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Contributing to mountebank

Congratulations! You're here because you want to join the millions of open source developers contributing to mountebank. The good news is contributing is an easy process. In fact, you can make a difference without writing a single line of code. I am grateful for all of the following contributions:

  • Submitting an issue, either through github or the support page
  • Commenting on existing issues
  • Answering questions in the support forum
  • Letting me know that you're using mountebank and how you're using it. It's surprisingly hard to find that out with open source projects, and provides healthy motivation. Feel free to email at
  • Writing about mountebank (bonus points if you link to the home page)
  • Creating a how-to video about mountebank
  • Speaking about mountebank in conferences or meetups
  • Telling your friends about mountebank
  • Starring and forking the repo. Open source is a popularity contest, and the number of stars and forks matter.
  • Convincing your company to let me announce that they're using mountebank, including letting me put their logo on a web page (I will never announce a company's usage of mountebank without their express permission or a pre-existing public writeup).
  • Writing a client library that hides the REST API under a language-specific API
  • Writing a build plugin for (maven, gradle, MSBuild, rake, gulp, etc)

Still want to write some code? Great! You may want to keep the source documentation handy, and you may want to review the issues.

I have two high level goals for community contributions. First, I'd like contributing to be as fun as possible. Secondly, I'd like contributions to follow the design vision of mountebank. Unfortunately, those two goals can conflict, especially when you're just getting started and don't understand the design vision or coding standards. I hope this document helps, and feel free to make a pull request to improve it! If you have any questions, I am more than happy to help at, and I am open to any all suggestions on how to make code contributions as rewarding an experience as possible.

Designing mountebank

The code in mountebank is now a few years old, and maintaining consistency of design vision is key to keeping the code maintainable. The following describe key concepts:

API changes

I consider the REST API the public API from a semantic versioning standpoint, and I aspire never to have to release a v2 of mountebank. In my opinion, the API is more important than the code behind it; we can fix the code, but we can't change the API once it's been documented. Therefore, expect more scrutiny for API changes, and don't be offended if I recommend some changes. I often agonize over the names in the API, and use tests to help me play with ideas.

Before API changes can be released, the documentation and contract page need to be updated. The contract page is, I hope, friendly for users, but a bit unfriendly for maintainers. I'd love help fixing that.

Protocol Agnosticism

Most of mountebank is protocol-agnostic, and I consider central to its design. In general, every file outside of the protocol folders (http, tcp, etc) should not reference any of the request or response fields (like http bodies). Instead, they should accept generic object structures and deal with them appropriately. This includes all of the core logic in mountebank, including predicates, behaviors, and response resolution. To help myself maintain that mentality, I often write unit tests that use a different request or response structure than any of the existing protocols. This approach makes it easier to add protocols in the future and ensures that the logic will work for existing protocols.

I aim in a future version to completely separate the protocol implementations as separate plugins, making protocol extension as easy as pie.

Aim for the broadest reach

Many development decisions implicitly frame a tradeoff between the developers of mountebank and the users. Whenever I recognize such a tradeoff, I always favor the users. Here are a few examples, maybe you can think of more, or inform me of ways to overcome these tradeoffs in a mutually agreeable manner:

  • The build and CI infrastructure is quite complex and slow, but I'd prefer that over releasing flaky software
  • I aim for fairly comprehensive error handling with useful error messages to help users out
  • Windows support can be painful at times, but it is a core platform for mountebank

Coding mountebank

I do everything I can to resist code entropy, following the "no broken windows advice given by the Pragmatic Programmers. Like any old codebase mountebank has its share of warts. The following help to keep the code as clean as possible:

Pull Requests

While all pull requests are welcome, the following make them easier to consume quickly:

  • Smaller is better. If you have multiple changes to make, consider separate pull requests.
  • Provide as much information as possible. Consider providing an example of how to use your change in the pull request comments
  • Provide tests for your change. See below for the different types of testing in mountebank
  • Provide documentation for your change.

Developer Workflow

The following steps will set up your development environment:

  • npm install
  • npm install -g grunt-cli
  • grunt airplane

Note that running ./build (Linux/Mac) or build (Windows) will run everything for you even without an npm install, and is what CI uses. The grunt airplane command is what I use before committing. You can also run grunt, which more accurately models what happens in CI, but there are some tests that may pass or fail depending on your ISP. These tests that require network connectivity and verify the correct behavior under DNS failures. If your ISP is kind enough to hijack the NXDOMAIN DNS response in an attempt to allow you to conveniently peruse their advertising page, those tests will fail.

When you're ready to commit, do the following

  • Look at your diffs! Many times accidental whitespace changes get included, adding noise to what needs reviewing.
  • Use a descriptive message explaining "why" instead of "what" if possible
  • Include a link to the github issue in the commit message if appropriate

JavaScript OO

Try to avoid using the new and this keyword, unless a third-party dependency requires it. They are poorly implemented (most JavaScript developers don't know what the new keyword does, and every time I know it, I forget it 30 seconds later). In a similar vein, prefer avoiding typical JavaScript constructor functions and prototypes.

Instead prefer modules that return object literals. If you need a creation function, prefer the name create, although I've certainly abused that with layers and layers of creations in places that I'm none too proud about. Although I'm of mixed opinion on this, I've tended to capitalize objects with a create method to emulate the style for standard JavaScript constructors.

The code was written using ES4 and left that way for a long time to support node v4 and earlier. Now that node v4 is no longer supported, prefer using ES2015 features.

Requiring Packages

In the early days, the mb process started up quite quickly. Years later, that was no longer true, but it was like boiling a frog, the small increase that came from various changes were imperceptible at the time. The root cause was adding package dependencies - I had a pattern of making the require calls at the top of each module. Since that was true for internal modules as well, the entire app, including all dependencies, was loaded and parsed at startup, and each new dependency increased the startup time.

The pattern now is, where possible, to scope the require calls inside the function that needs them.


In the spirit of being as lazy as possible towards maintaining code quality, I rely on linting heavily. You are welcome to fix any tech debt that you see in SaaS dashboards:

There are several linting tools run locally as well:

  • eslint - I have a strict set of rules. Feel free to suggest changes if they interfere with your ability to get changes committed, but if not I'd prefer to keep the style consistent.
  • custom code that looks for unused packages and only calls left in the codebase

Testing mountebank

I almost never manually QA anything before releasing, so automated testing is essential to maintaining a quality product. There are four levels of testing in mountebank:

Unit tests

These live in the test directory, with a directory structure that mostly mimics the production code being tested (except in scenarios where I've used multiple test files for one production file, as is the case for predicates and behaviors). My general rule for unit tests is that they run in-process. I have no moral objection to unit tests writing to the file system, but I aim to keep each test small in scope. Your best bet is probably copying an existing test and modifying it. Nearly all (maybe all) unit tests are protocol-agnostic, and I often use fake protocol requests during the setup part of each test.

Functional tests

These live in the functionalTest directory, and are out-of-process tests that verify three types of behavior:

  • Protocol-specific API behavior, in the functionalTest/api directory. Each of these tests expects mb to be running (sometimes with the --mock --debug --allowInjection flags) and calls its API.
  • Command line behavior, in the functionalTest/commandLine directory. Each of these tests spins up a new instance of mb and verifies certain behaviors
  • Website integrity, in the functionalTest/html directory. These expect mb to be running and validate that there are no broken links, that each page is proper HTML, that the feed works, that the site map is valid, and that the documentation examples are valid. That last point is unique enough that I consider it to be an entirely different type of test, described next.

Documentation tests

The functionalTest/html/docsIntegrityTest.js file crawls the website and looks for special HTML tags that indicate the code blocks within are meant to be executed and validated. At first I wrote these tests as a check on my own laziness; I know from experience how hard it is to keep the docs up-to-date. They proved quite useful, however, as a kind of BDD style outside-in description of the behavior, motivating me to rewrite the framework to make it easier to maintain. I'd like in the future to open source the docs tester as a separate tool.

You start by writing the docs, with the examples, in the appropriate file in src/views/docs. Wrap a series of steps in a testScenario tag with a name attribute to disambiguate it from other scenarios on the same page (multiple test scenarios on the same page are run in parallel). A step is a request/response pair, where the response is optional, and is wrapped within a step tag. The request and response must be within a code element, and are usually also wrapped in a pre block if they are meant to be visible on the page. To validate the response, wrap the response code block within an assertResponse code block.

The simplest example is on the overview page, documenting the hypermedia on the root path of the API. You can see the actual documentation on the website. It makes a request to GET / and validates the response headers and JSON body:

<testScenario name='home'>
    <step type='http'>
<pre><code>GET / HTTP/1.1
Host: localhost:<%= port %>
Accept: application/json</code></pre>

<pre><code>HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Vary: Accept
Content-Type: application/json; charset=utf-8
Content-Length: 226
Date: <volatile>Sun, 05 Jan 2014 16:16:08 GMT</volatile>
Connection: keep-alive

  "_links": {
    "imposters": { "href": "http://localhost:<%= port %>/imposters" },
    "config": { "href": "http://localhost:<%= port %>/config" },
    "logs": { "href": "http://localhost:<%= port %>/logs" }

The actual response is string-compared to the expected response provided in the assertResponse code block. To make the expected responses both easier to read from a documentation standpoint and avoid problems of dynamic data, the following transformations are applied before the comparison:

  • Any JSON listed is normalized, so you don't have to worry about whitespace within JSON
  • You can wrap any data that differs from response to response in a volatile tag, as shown with the Date header above
  • If you want to display something other than what's tested, you can use the change element
  • If you don't want to document the entire response to focus in on the key elements, you can set the partial attribute on the assertResponse element to true. The proxies page makes heavy use of this feature.

You can see an example using the change element on the getting started page. The docs indicate that the default port is 2525, and so it intentionally displays all the example requests with that port, even though it may use a different port during test execution:

    <step type='exec'>
<pre><code>curl -X DELETE http://localhost:<change to='<%= port %>'>2525</change>/imposters/4545
curl -X DELETE http://localhost:<change to='<%= port %>'>2525</change>/imposters/5555</code></pre>

The two examples just shown include two types of steps. The following are supported:

  • http is the most common one, representing HTTP requests and responses.
  • exec is used for command line executions like the one shown above
  • smtp is used for SMTP examples, as on the mock verification page. It expects a port attribute indicating the port of the imposter smtp service
  • file is used to create and delete a file, as on the lookup examples on the behavior page. It expects a filename attribute. If the delete attribute is set to "true", the file is deleted.

As the doc tests get unwieldy to work with at times, I will often comment out all files except the one I'm troubleshooting in functionalTest/html/docsIntegrityTest.js.

Performance tests

I only have a few of these, to ensure a flat memory usage under normal circumstances and to ensure that the application startup time doesn't increase over time (as was happening as more package dependencies were added). Performance testing is a key use case of mountebank, so if you have experience writing performance tests and want to add some to mountebank, I'd be eternally grateful. These are run in a special CI job and not as part of the pre-commit script.


I was somewhat of a JavaScript newbie when I started mountebank, and even now, I don't actually code for a living so I find it hard to keep my skills up-to-date. If you're a pro, feel free to skip this section, but if you're like me, you may find the tips below helpful:

  • mocha decorates test functions with an only function, that allows you to isolate test runs to a single context or a single function. This works on both describe blocks and on it functions. You'll notice that I use a promiseIt function for my asynchronous tests, which just wraps the it function with promise resolution and error handling. promiseIt also accepts an only function, so you can do promiseIt.only('test description', function () {/*...*/});
  • Debugging asynchronous code is hard. I'm not too proud to use console.log, and neither should you be.
  • The functional tests require a running instance of mb. If you're struggling with a particular test, and you've isolated it using the only function, you may want to run mb with the --loglevel debug switch. The additional logging exposes a number of API and socket events.

A combination of only calls on tests with console.logs alongside a running instance of mb is how I debug every test where it isn't immediately obvious why it's broken.

Configuring Your IDE

I use IntelliJ to develop. I've found it convenient to set up the ability to run tests through the IDE, and use several configurations to run different types of tests:

Test configurations

The screenshot below shows how I've set up the ability to run unit and functional tests as part of what I've called the all configuration:

Configuration details

That configuration assumes mountebank is running in a separate process. I also have a configuration that removes the functional tests from the 'Application Parameters' line, which runs the unit tests without any expectation of mountebank running. Combined with the only function described in the Debugging section above, I'm able to do a significant amount of troubleshooting without leaving the IDE.

I use nvm to install different versions of node to test against.

The Continuous Integration Pipeline

The pipeline is orchestrated in CircleCI, although it uses TravisCI and Appveyor for OSX and Windows tests. I've had bugs in different operating systems, in different versions of node, and in the packages available for download. The CI system tests as many of those combinations as I reasonably can.

Every successful build that isn't a pull request deploys to a test site that will have a link to the artifacts for that prerelease version.

Releasing mountebank

Very few of you will have to worry about this, but in case you're curious, here's the process. CircleCI does most of the heavy lifting.

  • Make sure the previous builds pass across all operating systems, install types, and node versions
  • Review major / minor version.
  • Update the releases.json with the latest release
  • Add views/releases/vx.x.x with the release notes. Make sure to use absolute URLs so they work in aggregators, etc
  • Make sure all contributors have been added to package.json
  • commit
  • push
  • wait for the build to pass
  • git tag -a vXX.YY.ZZ -m 'vXX.YY.ZZ release'
  • git push --tags
  • update version in package.json to avoid accidental version overwrite for next version

Getting Help

The source documentation is always available at Firebase.

I'm also available for questions. Feel free to reach me at