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Conduct load tests with Clojure, obtaining results as Clojure data or graphical charts. This tool targets stateful applications needing scripted logic in tests.


Add the following to your project.clj :dependencies:

Clojars Project

This tool name used to be clj-gatling. The project started as a simple wrapper to Gatling. However, today the library has evolved into a separate tool with its own design. Gatling Highcharts is now only an optional dependency that is used for generating graphical results. So the name was misleading and project was renamed to Trombi (It is Finnish word meaning Tornado).

Note for clj-gatling users! trombi is backwards compatible and you can still run the existing tests without any code change. However, trombi does not include all the dependencies by default anymore. See section Additional Dependencies.


Trivial example

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

(require '[trombi.core :as trombi])
(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 prints some statistics to stdout and returns a result like this:

{:ok 90 :ko 10 :response-time {:global {:min 20
                                        :max 500
                                        :mean 154}}}

Where ok means number of succesful requests and ko number of failed ones. Response times are in milliseconds.

If you want to get see a graphical report instead you can call trombi with additional options and add additional deps. (See sections Reporters and Additional Dependencies for more details)

(require '[trombi-gatling-highcharts-reporter.core])
(trombi/run your-simulation {:concurrency 100 :reporters [trombi-gatling-highcharts-reporter.core/reporter]})

This call will use Gatling Highcharts reporter and will generate a graphical report. Location of the report is returned and can also be found from stdout output.

Calling run function will block while simulation is running. If you want to more control you can also call run-async function. It takes same parameters as the synchronous call. However, it returns immediately and returns map with following keys:

  • results: A promise that is delivered once the simulation finishes.
  • force-stop-fn: Function that stops the execution of the simulation. Function does not take any parameters. Stopping does not kill scenarios/requests that are in progress. They will be finished before the exit When simulation is force stopped trombi does not guarantee that results are very reliable. So it is better to ignore the results when you finish the simulation with force stop.


trombi runs simulations to simulate load. A simulation consists of one or multiple scenarios that will be run in parallel. One scenario contains one or multiple steps that are run sequentially. One simulation is configured to run with a given number of virtual users or with given rate of new virtual users per second. 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 millisecond response times.

A 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 trombi 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 trombi tests.

If the function returns a false trombi counts the step as a failed. If a function throws an exception it will be considered as a failure too. trombi 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! trombi 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 trombi 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 trombi 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 trombi 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 trombi.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 trombi tries to use. Default to 1.
 :concurrency-distribution ;Function for defining how the concurrent users are distributed during the simulation. Optional.
 :rate 100 ;Number of new requests to add per second. Note! If rate is given, concurrency value will be ignored.
 :rate-distribution ;Function for defining how the rate is adjusted during the simulation. Optional.
 :progress-tracker ;Function used for tracking simulation progress. Optional.
 :reporters ;List of reporters to use. Optional. If omitted short summary reporter and highchart reporter are used.
 :requests 10000 ;Total number of requests to run before ending the simulation. Defaults to the number of steps in simulation.
 :duration (java.time.Duration/ofMinutes 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 trombi 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. The concurrency and rate distribution functions both have a legacy (version < 0.17.0) and new possible format.

In legacy mode, when the user-provided function is binary (2-arity), your function will be called with:

  • The progress through the simulation (as defined by either duration or requests), as a floating point number that goes from 0.0 to 1.0.
  • The scenario-level context.


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

In new mode, the user-provided function should be unary (1-arity). The single argument provided will be a map, which can be destructured at will, and allows extensibility for new arguments without breaking backwards compatability. Currently, the provided keys are:

  • progress: The percentage progress through the simulation (as defined by either duration or requests), as a floating point number that goes from 0.0 to 1.0.
  • duration: The elapsed time the simulation has been running for.
  • context: The scenario-level context.


(fn [{:keys [progress duration context]}]
   (if (< (.toSeconds duration) 10)

Your distribution function should return a floating point number from 0.0 to 1.0. The concurrency/rate at that point in time will be the requested concurrency/rate multiplied by the returned number.

Progress tracker

By default, trombi will write the progress periodically (every 200 milliseconds) to console output.

If you want to disable this functionality you can specify option :progress-tracker (fn [_]).

Following keys are passed to progress tracker function:

  • progress: Progress as a floating point number between 0.0 and 1.0.
  • sent-requests: Number of requests sent so far
  • total-concurrency: How many concurrent requests are in progress at the moment
  • default-progress-tracker: Function for default behaviour. This can be used to also call the default tracker from user-provided progress tracker
  • force-stop-fn: Function that stops the execution of the simulation. Function does not take any parameters. Stopping does not kill scenarios/requests that are in progress. They will be finished before the exit.


(fn [{:keys [progress sent-requests total-concurrency default-progress-tracker force-stop-fn] :as params}]
  (println "Progress:" progress ", sent requests:" sent-requests ", total concurrency:" total-concurrency)
  (default-progress-tracker params) ;Call default behaviour

Tuning parallelism

Internally trombi uses core.async, which has a fixed size thread pool. For load test scripts that use a high performance, asynchronous, non-blocking I/O (e.g. http-kit) library this is not a big issue. However, for libraries that require a thread per get request (e.g. clj-http) this is a real limitation.

The latest version of core.async supports setting the thread pool size using system property clojure.core.async.pool-size. With that the thread pool could be set to match the concurrency used in the simulation.


By default, trombi generates one report which is a short summary report.

When you call the trombi/run function it will return all the reports with reporter keys (e.g :short for the short summary reporter). Reporters also generate report to stdout and some reporters may generate results to file.

If you don't want to use the default reports you can specify a list of reporters with the :reporters key in the options. Available reporters are following:

  • trombi.reporters.short-summary/reporter This reporter returns a summary with number of successful and failed requests. In addition to that global min, max and mean is reported.
  • trombi.reporters.raw-reporter/in-memory-reporter This reporter returns all the raw results (scenarios & requests with their start and end times). It stores results in memory.
  • trombi.reporters.raw-reporter/file-reporter This reporter returns all the raw results (scenarios & requests with their start and end times). It stores results to file.
  • trombi-gatling-highcharts-reporter.core/reporter Generates a Gatling Highchart html report (See Additional Dependencies).

You can also specify your own custom reporter. Check as an example.

Additional dependencies

Trombi by default uses only few libraries. However, it supports some features that might need additional dependencies.

If you want to use Gatling Highcharts reporter you have to include the depenency [com.github.mhjort/trombi-gatling-highcharts-reporter "1.0.0"].

If you want to use http-kit as your http client library you have to include the dependency [http-kit "2.6.0"].

Note that in trombi clj-time dependency is not included anymore because from Java SE 8 onward, users are asked to migrate to java.time (JSR-310) However, trombi is backwards compatible and if you still want to use clj-time please add [clj-time "0.15.2"] as a dependency.


See example project here: metrics-simulation

Here is a presentation on how to test stateful applications with Trombi

Tuning the test runner

In load testing the goal is to generate load to the system under the test. However, sometimes the test runner can be also a bottleneck. trombi has been built on the idea that the request functions should be non-blocking. This way the test runner does not need to use that many threads and it is possible to generate huge amount of requests from the single machine. To track this behaviour there is now an experimental support for tracking active thread count in test client. By setting :experimental-test-runner-stats? true you can get statistics about thread count during the test simulation. In the end trombi will produce following output to console:

Test runner statistics: {:active-thread-count {:average 30, :max 33}}

In general these numbers should be lower than the number of concurrency in the simulation. And when increasing the concurrency these numbers should not increase accordingly.

Change History

Note! Version 0.8.0 includes a 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 may be deprecated in future. You can see documentation for old versions here.


This is compatible with Jenkins Gatling Plugin.


AFAIK there are no other load testing tool where you can specify your tests in Clojure and which is suitable for testing large stateful applications. In my opinion Clojure syntax is very good for this purpose.

Design Philosophy

Real life scenarios

Idea is to test the system/application by simulating how multiple users access the application concurrently. After the simulation you get comprehensive results: both as Clojure data and nice graphs if you want.

If you want to just test single web page with huge amounts of requests simpler tools like Apache Benchmark can do that. Of course, trombi can do that also but it might be an bit of an overkill for that job.


I am not a fan of complex DSLs. trombi 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 experimental tool that can run trombi in a distributed manner. It uses AWS Lambda technology for running distributed load tests in the cloud.


Use GitHub issues and Pull Requests.


By default Leiningen is used for development. There is also experimental support for deps.edn (See Makefile).


Copyright (C) 2014-2023 Markus Hjort

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

Can you improve this documentation? These fine people already did:
Markus Hjort, Andrea Crotti, mhjort, Dan Meyers, Remi Lefebvre, Antonis Kalou, Oliver Powell & Jussi Virtanen
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