Application code is stored in modules. A module is a map which can have any of the following keys:

(def module
  {:static {...}
   :routes [...]
   :api-routes [...]
   :schema {...}
   :tasks [...]
   :on-tx (fn ...)
   :queues [...]})

Each of these keys are discussed on subsequent pages:

To demonstrate, a "hello world" module might look like this:

(defn hello [ctx]
    [:p "Hello world"]]])

(def module
  {:routes [["/hello" {:get hello}]]})

Your app's modules are bundled together and stored in the system map:

(def modules
   (biff/authentication-module {})


(def initial-system
  {:biff/modules #'modules

When your app starts, this system map is passed through a sequence of components:

(defonce system (atom {}))

(def components

(defn start []
  (let [new-system (reduce (fn [system component]
                             (log/info "starting:" (str component))
                             (component system))
    (reset! system new-system)

Each component is a function which takes the system map as a parameter and returns a modified version. After the system map is passed through all the components, the system will be running and your module code will be called as appropriate.

For example, any routes you define in your modules will be compiled into a handler function, and this handler will be passed to the use-jetty component. This component starts the Jetty web server and passes requests to your handler.

(defn use-jetty [{:keys [biff/handler] :as system}]
  (let [server (jetty/run-jetty (fn [request] (handler (merge system request)))
                                {:host "localhost"
                                 :port 8080
                                 :join? false})]
    (update system :biff/stop conj #(jetty/stop-server server))))

This component also merges the system map with incoming requests, so your routes can get access to resources like the database:

(defn hello [{:keys [biff/db] :as ctx}]
  (let [n-users (ffirst (xt/q db
                              '{:find [(count user)]
                                :where [[user :user/email]]}))]
      [:p "There are " n-users " users."]]]))

(def module
  {:routes [["/hello" {:get hello}]]})

Biff often uses ctx (or "context map") as a more general term, since it often includes both the system map and other things like the Ring request.

Your project includes a refresh function which can be used during development if you modify any of the components:

(defn refresh []
  (doseq [f (:biff/stop @system)]
    (log/info "stopping:" (str f))
  ( :after `start))

Calling refresh should be rare. Biff is designed so that as much as possible, changes take effect immediately without needing to restart the system (through the use of late binding).

Modifying the framework

Biff is designed to give you strong defaults while still allowing you to change just about anything without too much hassle. The goal is that Biff helps you to launch quickly and that it doesn't get in the way later on as your needs evolve.

You can modify your app's framework code by supplying a different set of component functions. For example, if you need to change the way configuration is stored, you can replace biff/use-aero-config with your own component:

(defn use-custom-config [system]
  (let [config ...]
    (merge system config)))

(def components

If you'd like to modify one of Biff's default components beyond what its options allow, it's fine and recommended to copy the source code for the component into your own project. For example, the biff/use-xtdb component only supports using RocksDB, LMDB, and/or JDBC as the storage backend. If you want to use Kafka, you could copy the biff/use-xtdb source into your project and make the needed changes.

Biff also provides a single default module, biff/authentication-module, which defines the backend routes needed for Biff's email-based authentication. If you need to modify it substantially, you can similarly copy the source code into your project or replace it altogether.

What about the frontend?

Biff doesn't need to add much frontend architecture thanks to htmx. htmx allows server-side frameworks like Django, Rails, and Biff to to be used for moderately interactive apps, while still keeping most of your code on the backend. See Understanding htmx.

Have a question? Join the #biff channel on Clojurians Slack, or ask on GitHub.

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