Kemal Modular Router support PR - Looking for more Feedback

sdogruyol · January 30, 2026, 3:19pm

After a long time I’ve opened a PR to implement Modular Routing for Kemal
I’d be super happy to hear your feedback.

P.S: Please drop a in the PR if you like it

github.com/kemalcr/kemal

Add modular Router support

master ← modular-router

opened 07:37PM - 29 Jan 26 UTC

sdogruyol

+776 -0

## Summary This PR introduces a modular router feature to Kemal, inspired by …Flask's Blueprint and Express.js's Router. It enables organizing routes into logical groups with shared prefixes and scoped middleware, while maintaining full backward compatibility with the existing DSL. ## Motivation As Kemal applications grow, organizing routes becomes challenging with the flat global DSL. This feature allows: - Grouping related routes under common prefixes - Applying filters (middleware) to specific route groups only - Better code organization with separate router files - API versioning support ## New Features ### 1. Basic Router with HTTP Methods ```crystal router = Kemal::Router.new router.get "/users" do |env| "list users" end router.post "/users" do |env| "create user" end router.put "/users/:id" do |env| "update user #{env.params.url["id"]}" end router.patch "/users/:id" do |env| "patch user #{env.params.url["id"]}" end router.delete "/users/:id" do |env| "delete user #{env.params.url["id"]}" end router.options "/users" do |env| env.response.headers["Allow"] = "GET, POST, OPTIONS" end mount "/api", router # Results: # GET /api/users # POST /api/users # PUT /api/users/:id # PATCH /api/users/:id # DELETE /api/users/:id # OPTIONS /api/users ``` ### 2. Router-Scoped Filters ```crystal api = Kemal::Router.new # Before filter for all routes in this router api.before do |env| env.response.content_type = "application/json" end # After filter for all routes api.after do |env| env.response.headers["X-Response-Time"] = "10ms" end # Method-specific filters api.before_post do |env| # Only runs before POST requests env.set "validated", "true" end api.after_get do |env| # Only runs after GET requests end # Path-specific filter api.before "/admin" do |env| halt env, 403, "Forbidden" unless admin?(env) end api.get "/users" do |env| {users: []}.to_json end api.get "/admin" do |env| {admin: true}.to_json end mount "/api", api ``` ### 3. Nested Routing with namespace ```crystal api = Kemal::Router.new api.namespace "/users" do get "/" do |env| "list all users" end get "/:id" do |env| "show user #{env.params.url["id"]}" end post "/" do |env| "create user" end put "/:id" do |env| "update user #{env.params.url["id"]}" end delete "/:id" do |env| "delete user #{env.params.url["id"]}" end end api.namespace "/posts" do get "/" do |env| "list all posts" end get "/:id" do |env| "show post #{env.params.url["id"]}" end end mount "/api/v1", api # Results: # GET /api/v1/users # GET /api/v1/users/:id # POST /api/v1/users # PUT /api/v1/users/:id # DELETE /api/v1/users/:id # GET /api/v1/posts # GET /api/v1/posts/:id ``` ### 4. Using group (alias for namespace) ```crystal router = Kemal::Router.new router.group "/admin" do before do |env| halt env, 401, "Unauthorized" unless authenticated?(env) end get "/dashboard" do |env| "admin dashboard" end get "/settings" do |env| "admin settings" end end mount router # Results: # GET /admin/dashboard (with auth filter) # GET /admin/settings (with auth filter) ``` ### 5. Deeply Nested Routers ```crystal router = Kemal::Router.new router.namespace "/api" do namespace "/v1" do namespace "/users" do get "/" do |env| "deeply nested users list" end get "/:id" do |env| "user #{env.params.url["id"]}" end end end end mount router # Results: # GET /api/v1/users # GET /api/v1/users/:id ``` ### 6. Mounting Sub-Routers ```crystal # Define separate routers users_router = Kemal::Router.new users_router.get "/" do |env| "users list" end users_router.get "/:id" do |env| "user #{env.params.url["id"]}" end posts_router = Kemal::Router.new posts_router.get "/" do |env| "posts list" end # Combine them api = Kemal::Router.new api.mount "/users", users_router api.mount "/posts", posts_router mount "/api";, api # Results: # GET /api/users # GET /api/users/:id # GET /api/posts ``` ### 7. WebSocket Support ```crystal chat = Kemal::Router.new chat.ws "/room/:id" do |socket, env| room_id = env.params.url["id"] socket.on_message do |msg| socket.send "Room #{room_id}: #{msg}" end socket.on_close do puts "Connection closed" end end ``` ### 8. Router with Initial Prefix ```crystal v2_api = Kemal::Router.new("/v2") v2_api.get "/status" do |env| {version: "2.0", status: "ok"}.to_json end mount "/api", v2_api # Result: GET /api/v2/status ``` ### 9. Mount Without Prefix ```crystal router = Kemal::Router.new router.get "/health" do |env| "OK" end mount router # No prefix # Result: GET /health ``` ### 10. Backward Compatibility ```crystal # Old DSL still works exactly as before get "/" do |env| "Hello World" end before_all do |env| env.response.headers["X-Frame-Options"] = "DENY" end # New router alongside old DSL api = Kemal::Router.new api.get "/status" do |env| {status: "ok"}.to_json end mount "/api", api Kemal.run # Both work: # GET / -> "Hello World" # GET /api/status -> {"status": "ok"} ``` ### 11. Filters with Namespace ```crystal api = Kemal::Router.new api.namespace "/admin" do # This filter only applies to routes inside this namespace before do |env| halt env, 401 unless admin?(env) end get "/dashboard" do |env| "admin only" end end api.get "/public" do |env| "anyone can access" end mount "/api", api # /api/admin/dashboard -> requires admin (filter applied) # /api/public -> no filter ```

ralsina · January 30, 2026, 5:30pm

Would love to have this. The API looks fine, have not looked at the implementation

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