Step-by-step guide for converting Spring Boot Cassandra applications to use Azure Cosmos DB with Spring Data Cosmos

# Comprehensive Guide: Converting Spring Boot Cassandra Applications to use Azure Cosmos DB with Spring Data Cosmos (spring-data-cosmos)

## Applicability

This guide applies to:
- ✅ Spring Boot 2.x - 3.x applications (both reactive and non-reactive)
- ✅ Maven and Gradle-based projects
- ✅ Applications using Spring Data Cassandra, Cassandra DAOs, or DataStax drivers
- ✅ Projects with or without Lombok
- ✅ UUID-based or String-based entity identifiers
- ✅ Both synchronous and reactive (Spring WebFlux) applications

This guide does NOT cover:
- ❌ Non-Spring frameworks (Jakarta EE, Micronaut, Quarkus, plain Java)
- ❌ Complex Cassandra features (materialized views, UDTs, counters, custom types)
- ❌ Bulk data migration (code conversion only - data must be migrated separately)
- ❌ Cassandra-specific features like lightweight transactions (LWT) or batch operations across partitions

## Overview

This guide provides step-by-step instructions for converting reactive Spring Boot applications from Apache Cassandra to Azure Cosmos DB using Spring Data Cosmos. It covers all major issues encountered and their solutions, based on real-world conversion experience.

## Prerequisites

- Java 11 or higher (Java 17+ required for Spring Boot 3.x)