Dogma

Build message-based applications in Go.

Overview

Dogma is a toolkit for building message-based applications in Go.

In Dogma, an application implements business logic by consuming and producing messages . The application is strictly separated from the engine, which handles message delivery and data persistence.

Features

• Built for Domain Driven Design: The API uses DDD terminology to help developers align their understanding of the application's business logic with its implementation.

• Flexible message format: Supports any Go type that can be serialized as a byte slice, with built-in support for JSON and Protocol Buffers.

• First-class testing: Dogma's testkit module runs isolated behavioral tests of your application.

• Engine-agnostic applications: Choose the engine with the best messaging and persistence semantics for your application.

• Built-in introspection: Analyze application code to visualize how messages traverse your applications.

Related repositories

• testkit: Utilities for black-box testing of Dogma applications.
• projectionkit: Utilities for building projections in various popular database systems.
• example: An example Dogma application that implements basic banking features.

Concepts

Dogma leans heavily on the concepts of Domain Driven Design. It's designed to provide a suitable platform for applications that make use of design patterns such as Command/Query Responsibility Segregation (CQRS), Event Sourcing and Eventual Consistency.

The following concepts are core to Dogma's design, and should be well understood by any developer wishing to build an application:

• Message
• Message handler
• Application
• Engine
• Aggregate
• Process
• Integration
• Projection

Message

A message is a data structure that represents a command, event or timeout within an application.

A command is a request to make a single atomic change to the application's state. An event indicates that the state has changed in some way. A single command can produce any number of events, including zero.

A timeout helps model business logic that depends on the passage of time.

Messages must implement the appropriate interface: Command, Event or Timeout. These interfaces serve as aliases for dogma.Message, but may diverge in the future.

Message handler

A message handler is part of an application that acts upon messages it receives.

Each handler specifies the message types it expects to receive. These message are routed to the handler by the engine.

Command messages are always routed to a single handler. Event messages may be routed to any number of handlers, including zero. Timeout messages are always routed back to the handler that produced them.

Dogma defines four handler types, one each for aggregates, processes, integrations and projections. These concepts are described in more detail below.

Application

An application is a collection of message handlers that work together as a unit. Typically, each application encapsulates a specific business (sub-)domain or "bounded-context".

Engine

An engine is a Go module that delivers messages to an application and persists the application's state.

A Dogma application can run on any Dogma engine. The choice of engine brings with it a set of guarantees about how the application behaves, for example:

• Consistency: Different engines may provide different levels of consistency guarantees, such as immediate consistency or eventual consistency.

• Message delivery: One engine may deliver messages in the same order that they were produced, while another may process messages out of order or in batches.

• Persistence: The engine may offer a choice of persistence mechanisms for application state, such as in-memory, on-disk, or in a remote database.

• Data model: The engine may provide a choice of data models for application state, such as relational or document-oriented.

• Scalability: The engine may provide a choice of scalability models, such as single-node or multi-node.

This repository is not itself an engine implementation. It defines the API that engines and applications use to interact.

One example of a Dogma engine is Veracity.

Aggregate

An aggregate is an entity that encapsulates a specific part of an application's business logic and its associated state. Each instance of an aggregate represents a unique occurrence of that entity within the application.

Each aggregate has an associated implementation of the dogma.AggregateMessageHandler interface. The engine routes command messages to the handler to change the state of specific instances. Such changes are represented by event messages.

An important responsibility of an aggregate is to enforce the invariants of the business domain. These are the rules that must hold true at all times. For example, in a hypothetical banking system, an aggregate representing a customer's account balance must ensure that the balance never goes below zero.

The engine manages each aggregate instance's state. State changes are "immediately consistent" meaning that the changes made by one command are always visible to future commands routed to the same instance.

Aggregates can be a difficult concept to grasp. The book Domain Driven Design Distilled, by Vaughn Vernon offers a suitable introduction to aggregates and the other elements of domain driven design.

Process

A process automates a long running business process. In particular, they can coordinate changes across multiple aggregate instances, or between aggregates and integrations.

Like aggregates, processes encapsulate related logic and state. Each instance of a process represents a unique occurrence of that process within the application.

Each process has an associated implementation of the dogma.ProcessMessageHandler interface. The engine routes event messages, which produces commands to execute.

A process may use timeout messages to model business processes with time-based logic. The engine always routes timeout messages back to the process instance that produced them.

Processes use command messages to make changes to the application's state. Because each command represents a separate atomic change, the results of a process are "eventually consistent".

Integration

An integration is a message handler that interacts with some external non-message-based system.

Each integration is an implementation of the dogma.IntegrationMessageHandler interface. The engine routes command messages to the handler which interacts with some external system(s). Integrations may optionally produce event messages that represent the results of their interactions.

Integrations are stateless from the perspective of the engine.

Projection

A projection builds a partial view of the application's state from the events that occur.

Each projection is an implementation of the dogma.ProjectionMessageHandler interface. The engine routes event messages to the handler which typically updates a read-optimized database of some kind. This view is often referred to as a "read model" or "query model".

The projectionkit module provides engine-agnostic tools for building projections in various popular database systems, such as PostgreSQL, MySQL, DynamoDB and others