Glossary

Adapter

In Hexagonal Architecture, an adapter is a component that connects the application’s core (or business logic) to external systems, such as databases, user interfaces, or third-party services. Adapters serve as a bridge between the inner layers of the application and the outside world.

Architecture Patterns With Python

Architecture Patterns With Python: Enabling Test-Driven Development, Domain-Driven Design, and Event-Driven Microservices is a book from Harry J.W. Percival and Bob Gregory that expose design pattern coming from DDD.

Breaking Change

In programming, updating contracts can involve breaking changes. Not every changes are breaker. Breaking changes should be well known and documented by software maintener. Usually, breaking changes are:

• adding a new required field (or marking an optional field required)

• removing required field

• renaming field

• removing or renaming enum values

Non breaking changes are:

• adding a new type of operation (api method, command, event)

• adding an optional field

• adding an enum value

Those rules are used to define when a new version is required. When a new version of an operation is created, a new handler is created for that version, the old one is kept as is, or updated if necessary, reused in case of Event Replay.

casualcms

A CMS Engine https://github.com/mardiros/casualcms/

Command

A command is an object that represent an intention of mutation of the Domain Model. This is an atomic change. Commands are timestamped, ordered, and immutable. In Event Sourcing, we store those commands as the source of truth.

DDD

Domain Driven Design acronym.

Domain Model

In DDD, the domain model represent the layer part of the software that focus on representing the structure of the business rules of the application. In the domain model layer, there is no implementation details involved, neither database storage engine, nor graphical user interface representation.

Domain Driven Design

A software design approach focusing on modeling software to match a domain according to input from that domain’s experts.

The term was coined by Eric Evans in his book of the same title published in 2003.

https://en.wikipedia.org/wiki/Domain-driven_design

Event

An event is an object that represent a mutation of the model has happened. Like commands, Events are timestamped, ordered and immutable. In Event Sourcing, we store those events as the source of truth and we use an Event Stream in order to propagate events to subscribers that can update or react by the reception of an event.

Event Replay

In the Event Sourcing world, we replay all events from an Event Store in the proper order in order to create an application state. We may also replay the event to a specific timestamp in order to reproduce a bug, or to recreate a new database schema and fill out it by replaying those events with appropriate event handlers.

Event Store

A storage backend that save all the events ordered.

Event Sourcing

A software development strategy based on storing model mutations instead of a current model state.

Event Stream

In software development, an event stream is a PUB/SUB component where publisher submit their internal changes publicaly, and subscribers react when receiving those events. Usually most services are publisher and subscriber, they receive events, they update their internal state and publish an event due to the update.

Gandi

A domain name registrar. https://www.gandi.net/

Hexagonal Architecture

An architectural pattern used in software design to create loosely coupled application components that can be easily connected. The testability of hexagonal application is improved by making components exchangeable on any level of abstraction.

The term was coined by Alistair Cockburn in 2005. https://en.wikipedia.org/wiki/Hexagonal_architecture_(software)

Message

A Command or an Event.

Message Bus

The bus of message is the software component that dispatch commands and events to handlers in order to update the Domain Model, using a Unit Of Work.

purgatory

A circuit breaker implementation https://github.com/mardiros/purgatory/

Service Handler

A hook function which is made to update the application state, or to react to a message from the bus. This hook is called by the message bus and has two parameters, a Message, and a Unit Of Work. When the message is received, the service handler will update application state throw the Unit Of Work. Thus, if the message is a Command, the service handler may return an object. For instance a CreateObject can return an Object created by the service handler. Finally, a Service Handler can also create sub message to process, those message will run inside the same transaction of the Unit Of Work.

Ubiquitous Language

In the Domain Driven Design book, Eric introduce the approach by communication issue, where stake holder and developper does not share the same language. When you work as a team, we have to establish the same language for every team player, to get the best communication between people. The vocabulary used to discribe a the business and shared by all the team is named the Ubiquitous Language. This is the first step of the DDD, following design patterns like unit of work is not enough to practice DDD.

Unit Of Work

The unit of work is an object which is responsible to represent a transaction at the business layer. The messagebus https://en.wikipedia.org/wiki/Unit_of_work

Repository

In DDD, a repository is an abstraction to store domain model objects.