Form Application Architecture

Our best practice architecture for developing complex online form applications.

⚠️ TODO: This repository is a work in progress.

Indication

You need to build a transactional service where users answer a series of questions, and the questions shown and their order depend on answers to previous questions.

Motivation

Many of our past approaches to developing large, form-based applications suffered from pain points, e.g.:

• Questions get re-ordered constantly and developers keep playing catch-up with user research and service design
• A lack of shared language within the (dev and wider) team to describe the application and content structure
• Complex logic determining which steps to show, and in which order, and no "natural" place to put that logic in typical MVC frameworks
• We keep reinventing the wheel with every new service
• Difficult to onboard new team members due to complex custom code

Our primary guiding intention for this architecture was to allow any developer who is new to a project using it to be productive immediately. We avoid behind-the-scenes magic and prefer slight verbosity and duplication where it makes sense, in line with the following guideline from the MoJ development principles:

Code should be correct, clear, and concise – in that order.

Scope

This architecture is best suited to online services that guide users through a large number of steps, with business logic determining which steps are shown and in which order. It includes a simple domain language to talk about form apps, as well as a set of lightweight implementation patterns.

What this is not

We deliberately did not create a domain-specific language (DSL) or drop-in components because that would tie this architecture down to a specific programming language or web framework. This way developers who are new to a project do not have to learn (or remember, if they come back to it later) a set of potentially leaky abstractions.

Counterindications

A different (potentially simpler) architecture may be preferable if your service:

• Has a small number of steps
• Has a fixed, linear flow through the steps with no business logic considerations in the step transitions
• Primarily has non-flow user interaction (although you may want to consider extracting the step flow parts of your app into a microservice using these patterns)

Technology Choice

The reference implementation of this architecture is in Ruby on Rails, however there is no reason some or all of these patterns could not be used in other MVC web frameworks.

Structure

Figure 1: Architectural diagram

Structural elements

Intents

An intent is a user activity from when they first visit your service to when they've achieved what they intended to do.

Examples:

• Appeal an HMRC decision with the Tax Tribunal
• Book a prison visit
• Apply for a fishing licence

Many services will just have one intent, but they may have more. Intents are distinct activities and the user will usually complete exactly one intent as part of using your service.

Tasks

A task is a logical group of questions that belong together.

Examples:

• Determine the type of your appeal
• Check your eligibility for this service
• Enter your personal details

Tasks can be shared between intents – in the Appeal to the Tax Tribunal service, there is a task that captures a users personal details, and if they are represented by someone else, that entity's details too. This task is used in both the "Appeal an HMRC decision" and the "Apply to close an ongoing enquiry" intents.

Steps

An individual question or a set of closely related fields.

Examples:

• What kind of tax is your appeal about?
• How much of a penalty have you been charged?
• What kind of fish do you want to fish?
• Enter your address details
• Check your answers

A step is usually a form, but it can also be a static page. For example, the first step in a task could contain an overview of what the user is supposed to do in the following steps, and the last step in a task could be a "Check your answers" page listing all the answers given by the user. A step could also be an end point of your service, e.g. a page informing the user that based on their answers, they are not eligible to use your service.

Components

Session Model

There is no prescribed way of keeping track of your session state, although it would be preferable to not spread it out too much as the combined session state is passed into form objects and decision trees in the StepController#update_and_advance method.

The simplest way to start out will be to keep track of your session state in a single ActiveRecord model, and provide a method of retrieving an instance based on the user session in your ApplicationController. For example, in a service handling passport applications, you may have a PassportApplication model whose ID is stored in the session cookie, and a current_passport_application method that retrieves the relevant object as and when required.

In the Appeal to the Tax Tribunal service, we decided on staying with this approach even as the number of fields on the object grew to around ~40 – there were no particular performance considerations as it was a low volume service, and a single database model was a reasonable place to keep track of this data.

Controllers

Controllers come in two forms, depending on whether the step involves user input or not (in other words, whether or not there is a form object for the step).

A static step controller will only have a single #show (GET) action, which will show a page such as a start or end page for a given task.

A step controller with user input will have an #edit (GET) and an #update (PUT) action:

• #edit initialises a form object with current state and renders the view
• #update runs the StepController#update_and_advance method

⚠️ TODO: Code!

Value Objects

We use value objects extensively in this architecture to encapsulate primitives in a more semantically meaningful way.

A value object is immutable, and is equal to another value object if and only if they are of the same type and contain the same primitive value(s). For example, a Duration(5 days) is equal to a Duration(5 days), but not to a Duration(5 weeks) or a DeliveryTimeEstimate(5 days):

def ==(other)
  other.is_a?(self.class) && other.value == value
end

An example of a simple value object to represent a user's preferred method of contact could look like this:

class PreferredContactType < ValueObject
  VALUES = [
    EMAIL = new(:email),
    PHONE = new(:phone),
    POST = new(:post)
  ].freeze

  def self.values
    VALUES
  end
end

ActiveRecord's #composed_of is a quick and easy way to persist value objects against a model object, and in most cases your value objects will simply wrap a single symbol, e.g. the user's choice out of a range of options.

For this, we add a simple helper method to ApplicationRecord to handle transparent serialisation and deserialisation of value objects against your session model:

def self.has_value_object(value_object, constructor: nil, class_name: nil)
  composed_of value_object,
    allow_nil:   true,
    mapping:     [[value_object.to_s, 'value']],
    constructor: constructor,
    class_name:  class_name
end

This allows you to simply add something like the following to your model:

class Inquiry < ApplicationRecord
  has_value_object :preferred_contact_type
end

Form Objects

We use form objects as a layer of abstraction between a controller handling user input and the model. They handle:

• setting up the range of choices for multiple- or single-choice forms
• validation of user input
• casting choisen option(s) into value object(s) if applicable
• persisting new state against the session model

⚠️ TODO: Code!

Decision Trees

Decision trees encapsulate the business logic of moving between steps – given a step that has just been completed, the answer(s) given by the user, and the current state of the session model, they determine what step to show next.

The decision tree's #destination method receives the session state and current step as an input from the StepController#update_and_advance method, and can return any object suitable for Rails routing (e.g. a {controller:, action:} hash).

⚠️ TODO: Code!

Further considerations

Stumbling blocks? Quirks? Notes on testing Notes on generators

Reference implementation

Add sample code to this repo

MoJ projects using this architecture

• Appeal to the Tax Tribunal - Data Capture

Further reading

Add more links