Command Buses Demystified: A Look at the Tactician Package

Command Buses have been getting a lot of community attention lately. The topic can be quite overwhelming at first when trying to understand all the concepts and terminology, but in essence - what a Command Bus does is actually incredibly simple.

In this article, we’ll take a closer look at variations of the Command Pattern; their components; what a Command Bus is; and an example application using the Tactician package.

Overview

So, what exactly is a Command Bus?

The role of the Command Bus is to ensure the transport of a Command to its Handler. The Command Bus receives a Command, which is nothing more than a message describing intent, and passes this onto a Handler which is then responsible for performing the expected behavior. This process can therefore be looked upon as a call to the Service Layer - where the Command Bus does the plumbing of the message in-between.

Before the introduction of the Command Bus, Service Layers were often a collection of classes without a standard way of being invoked. Command Buses solve this problem by providing a consistent interface and better defining the boundary between two layers. The standard interface also makes it possible for additional functionality to be added by wrapping with decorators, or by adding middleware.

Therefore, there can be more to a Command Bus than calling a Service Layer. Basic implementations may only locate a Handler based on naming conventions, but more complex configurations may pass a Command through a pipeline. Pipelines can perform additional behavior, such as: wrapping the behavior in a transaction; sending the Command over a network; or perhaps some queueing and logging.

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by Andrew Cairns via SitePoint