EventBridge events, consumers, failure handling     |
| [DevOps](/devops)                 | CI/CD, monitoring, scaling, infrastructure as code  |
| [Hard Parts](/hard-parts)         | Technical challenges and their solutions            |

***

## Day 1 Scope

This design is time-boxed to **a single day of implementation**. Below is what would be delivered versus what's deferred.

<CardGroup cols={2}>
  <Card title="Day 1 Deliverables" icon="check">
    * User CRUD endpoints (`/users/me`)
    * Single primary email per user
    * Cognito integration with post-confirmation trigger
    * Basic EventBridge publishing (`user.created`, `user.deleted`)
    * Terraform: Cognito User Pool, DynamoDB table, EventBridge bus
    * Serverless Framework: Lambda handlers, API Gateway with authorizer
    * Manual deployment to staging
  </Card>

  <Card title="Deferred to Future Iterations" icon="clock">
    * Multiple emails per user with verification flow
    * Transactional outbox pattern (accept at-least-once initially)
    * Full CI/CD pipeline with integration tests
    * Comprehensive CloudWatch dashboard
    * Provisioned concurrency optimization
    * Admin endpoints (`GET /users/{userId}`)
  </Card>
</CardGroup>

### Trade-off Rationale

| Decision       | Day 1 Approach                   | Full Implementation              | Why Defer?                                          |
| -------------- | -------------------------------- | -------------------------------- | --------------------------------------------------- |
| Multi-email    | Single email only                | Full CRUD with verification      | Reduces transaction complexity                      |
| Event delivery | Direct publish (may lose events) | Outbox pattern                   | Acceptable for MVP; add if reliability issues arise |
| CI/CD          | Manual `serverless deploy`       | GitHub Actions with staging/prod | Pipeline setup takes 2-3 hours                      |
| Monitoring     | CloudWatch Logs + basic alarms   | Full dashboard + X-Ray           | Can add incrementally                               |

<Tip>
  The goal is a **working, deployable service** by end of day. Complexity is added only when it solves a real problem, not preemptively.
</Tip>