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Migrating to microservices using a Domain Driven Design

Migrating to Microservices may seem like a daunting task. That is because organizing your reusable components and services is key to a successful implementation of a service-based architecture. Because it is so key, it should be the first step you take in your modern architecture journey.  This is often called Domain Driven Design (DDD).   The good news is that you can start the process by looking at your current monolithic application in terms of components and Domains.  From that perspective you can slowly and successfully see what migrating to microservices will look like.  Domains are critical when you begin to decompose functions into independently deployable services.

To understand this first step in migrating to microservices, lets break down a monolithic into components. If we take a simple web store application, you may have a .jar file, the database, the infrastructure components and environment variable settings.  While the .jar file may be your primary focus, you need to start seeing the database, infrastructure and environment variables as independent components of your overall application. You can build a strategy for managing each of these layers as individual services into a Domain.

Your monolithic Domains may include:

  • Infrastructure Components (java runtime, Tomcat, Environment. Variables)
  • Database and SQL (.sql)
  • The Website (.jar)

To visualize this, we can use a triangle that shows the lowest to highest level of dependencies.  This is a simple organizational method that can be useful for our more complex service-based architectures.

Monolithic Dependencies

Monolithic Dependencies

Migrating Microservices with DeployHub Domains

One of the goals for Domain Driven Design is taking an application and breaking it down into its smaller parts. We then organize those parts in a fashion that will allow reuse.

If we think about the potential microservices for a simple store website, we'll have:

  • the website frontend,
  • a cart,
  • credit card processing,
  • a check out system,
  • advertisements and recommendations,
  • sign-up,
  • login,
  • shipping,
  • taxes.

We start by expanding the triangle. Think about what's most common. Like our monolithic, the lowest level is infrastructure and database. In our service-based architecture, these are followed by credit card processing and the login/sign up. Above that, we may have the cart and on top of that are the more specific pieces such as the ads and recommendations. At the peak of the triangle is the frontend.

Microservice Domains

Microservice Domains 

This triangle shows how we build up from the most common pieces and represents how we would define Domains. Each section of our triangle becomes a Domain.

Domains and Sharing

Say we have another team that's writing a store website, Website B. With DeployHub they find common pieces based on the Domains. Once you define your Domains, your developers can begin publishing and deploying microservices under the Domain structure making it possible for other teams to easily find and then reuse the services. When our next team creates a new store, they begin the process of re-use.

Shared Microservices

Shared Microservices

Between Website A and B we have accomplished a level of reuse for all the shareable layers from the infrastructure to the cart. This allows us to write the login, sign up, the cart, and the credit card processing once. 

The OOPS in Object-Oriented Programming - Let's Not Repeat that Mistake

Object Oriented Programming (OOPS) attempted to achieve a similar level of reuse. Part of the problem of OOPS was the inability for teams to accurately run builds where linking was carefully managed from a central ‘common' location. To solve this problem, developers would add the ‘common' object, like our login service, to their local repository so the build could use it. Their build would always reference that version of the library. Updates to the library would be private. There would be little, if any, contribution of updates to the ‘common' version. This mistake could be easily repeated in microservices. That would be unfortunate and represent a failure in the implementation of a service-based architecture.

The question always is ‘how backward compatible is the new versions?' Are we breaking any signatures on the methods? If we're not breaking any of the interfaces, then can we consume those latest versions? And which application is consuming what version of a service?

For example, website B is going to use v2 of the login and website A will use v3. We need to track which Website is using which version. If you want to deprecate v2, it will require Website B move to v3. What is needed is a method for impact analysis to accurately make these types of decisions. For this reason, microservice configuration management and the ability to organize domains will become increasingly important for microservice development.

For more information, see Steve's presentation on this topic. 

To learn more about containerized infrastructure and cloud native technologies, consider coming to KubeCon + CloudNativeCon NA, November 18-21 in San Diego.


About the Author

Steve Taylor, CTO DeployHub

steve taylor 

Steve is seen as a visionary and industry leader in the area of software development, DevOps process improvement and software build and release automation. Lets just say he was designing continuous delivery pipelines for Fortune 1000 companies before the term 'continuous integration' was ever used.  He is the  master mind behind OpenMake Meister, a build automation solution that has served its customers for over 20 years.  Steve is also the architect of DeployHub, the first microservice sharing platform that also versions and maps microservices and their configurations. Steve is the primary contributor to Ortelius, the open source solution for microservice configuration management.  Steve is expert in all things Kubernetes and Service Mesh.  He is driving the Ortelius feature map to support the new challenges that microservices create, particular to the Kubernetes pipeline. In his free time, Steve is the Chief to the Turquoise Trail Volunteer Fire Department where he serves as a super hero.

Published Thursday, October 10, 2019 7:39 AM by David Marshall
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