Conditional routes in router.ex

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Hello,

I’m currently in the process of rewriting parts of a Phoenix application. The frontend is primarily a React single-page app, but we’re transitioning to LiveView. To ensure a smooth rollout, I’d like to migrate specific pages from React to LiveView for a small subset of users first.

At the moment, we’re using a URL prefix approach — for example, /live/entity/123 instead of /entity/123. However, many of our links (e.g., from emails or external systems) point to the original /entity/123 routes, and updating all those references isn’t feasible.

Ideally, I’d like to implement a mechanism in the router that conditionally serves either the LiveView or the React SPA based on a value in the session or assigns (e.g., a feature flag or user group).

Is this kind of conditional routing possible in the Phoenix router? I couldn’t find a clear way to do it. Alternatively, are there other recommended approaches to handle this kind of progressive migration?

Thanks in advance!

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The router cannot do so. All the router looks at to decide on a route is method, path segments and hostname – as becomes obvious here: Phoenix.Router — Phoenix v1.8.0-rc.3

You could however dig into implementation details and wrap Phoenix.LiveView.Plug – which is the plug used with the Phoenix.LiveView.Router.live macro – and however you make your react frontend work and let that wrapping plug decide on either or for a given path.

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