Architecture and Code

OnDemand Loop is a Ruby on Rails application that runs as an Open OnDemand Passenger app. The project follows standard Rails conventions as long as they do not interfere with the modern simple design of the application.

Instead of a traditional database it stores the data required to function in YAML files on disk. A small set of models like Project, DownloadFile, UploadBundle, and UploadFile inherit from ApplicationDiskRecord which provides YAML based persistence.

Transfers are processed by background services started by DetachedProcessManager. Each service loops over pending download or upload requests and spawns connector specific processors. Status updates are recorded back to the YAML metadata so the web UI can poll and display progress.

The web interface communicates with these services via simple command endpoints exposed through the CommandRegistry. This design keeps the runtime lightweight while avoiding the need for a separate job queue.

Connectors encapsulate repository-specific logic and are loaded dynamically through ConnectorClassDispatcher. They make Loop extensible by providing custom processors and controllers for each repository. The reference implementation is the Dataverse connector, with Zenodo as an additional implementation. See the Connectors section for the exact features a connector must implement.

Development Environment

OnDemand Loop is designed to run entirely within Docker containers.
The included Makefile handles image builds, container orchestration, and test execution. This means there is no need to install Ruby, Node.js, or any dependencies locally—just:

• Docker
• Docker Compose
• GNU Make

With these installed, you can build, run, and test the application using a single set of commands.

Requirements

OnDemand Loop is built to align with Open OnDemand's environment, ensuring compatibility with its supported Ruby and Node.js versions.
The application has been developed and tested against the following stack:

• Rocky Linux 8
• Open OnDemand v3.1.7
• Ruby 3.1
• Node.js 18

For a full compatibility matrix and upgrade notes, see the Open OnDemand section.

Project Layout

The repository root contains the following directories:

Folder	Purpose
application/	Complete Rails app with app/, config/, lib/, tests, and Gem/Node dependencies.
docker/	Docker build assets including Dockerfile.builder and the OOD NGINX loop.conf.
docs/	Project documentation including this guide.
scripts/	Shell scripts used by the Makefile for building, testing, versioning, and generating docs.
config/	Environment files for Docker Compose. Currently contains .env.
tools/	Helper files for project tooling.

Other important files live at the root:

• Makefile – orchestrates Docker builds and helper commands.
• docker-compose.yml – defines the local environment container stack.
• .github/ – GitHub Actions workflows for CI, guide and releases.

Inside application/app important subfolders include:

Path	Purpose
controllers/	Rails controllers plus connector specific subfolders.
views/	Rails views with the front end templates.
models/	Disk based models such as Project, DownloadFile and UploadBundle.
helpers/	Rails helpers with utilities to to use in the views.
connectors/	Connector classes loaded via ConnectorClassDispatcher.
services/	Business logic for downloads, uploads and repository APIs.
process/	Lightweight background execution framework used by the services.

The main configuration class for the application is lives under config/configuration_singleton.rb. This globally accessible object defines all adjustable settings which can be overridden via YAML or environment variables (see the Admin Guide for details).

application/test include all the unit tests and integration tests for the application. The application uses standard Rails testing with Minitest with Mocha.

The static fixtures for the tests are stored under application/test/fixtures