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Mike Solomon for Meeshkan

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Comparing Prism, Hoverfly, and HMT- Part 1

This is the first article of a two-part series comparing Prism, Hoverfly, and HMT. This article presents a high-level comparison. The second one shows they can be used with a specific API.

HMT team
Our team during a brief, spontaneous moment of levity.

At Meeshkan, we reverse engineer software. Our mission is to build a tool that can clone even the most complex systems with a high degree of accuracy.

We have recently begun to open-source more of our stack as we get closer to achieving our mission. In doing so, we've come up against some strong incumbents such as Prism and Hoverfly. In this article, I'd like to present both of those tools to our new open-source project HMT.


Prism is an open-source component of the stack. Stoplight is, in my experience, the best editor for building OpenAPI specifications. The UI is gorgeous. If you are authoring an OpenAPI spec and don't like writing YAML or JSON, you should try Spotlight.

Using Prism, you can take your OpenAPI spec built on Stoplight (or built with any other tool) and run:

$ prism mock my-spec.yml

Prism will create a server that mocks all of the endpoints of your OpenAPI spec. For example, imagine that my-spec.yml contains a path /users. You can call http://loalhost:3000/users to get a list of mock users that conforms to the spec in my-spec.yml.

Conclusion:: Prism is a great tool to turn an OpenAPI spec into a mock server.


Hoverfly creates a mock server based on recordings of a live service.

The most straightforward way to record traffic with Hoverfly is to use it as a proxy. To do this, you start hoverctl and instruct it to run in capture mode. Then, you can use curl or Postman to send your HTTP traffic through the Hoverfly proxy. It is also possible to set up a proxy in specific languages. For example, in NodeJS, you can set the HTTP_PROXY and HTTPS_PROXY environment variables.

From here, you can export the recording to a simulation file and stop hoverctl when you're done.

$ hoverctl start
$ hoverctl mode capture
$ curl --proxy http://localhost:8500
$ hoverctl export simulation.json
$ hoverctl stop

After this, hoverfly consumes the simulation file in order to serve mock data. In this case, it will serve the recording we made of /a/b.

$ hoverctl start webserver
$ hoverctl import simulation.json
$ curl http://localhost:8500/a/b
$ hoverctl stop

In this way, Hoverfly allows you to record and store various fixtures from a mock API and serve them back. Hoverfly uses a custom JSON format to store its recordings. In this JSON file, you can do things like specify matching if your URLs have wildcards.

Hoverfly is extensible through a middleware system that communicates through stdin and stdout. Middleware can do things like change a response's status code and even add latency.

Conclusion: Hoverfly is a good tool when you need to serve back recordings of an API for testing.


If Hoverfly and Prism had a baby, it would be HMT. HMT allows developers to build a mock server from recordings and OpenAPI specs. It exposes a Python API for building a persistance layer behind mocks. We also use it to tie together ML models to simulate realistic server traffic.

HMT ingests recordings in the http-types format. You can build these HMT recordings using middleware or a proxy. For example, let's say that we have an express server and want to do our recording from that. We can use the HMT express middleware to save recordings.

    transport: S3Transport({
      bucket: "my-recording-bucket",
      key: "/ myservice",
      prefix: "recording_",

It's also possible to use HMT as a reverse proxy for recording.

$ hmt record --log_dir ./logs
$ curl http://localhost:8000/http/

Then, we use hmt build to turn these recordings into an OpenAPI spec. In the example below, we mix the recordings with a preexisting spec. The result is an enhanced OpenAPI spec.

$ hmt build --mode replay -i recording.json -a spec.yml -o enhanced_spec

If you use gen mode, then HMT will infer a spec from recordings instead of serving them rote.

$ hmt build --mode gen -i recording.json -a spec.yml -o enhanced_spec

Like Hoverfly and Prism, HMT can be started in server mode.

$ hmt mock -i enhanced_spec/

Conclusion: HMT blends together recordings and OpenAPI specs into an enhanced OpenAPI spec. We'd love to hear your feedback on our GitHub issues page. Also, please don't hesitate to leave a comment with feedback!

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