Merge route trees instead of routers. Routers are separate functions, if you middlewares have side-effects, both router functions will execute them.

Or... ring-handler is a function at least,hmph.

Aha and merge takes routes from both routers and then creates one router with route trees from both

The merge-routers function is from here: https://github.com/metosin/reitit/blob/master/doc/advanced/composing_routers.md#merging-routers

If you just want to separate some routes per namespace etc, you don't need to create router for both, you could just define parts of route tree data. And you don't need to use router to add route-data to every route under route tree branch:

(def tree1 [["/one" ...]])
(def tree2 [["/two" {:middleware [middleware]} ...]])
(def router (ring/router [tree1 tree2])

Yes, admittedly a better solution, but the problem is that in the real app there are actually two routers which needs to be merged into a single one.

I'm not sure, but I think when you create router, in your code, the :data option is "compiled" into every route inside router. Then on the merge the routes from r/routes call already include the middleware, and now the options merge adds another middleware when the new router is created to each route.

Try (apply merge (map #(dissoc (r/options %) :data) routers)) to drop :data option from the merge, as they are already applied to routes (if I guess correctly what is happening.)

Thanks. That makes the test pass. But that will also not apply the middleware to the "/one" route, right..?

True

I'll see if I can rearrange some things and apply the router wide middleware after the merging has taken place.

Router creation does "resolve-routes" and that will apply route data merge, including route data from router opts: https://github.com/metosin/reitit/blob/master/modules/reitit-core/src/reitit/impl.cljc#L72-L74

Other router options probably don't affect the r/routes result the same way

Ok, thanks. Good to know. Will fiddle with it some more.

Hmm, can I create a new router from an existing router while also adding some route data?

yeah, but for the meta merge will concat e.g. vector values

Ok, I think I have something which looks like it could work. Thanks for your help.

if route already has {:middleware [1]} and router options has {:data {:middleware [2]}} the resulting route in the router has :middleware [1 2]

Right. I've been down that path of debugging a little. I saw no option for doing some type of distinct meta merge so I ended up with duplicates in my vectors.

Doing something along the lines of

merged-routers (merge-routers
                          router1
                          router2
                          (ring/router
                           []
                           {:data {:middleware [middleware]}}))

[:sequential {:decode/string (fn [s] (str/split s #","))} :int]