How do SFRA controllers differ from legacy pipeline-based development?

Legacy pipelines were XML-based visual workflows consisting of interconnected nodes (start nodes, script nodes, decision nodes) that controlled storefront logic in SiteGenesis. They were difficult to unit-test, version-control, and extend without forking the entire pipeline.

SFRA controllers are JavaScript modules following an Express.js-inspired pattern:

1. Routes are registered with server.get('routeName', middleware, handler) or server.post()
2. Each step is a middleware function that receives (req, res, next) and calls next() to pass control forward
3. The final middleware calls res.render('templatePath', viewModel) or res.json(data)
4. Custom cartridges extend routes via server.extend() combined with server.append() or server.prepend()

Controllers are plain JavaScript files, making them easy to lint, unit-test, and track in git. Pipelines are deprecated but still supported in legacy implementations.