Question

This works, but I’m curious what the performance penalty is for dynamically passing in a namespace and resolving it using ns-resolve…

(ns bulbs.vertices)

(defn create
  [config data]
  ((ns-resolve (:ns config) 'create-vertex) config data))

And then call it like this…

(ns bulbs.neo4jserver.graph
  (:require [bulbs.vertices :as vertices])
  (:require [bulbs.neo4jserver.client :as client]))

(defn graph
  [& [config]]
  (let [config (client/build-config config {:ns 'bulbs.neo4jserver.client})]
    (fn [func & args]
      (apply func config args))))

(def g (graph))

(g vertices/create {:name "James"})

Answer 1

unless your ns-resolve is part of a loop (and unless you need to dynamically resolve a different function in every iteration there is absolutely no need for this), I wouldn’t worry about a performance penalty.

But yes, there is a performance penalty:

user=> (time (dotimes [n 1000000] (let [f (ns-resolve 'clojure.core 'inc)] (f n))))
"Elapsed time: 175.386 msecs"
nil
user=> (time (dotimes [n 1000000] (let [f inc] (f n))))
"Elapsed time: 27.022 msecs"
nil

If you really really really need the ns-resolve magic (but see the replies to your other question discouraging you from doing this in your particular situation), and if the function you’re resolving is used in a loop, take the resolve out of that loop:

user=> (time (let [f (ns-resolve 'clojure.core 'inc)] (dotimes [n 1000000] (f n))))
"Elapsed time: 48.538 msecs"
nil