An appender's configuration is managed by a single function, timbre/set-config!, which accepts a list of keys identifying the type of appender, and the appender map, which is the actual configuration for the appender.

Taking a look at the init function in hipstr.handler, the first thing we see is a call to configure Timbre:

(timbre/set-config!
  [:appenders :rotor]
  {:min-level :info
    :enabled? true
    :async? false ; should be always false for rotor
    :max-message-per-msecs nil
    :fn rotor/appender-fn})

The preceding snippet configures the rotor appender, a type of file appender that creates a new log file after the current log file exceeds a specific size. Timbre's set-config! has a similar signature and behavior to Clojure's assoc-in (https://clojuredocs.org/clojure.core/assoc-in) function, but without having to define the target map to mutate.