LimeSurvey architecture

This is the main public document for LimeSurvey's architecture

Overview

todo, constraints

This is the architectural documentation for the LimeSurvey application. It does not document the ecosystem surrounding it.

The architectural decision roundtrip should look like this:

This document should be reviewed, updated and discussed at least yearly.

Vision

Stable software

Easy to change

Up to date with modern paradigms

Technology stack

Programming languages:

• PHP 7.2+
• JavaScript ES6 (admin) and IE11 compatible (survey taking)
• CSS, HTML
• XML, JSON

Frameworks:

• Yii 1.1
• jQuery
• Vue 2
• SASS

Tools:

• Git
• GitHub Actions
• Psalm
• CodeSniffer
• MessDetector
• Scrutinizer
• Selenium, geckodriver
• PHPUnit
• Jest (JavaScript unit testing)

Dependency management:

• Manually (should be refactored)
• Yarn 1
• npm
• Rollup
• Composer

Libraries:

• CKEditor
• kcfinder
• PDF?
• Sodium
• Twig
• PHPMailer
• pclzip

PHP extensions

• SimpleXML
• ZIP
• GD
• Sodium

Risks

As defined in the code quality guide.

Risk identification, analysis, mitigation, monitor.

todo, probability, impact, supporting knowledge

todo, security risks

For common lists of risks, see Pandian

Product risk

These risks are considering the individual LimeSurvey user or admin.

Possible mitigations are in sub-lists.

Risk	Probability	Impact	Mitigation
Data loss (known)	Low	Medium to high	Unit-tests and integrity tests in the CI
			Internal QA culture
			Possibility to enable extensive logging, from which you can reconstruct answers
			Regular backups
Data loss (unknown)			?
Unauthorized data access			Full test-suite of the permission system
Blocking bugs (that could lead to data loss)'			Full test-suite of the high-priority functionality
			Full manual QA testing before each major and minor release (not for patch releases)
Low user experience (UX)	High	Medium to high	Regular feedback surveys from users
			Regular reviews by professional UX designer
			High changeability in the frontend of the product

Project risk

todo, pull request process

Risk	Probability	Impact	Mitigation
Communication	Medium	Low to medium	Weekly meetings
			Yearly LimeCon
			Virtual LimeCon
Regressions	High	Medium to high	Functional testing for each new feature
			Continuous effort to expand the test suit
			Effort to keep an eye on incoming bug reports
			Effort to create bug statistics and improve bug statistics

Business risk

Internal to LimeSurvey GmbH

Quality attributes

Also called non-functional requirements.

Considering the risks above, the most important quality attributes could be considered:

• Testability
• Changeability
• Usability, or user experience (UX)

They can be improved by using dependency injection, and having clearer separation between the abstraction layers, including splitting the monolith into separate modules.

Function composition and the "functional core, imperative shell" pattern could also help.

Strategies to improve testability:

• Explicit dependencies
• "Functional core, imperative shell"
• Make testability explicit when designing new features
• Make testability part of refactoring efforts

Strategies to improve changeability:

• Refactor functions into smaller parts
• Improve documentation
• Improve and extend the tools for continuous integration (Psalm, code duplication, metrics)
• Factor out widgets and/or JS components

To improve changeability, the system must become less fragile. This can only happen with good separation of concerns and a CI.

Quality attributes that have good performance:

• Performance and scale
• Availability
• Easy to deploy
• Security (but requires continuous and conscious efforts)

Views

Using the 4+1 view model from Kruchten, you can split the documentation into five parts:

• Logic
• Development
• Process
• Physical
• Scenario

Loosely interpreted, for us is will be:

• Domain diagram
• Class and layer diagram
• Flow diagrams
• Deployment diagrams
• Scenarios (possible omitted)

Domain

A simple domain diagram over LimeSurvey:

Source.

Layers

LimeSurvey uses a basic, monolithic MVC layered architecture.

• View layer, serving HTML, JSON or XML to the calling part (can be browser or local API machine)
• Controller layer, gluing parts together from the other layers
• Business logic layer, calculating business rules, should be fully unit-testable
• Database storage layer (also includes domain entity validation)

Refactoring should be focused on strengthening the separation between the layers.

Source.

Overview of future refactoring efforts.

Forbidden transgression between abstraction layers:

How splitting up a monolith into separate modules could look like:

Possible architecture of REST + SPA module:

todo, mobile app architecture

Class hierarchy

Model hierarchy, major groups:

• LSActiveRecord - base class
• Localization tables, L10n
• Survey tables - survey, quota
• Group tables
• Question tables - subquestion, answers, default answers, labels, condition, timing
• Participator tables - token, custom attributes, CPDB
• Plugin tables
• User tables
• Themes - question themes, survey themes, configuration
• Admin customization - menus, boxes, notification, tutorial
• Permission tables - user groups, permission

Flow and state

Most basic flow when administer a survey.

Overview over the survey taking state machine.

Deployment

Server, PHP-FPM, MySQL, browser, LimeSurvey installation

API design

todo

Security

todo

Permission system

Users

User groups