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Bandito: Easy conversion optimization for Ring apps.

Bandito is a small clojure library designed to help you easily run epsilon-greedy multi-armed bandit tests. It's designed to be used with Ring, and optionally a routing library like Compojure

Bandito will take care of the following for you:

  • Selecting your best-converting variant based on an epsilon-greedy algorithm.
  • Making sure a given user sees the same page consistently.
  • Storing the results in memory. Persisting your results to disk somehow is up to you.

Bandito requires that you use ring's session middleware.


Bandito is available from clojars, so you just need to put the following in your project :dependencies

[bandito "1.1.0"]


The current version is 1.1.0

To start doing tests, you need to make different variants. Bandito assumes that your variants can be expressed as different view functions. The view functions (i.e. handler functions) you provide must take a single argument representing a Ring request map, and respond with either a string or a Ring response map

Here's some test views we'll use for examples.

(defn view1 [req] "View #1")
(defn view2 [req] "View #2")
(defn view3 [req] "View #3")

Running experiments

Use init-experiment to create an experiment -- that is, a container for your results. You can define this in your top level or give it a lifecycle with component or mount, whatever you like. init-experiment an optional configuration map, and a hash-map of view functions identified by keywords. Use experiment-handler to create a ring handler from the experiment that will track views and conversions.

(require '[bandito.core :as bandito])

;; Define your experiment views. You can add new views on the end, but don't
;; remove them while the experiment is running!
(def viewmap
  {:view1 view1
   :view2 view2
   :view3 view3})

(def experiment (bandito/init-experiment viewmap))
(def experiment-view (bandito/experiment-handler experiment))

You can use experiment-view here as you would normally use view1, view2, or view3

An experiment is a clojure record with 4 properties:

  • :report, an atom containing a map representing the results of your experiment,
  • :config, a hash-map containing configuration options
  • :view-map, the view map described above

Logging conversions

In the view representing your conversion, include a call to convert! with the experiment and the request map (from which bandito will pull the view that was shown):

(defn conversion-view [req]
  (bandito/convert! experiment req)
  ; ... the rest of your view

Fetching and loading results

Bandito doesn't know or care how you store the results. You can grab the current results report at any time using as-map:

(bandito/as-map experiment)

;; How your test might look after running for a bit

;   {:view1 {:views 2 :conversions 0}
;    :view2 {:views 7 :conversions 1}
;    :view3 {:views 17 :conversions 3}}

To load persisted results, just update the :report atom within the experiment with a map of a form similar to the above:

(reset! (:report experiment) my-report-map)


Copyright © 2013-2016 Adam Bard (

Distributed under the Eclipse Public License, the same as Clojure.


Simple multi-armed bandit testing in Clojure for Ring web applications.



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