Conditional request (ETag & modified-since) support for Phoenix.

Usage

The library provides a replacement function for Phoenix.Controller.render/1-4 called PhoenixETag.render_if_stale/1-4 accepting exactly the same arguments. When called the function expects the view to implement an additional callback: stale_checks/2 similar to render/2 that is responsible for returning the etag value and/or last modified value for the current resource.

Additional helper PhoenixETag.schema_etag/1 is provided for generating etag values of of a single or multiple schema structs.

# controller

def show(conn, %{"id" => id}) do
  data = MyApp.load_data(id)
  PhoenixETag.render_if_stale(conn, :show, data: data)
end

# view

def stale_checks("show." <> _format, %{data: data}) do
  [etag: PhoenixETag.schema_etag(data),
   last_modified: PhoenixETag.schema_last_modified(data)]
end

Both the etag and last_modified values are optional. The first one will add an etag header to the response and perform a stale check against the if-none-match header. The second one will add a last-modified header to the response and perform a stale check against the if-modified-since header. If the headers indicate cache is fresh a 304 Not Modified response is triggered, and rendering of the response is aborted. If headers indicate cache is stale, render proceeds as normal, except the extra headers are added to the response.