You are developing a business-critical enterprise application. You need to deliver changes rapidly, frequently and reliably - as measured by the DORA metrics - in order for your business to thrive in today’s volatile, uncertain, complex and ambiguous world. Consequently, your engineering organization is organized into small, loosely coupled, cross-functional teams. Each team delivers software using DevOps practices as defined by the DevOps handbook. In particular, it practices continuous deployment. The team delivers a stream of small, frequent changes that are tested by an automated deployment pipeline and deployed into production.
A team is responsible for one or more subdomains. A subdomain is an implementable model of a slice of business functionality, a.k.a. business capability. It consists of business logic, which consists of business entities (a.k.a. DDD aggregates) that implement business rules, and adapters, which communicate with the outside world. A Java-based subdomain, for example, consists of classes organized into packages that’s compiled into a JAR file.
The subdomains implement the application’s behavior, which consists of a set of (system) operations. An operation is invoked in one of three ways: synchronous and asynchronous requests from clients; events published by other applications and services; and the passing of time. It mutates and queries business entities in one or more subdomains.
How to organize the subdomains into one or more deployable/executable components?
There are five dark energy forces:
There are five dark matter forces:
Design an architecture that structures the application as a single deployable/executable component that uses a single database. The component contains all of the application’s subdomains. Since there’s a single component, all operations are local.
Let’s imagine that you are building an e-commerce application that takes orders from customers, verifies inventory and available credit, and ships them. The application consists of several components including the StoreFrontUI, which implements the user interface, along with some backend services for checking credit, maintaining inventory and shipping orders.
The application is deployed as a single monolithic application. For example, a Java web application consists of a single WAR file that runs on a web container such as Tomcat. A Rails application consists of a single directory hierarchy deployed using either, for example, Phusion Passenger on Apache/Nginx or JRuby on Tomcat. You can run multiple instances of the application behind a load balancer in order to scale and improve availability.
This solution has a number of benefits:
This solution has a number of (potential) drawbacks:
These drawbacks become more severe as the application grows in size and complexity and the number of teams developing it increases.
A key challenge with the monolithic architecture is minimizing the potential drawbacks, especially as the application grows in size and complexity and the number of teams developing it increases. There are a few solutions that can reduce some of the drawbacks:
Increase maintainability and team autonomy by modularizing the monolith. Instead of the traditional layered architecture, the subdomains are organized into vertical slices consisting of presentation, business and persistent logic.
Accelerate the deployment pipeline by
The microservice architecture is an alternative pattern that addresses the limitations of the monolithic architecture.
Well known internet services such as Netflix, Amazon.com and eBay initially had a monolithic architecture. Most web applications developed by the author prior to 2012 had a monolithic architecture.
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