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Software systems are often built around technology choices, causing business logic to leak into databases and UIs. The result is a tightly coupled architecture that becomes hard to modernize. This document explores how the Hexagonal Architecture pattern helps teams build systems that can absorb changes gracefully, supported by a practical example: executing a money transfer in an online banking system.

Traditionally, monolithic applications run in a single process, use one data store, and run on servers that scale vertically. Modern services run across the network, use more than one data store, scale horizontally, and have independent fault domains. This guide provides guidance for implementing modernization design patterns that let you add new features, and migrate existing ones without disrupting the business.

Messaging systems are great at handling a flood of tiny messages. But the moment you push a large message through them, you pay the price. Messaging systems often charge a premium for larger message sizes. By reducing payload size in the bus, you can use cheaper tiers. Plus, smaller messages mean quicker consumers.

Every system today wants to embed a chat interface and benefit from LLMs. Employees want to chat with SharePoint. The intranet link is often lost. They just want to ask questions like: “What holiday do we have next?” or “What’s our bonus policy?”. This paper explains how to do that safely, using the Federated Identity and On-Behalf-Of (OBO) patterns, combined with an LLM.

By applying distributed processing, you can scale consumers out or in based on queue depth, even down to zero when idle. Consumers are treated as replicas. If one instance fails, others continue processing messages, ensuring your system remains available under stress.

Building your own identity system is hard. Users typically need to work with multiple applications hosted by different organizations they have a business relationship with. This pattern simplifies user management, strengthens security, and enables single sign-on (SSO) across multiple applications.

There are people that want to compromise your system. They can connect to your services if they are exposed to the internet. Adding a gateway into the request flow enables you to centralize security functionality like web application firewalls, DDoS protection, bot detection, request manipulation, authentication initiation, and authorization checks.