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Applications are systems. Systems are fascinating entities, and one of their characteristics is that we can observe them. Read more about that here. Also make sure you read about the rationale behind this library.

Dependencies Status

What's in the box?

This library helps you build distributed systems. Such a larger system could, for example, consist of multiple processes in different JVM, plus all connected browser instances, which, if you think about it, are an important part of the overall distributed systems. Going forward, there is also support planned for native apps. Except for a different presentation layer, the required code should be the exact same as for single-page web applications.

This library only contains the bare minimum for building and wiring systems. Additional functionality can be found in these repositories:

  • systems-toolbox-ui: This library gives you a simple way to build user interfaces using Reagent. This provided functionality is somewhat comparable to React & Redux. I'll elaborate on this soon. This library powers the user interfaces of the example applications.

  • systems-toolbox-sente: This library connects browser-based subsystems with a backend system using WebSockets. This library contains everything you need for serving your application via HTTP, including support for HTTPS, HTTP2, deployment in application servers, and serving REST resources. This library serves the sample applications and provides the communication with their respective backends.

  • systems-toolbox-kafka: This library connects different systems via Kafka so that they can form a larger, distributed system.

  • systems-toolbox-metrics: This library is a small example for how functionality can be implemented across subsystems. Here, we have a server-side component which gathers some stats about the host and JVM, plus a UI "widget" that can be embedded in a systems-toolbox-ui interface. You can see it in use in both example applications.


Artifacts are released to Clojars.

With Leiningen, add the following dependency to your project.clj:

Clojars Project

In addition, you also need to add the dependency for core.async, e.g. with Leiningen:

[org.clojure/core.async "0.6.532"]


This library targets both Clojure and Clojurescript and is written entirely in .clc. Accordingly, testing needs to happen on both the JVM and at least one of the JS runtimes out there. For testing on the JVM, you simply run:

$ lein test

On the JavaScript side, you have more options, for example:

$ lein doo node cljs-test once

Instead of once, you can also use auto to run the tests automatically when changes are detected. For more information about the options, check out the documentation for doo.

Both ways of testing run automatically on each new commit. On the JVM, we use CircleCI: CircleCI Build Status

On TravisCI, the tests then run in a JS environment, on Node.js: TravisCI Build Status

Check out the circle.yml and .travis.yml files when you need an example for how to set up your projects with these providers.


Right now, there are two example applications:

  • There's an example project in this repository that visualizes WebSocket round trip delay by recording mouse moves and showing two circles at the latest mouse position. One of them is driven by a message that only makes a local round trip in the web application, and the other one is driven by a message that is sent to the server, counted and sent back to the client. Thus, you will see the delay introduced by the by the client-server-client round trip immediately when you move the mouse. Also, there are some histograms for visualizing where time is spent. There's a live example of this application here.

Example Screenshot

  • Then, there's the toy example I mentioned above, BirdWatch. This application provided the inspiration for this library. A running demo instance can be seen here.

BirdWatch Screenshot

Feedback and question

Please feel free to open issues here or in any of the related projects when you have a question. Also, you can send the author an email, but issues are generally preferred as more users would benefit from the resulting discussion. Also, there's a chat on Join the chat at


Contributions always welcome! Unless it's nothing more than the fix of a typo though, the best approach is to start with an issue and a brief discussion of the problem or improvement, rather than start the process with a pull request. Cheers.

Project maturity

This project is quite young and APIs may still change. However, you can expect that minor version bumps do not break your existing system.


Copyright © 2015, 2016 Matthias Nehlsen

Distributed under the Eclipse Public License either version 1.0 or (at your option) any later version.


Tools and building blocks for building Systems in Clojure and ClojureScript







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