November 16, 2014

Service Design Patterns in Rails: Web Service Implementation Styles

This is the third post on Service Design Patterns in Rails. The first two where



 In these series of posts, I am trying to map the patterns described in the Service Design Patterns book by Robert Daigneau to a typical Rails application.

So far, I've proved that Rails already implements some of the patterns described in the book and thus, by only using the Rails framework, you are already following those patterns. This means you already follow certain good practices on implementing services without actually knowing about them.

However, we've also seen that some patterns are not implemented by default. The common thing on them is that they are related to the response: Response Mapper, Response Data Transfer Object Mapper, and Linked Service.

The chapter about Web Service Implementation Styles, is about five patterns:

  • Transaction script
  • Datasource adapter
  • Operation script
  • Command invoker
  • Workflow connector

Transaction script pattern means writing a service that, without any helper, nor model, accesses the database. This is not the Rails way, since in Rails we access the database through a domain model object. Personally, I think it is good that this pattern is not in the Rails framework. I am surprised to have find out this pattern in the book.

Datasource adapter means having a class that will get the data from the database, making the controller unaware of, for example, if the data is stored on a postgresql db or sqlite, and managing metadata, like the relationships between different entities (i.e. one-to-many, belong-to, many-to-many). Does it sound familiar? Yes! This looks like ActiveRecord, doesn't it?

Operation script means delegating work to a class that can be used by different web-services. Translating that to Rails means implementing features in the model so that different controllers can reuse it.

Command invoker means having a class that can be either call from a web service or put to a queue to be run later. For example, a class with a perform method can be run by calling the perform method, or queued to a delayed job queue. This may not be the preferred way of using delayed job, but certainly can be done (See Custom Jobs) . This class will then call domain model methods.

Workflow connector is something more complicated. It means designing a workflow where each step can be a web service. However, you need to maintain information on the state and be able to rollback (typically done by compensating, by calling a web service that performs the negative function).

I don't think you can implement a workflow in Rails with what the framework provides, nor with the typical gems. From what I know, you will need to add additional gems/tools.


So far, I've seen that rails implements the patterns for a basic REST API, which is cool. However, when it comes to more complex workflows where the services you implement are part of a bigger services infraestucture, rails does not implement patterns for that, like the Workflow Connector, the Response Mapper or the Linked Service.

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