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Create and run load tests using Clojure (and get fancy reports).


Add the following to your project.clj :dependencies:

[clj-gatling "0.15.0"]


Basic example

This will make 100 simultaneous http get requests (using http-kit library) to localhost server. Single request is considered to be ok if the response http status code is 200.

(require '[clj-gatling.core :as clj-gatling])
(require '[org.httpkit.client :as http])

(defn localhost-request [_]
  (let [{:keys [status]} @(http/get "http://localhost")]
    (= status 200)))

  {:name "Simulation"
   :scenarios [{:name "Localhost test scenario"
                :steps [{:name "Root"
                         :request localhost-request}]}]}
  {:concurrency 100})

Running the simulation shows statistics in console and generates a detailed html report. (clj-gatling uses gatling-highcharts for reporting)


clj-gatling runs simulations to simulate load. Simulation consists of one or multiple scenarios that will be run in parallel. One scenario contain one or multiple steps that are run sequentially. One simulation is configured to run with a given number of virtual users. As a result the tool returns response times (min, max, average, percentiles) and requests per second. Internally millisecond is used as a precision for benchmarks. Therefore this is not suited for testing systems with less than one millisecod response times.

Simulation is specified as a Clojure map like this:

{:name "Simulation"
 :pre-hook (fn [ctx] (do-some-setup) (assoc ctx :new-value value)) ;Optional
 :post-hook (fn [ctx] (do-some-teardown)) ;Optional
 :scenarios [{:name "Scenario1"
              :context ;Optional (default {})
              :weight 2 ;Optional (default 1)
              :skip-next-after-failure? false ;Optional (default true)
              :allow-early-termination? true ;Optional (default false)
              :pre-hook (fn [ctx] (scenario-setup) (assoc ctx :new-val value)) ;Optional
              :post-hook (fn [ctx] (scenario-teardown)) ;Optional
              :step-fn ;Optional. Can be used instead of list of steps
              :steps [{:name "Step 1"
                       :request step1-fn}
                      {:name "Step 2"
                       :sleep-before (constantly 500) ;Optional
                       :request step2-fn}]}
             {:name "Scenario2"
              :weight 1
              :steps [{:name "Another step"
                       :request another-step-fn}]}]}

Global simulation Hooks

You can define a pre-hook function that is executed once before running the simulation. Function takes in the context map. You can change the context (e.g. by adding new keys) by returning new map. Also you can define a post-hook function which is called after running the simulation.


You can define one or multiple scenarios. Scenarios are always run in parallel. Concurrent users are divided between the scenarios based on their weights. For example:

  • Simulation concurrency: 100
  • Scenario1 with weight 5 => concurrency 80
  • Scenario2 with weight 1 => concurrency 20

Scenario weight is optional key with default value 1. In that case the users are split evenly between the scenarios

Scenarios are also able to specific their own additional context via the optional :context key.

Scenario steps

Each scenario consists of one or multiple steps. Steps are run always in sequence. Step has a name and user specified function (request) which is supposed to call the system under test (SUT). Function takes in a scenario context as a map and has to return either directly as a boolean or then with core.async channel with a message of type boolean.

;Returning boolean directly
(defn request-returning-boolean [context]
  ;Call the system under test here
  true) ;Return true/false based on the result of the call
;Returning core.async channel
(defn request-returning-channel [context]
     ;Call the system under test here using non-blocking call
     true)) ;Return true/false based on the result of the non-blocking call

The latter is the recommended approach. When you use that it makes clj-gatling able to utilize (=share) threads and makes it possible to generate more load with one machine. However, the former is probably easier to use at the beginning and is therefore a good starting point when writing your first clj-gatling tests.

If the function returns a false clj-gatling counts the step as a failed. If a function throws an exception it will be considered as a failure too. clj-gatling also provides a global timeout (see Options) for a step. If a request function takes more time it will be cancelled and step is again considered as a failure.

Note! clj-gatling reports only step failures and successes. At the moment there is no support for different kinds of errors in reporting level. All errors are logged to target/results/<sim-name>/errors.log.

Context map contains all values that you specified as a simulation context when calling run method (See options) and clj-gatling provided user-id value. The purpose of user-id is to have a way to specify different functionality for different users. For example:

(defn request-fn [{:keys [user-id]}]
    (if (odd? user-id)

If your scenario contains multiple steps you can also pass values from a step to next a step inside an scenario instance (same user) by returning a tuple instead of a boolean.

;step 1
(defn login [context]
    (let [user-name (login-to-system)]
      [true (assoc context :user-name user-name)])))

;step 2
(defn open-frontpage [context]
    (open-page-with-name (:user-name context))))

If you want a step to not launch immediately after previous step you can specify a step key sleep-before. The value for that key is a user defined function that takes in the scenario context and has to return number of milliseconds to wait before starting request function for that step.

By default clj-gatling won't call the next step in scenario if a previous step fails (returns false). You can override this behaviour by setting skip-next-after-failure? to false at scenario level.

When clj-gatling terminates the simulation (either after the given duration or given requests) all running scenarios will still finish. If scenario has multiple steps and takes long to run the simulation may take some time to fully terminate. If you want to disable that feature in scenario level you can set allow-early-termination? to true.

Scenario hooks

Scenario 'pre-hook' function is executed before running a scenario with single virtual user. Scenario 'post-hook' function is executed after the scenario with user has finished. Post-hook will always be executed (even when previous step fails).

Dynamic scenarios

Sometimes a pre-determined sequence of steps does not provide enough flexibility to express the test scenario. In such a case, you may provide the key step-fn instead with a function taking the current context and returning a tuple specifying a step and a modified context. Returning a nil step marks the end of the scenario.

Note! If step-fn never returns nil the simulation will run endlessly. To prevent that you can use option :allow-early-termination?


Second parameter to clj-gatling.core/run function is options map. Options map contains following keys:

{:context {:environment "test"} ;Context that is passed to user defined functions. Defaults to empty map
 :timeout-in-ms 3000 ;Timeout for a request function. Defaults to 5000.
 :root "/tmp" ;Directory where cl-gatling temporary files and final results are written. Defaults to "target/results"
 :concurrency 100 ;Number of concurrent users clj-gatling tries to use. Default to 1.
 :concurrency-distribution ;Function for defining how the concurrent users are distributed during the simulation. Optional
 :requests 10000 ;Total number of requests to run before ending the simulation. Defaults to the number of steps in simulation
 :duration (clj-time.core/minutes 5) ;The time to run the simulation. Note! If duration is given, requests value will be ignored
 :error-file "/tmp/error.log"} ; The file to log errors to. Defaults to "target/results/<sim-name>/error.log".


If you only set the concurrency the clj-gatling will use same concurrency from the beginning till end. If you want to have more control for that (for example ramp-up period) you can specify your own concurrency distribution function like this one:

(fn [progress context]
   (if (< progress 0.1)

Progress is floating point number that goes from 0.0 to 1.0 during the simulation. Distribution function should return floating point number from 0.0 to 1.0. The concurrency at that point of time will be concurrency times the returned number.


See example project here: metrics-simulation


If you don't want to use built-in reporting by gatling-highcharts you can customize the reporting by implementing it yourself and using clj-gatling for load generation. You can do that by passing in :reporter map as an option. For example if you want to skip reporting totally pass this reporter:

{:writer (fn [simulation idx results]
            ;Do nothing
 :generator (fn [simulation]
              ;Do nothing

Results look like this:

[{:name "Scenario name"
  :id 1
  :start 1472898793468
  :end 1472898793908
  :requests [{:name "Request name"
              :id 1
              :start 1472898793468
              :end 1472898793908
              :context-after {:user-id 1}
              :context-before {:user-id 1}
              :result true}]}]

clj-gatling calls the writer function periodically (currently after each 20000 requests) and then generator once when the simulation is over. There is an example on how to use custom reporter in examples folder.

Change History

Note! Version 0.8.0 includes few changes on how simulations are defined. See more details in change history. All changes are backwards compatible. The old way of defining the simulation is still supported but maybe deprecated in future. You can see documentation for old versions here.


This is compatible with Jenkins Gatling Plugin.


AFAIK there are no other performance testing tool where you can specify your tests in Clojure. In my opinion Clojure syntax is very good for this purpose.

Design Philosophy

Real life scenarios

clj-gatling has same kind of an approach that Gatling has in this sense. Idea is to simulate a situation where multiple users use your application concurrently. Users do actions and based on the results do next actions etc. After the simulation you comprehensive results (nice graphs etc.)

If you want to just execute single request with high level of concurrency simpler tools like Apache Benchmark can do that. Of course, clj-gatling can do that also but it might be an bit of an overkill for that job.


I am not a fan of complex DSLs. clj-gatling tries to avoid DSL approach. Idea is that you should write just ordinary Clojure code to specify the actions and the actual scenario definition is an map.

Distributed load testing

Clojider is a tool that can run clj-gatling in a distributed manner. It uses AWS Lambda technology for running distributed load tests in the cloud.


Use GitHub issues and Pull Requests.


Copyright (C) 2014-2018 Markus Hjort

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


Load testing library for Clojure





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