Developing A Tool To Mock APIs With 100K Downloads As A Side Project

Published: January 30th, 2021
Guillaume Monnet
Founder, Mockoon
from Luxembourg
started September 2017
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Hello! Who are you and what business did you start?

Hi! I’m Guillaume. A passionate full-stack developer living in Luxembourg. I am working full-time in the banking industry and started working on a new side project three years ago: Mockoon.

Mockoon is a developer tool that allows you to easily and quickly mock any REST API. It’s an open-source desktop application that has been growing steadily for three years.

We recently passed the 100k downloads mark and 10k monthly active users. We are pre-monetization and focusing on growth.


What's your backstory and how did you come up with the idea?

I have quite an unusual profile for a developer as I studied law like most of my family. I got my master’s degree in intellectual property in 2008 and worked six years as a legal advisor in various companies like Yahoo! and PricewaterhouseCoopers.

Study the media you are going to post on and plan accordingly. You want to avoid launching on a website when the time is not right, especially when talking about Product Hunt!

However, I started coding in high school with C++ and never stopped. It quickly became a passion, and after spending most of my free time coding, I switched careers in early 2015. I am self-taught, and today I am lucky to be able to code for a living.

I came up with the Mockoon idea after discussing it with several colleagues in 2017. At that time, I was employed full-time as a front-end engineer, using a backend API that was under development. I started looking for some API mocking tools but got overwhelmed by the complexity of the existing ones. So, being a software engineer, I created a new one!

At first, I considered this project as a cool addition to my portfolio rather than a business idea. Still, as much as I wanted to have fun working on a personal project, I also wanted other developers to use it.

So, I tried to do everything well from the beginning. Having made a lot of mistakes on a previous startup (Sneepets, an Instagram for pets), I started reading books about entrepreneurship and startups. Thus, I tried to avoid replicating the same mistakes, especially working for two years on a project without showing it to anyone.

Sneepets, my first endeavor

Apart from my gut feelings, I had nothing yet to validate my idea. But I knew that creating a desktop application would add something new to the market. Something differentiating. The competitors in this field were mainly mocking APIs through online deployments.

So, I built an MVP in three months and posted it on various subreddits. The feedback was highly positive, which validated the project and filled me with joy. It is where it all began.

Take us through the process of designing, prototyping, and manufacturing your first product.

As said before, I wanted to avoid repeating past mistakes: reinventing the wheel, working without user feedback, over-engineering, etc.

So first thing, I built the prototype in three months, leaving aside all the bells and whistles that would not make any difference.

I leveraged my existing knowledge, using the language I knew best (Javascript, Electron), the libraries I was comfortable with (Angular, Electron, Express, Bootstrap), and the platform where I felt at home (Firebase).

To design the MVP I worked by trial and error and also followed the adage “eat your own dog food.” I was using the tool daily and tried to be my first unbiased user.

And it worked quite well. I showed it to my coworkers and they immediately liked it. I was pumped up and ready to launch.

I was also lucky to have no costs, Mockoon being a desktop application. For the binaries and website hosting, I used a combination of GitHub, Firebase hosting and Cloudflare, which reduced the hosting costs to 0$ for three years.

Fun fact, the interface and the website barely changed since the beginning. Small improvements here and there for sure, but no major redesign. They are working and converting well enough!

Mockoon application MVP screenshot

First version of the website

Describe the process of launching the business.

After testing the tool with my coworkers, I posted it on various relevant subreddits: frontend, web development, programming, etc.

The feedback was mostly positive, and the posts got many upvotes. I also launched on Product Hunt and was lucky to get around 160 votes, enough to keep Mockoon above the fold during the day, which makes a big difference in terms of traffic.

Another fun fact: I posted it on Product Hunt several days before, on a Saturday, if I remember well, and it flopped. But Product Hunt’s team emailed me and proposed to repost the tool during a day with a larger audience. And it worked.

So I guess luck played an important part.


Thanks to these posts, I got more articles mentioning the tool and a small but steady flow of visitors to the website.

If I had one piece of advice to give: study the media you are going to post on and plan accordingly. You want to avoid launching on a website when the time is not right, especially when talking about Product Hunt!

Since launch, what has worked to attract and retain customers?

At first, I wanted to sell the software after the beta period, at least for commercial usages. But quickly after launching, I open-sourced it.

A lot of developers requested it, and it seemed to be the right thing to do. As a developer, I use so many open-source tools and libraries, I felt like I had to give back to the community. And it was the best choice I ever made.

It also helped a lot with the growth and multiplied by ten the interactions with the users. People started contributing, fixing bugs, and adding new features. Also, it was an occasion to post again on social media. And the feedback was overwhelmingly positive.

Today, the number one source of traffic is SEO. Unfortunately, I realized that very recently and spent more than two years working on new features and doing zero marketing whatsoever. Zero like in zero blog posts written.

I felt the pain when I saw a drop in the number of visits after the Google Search algorithm update in April. Probably the lack of content didn’t help.

This year I started writing tutorials, blog posts, etc. I see the traffic slowly coming back, but this is, without doubt, the most difficult part, especially as a developer!


One piece of advice: the earlier you start, the better. Because if it is not something you are used to, you will have to learn a lot!

I also try to be more active on Twitter and LinkedIn. Honestly, the learning curve is steep. I usually don’t share much there. I feel like nobody wants to hear what I have to say. But you have to change your mindset to start interacting with people and sharing all the things relevant to your product or brand. It’s not easy. But I’m learning.

Finally, I noticed that most of the users are coming from word of mouth. When I took my current job, I was astonished to learn the second day I was there that two people in the team were using Mockoon. I heard them talking about the tool right next to me, while they didn’t know I created it. It was a fun and rewarding moment. Today, five people are using the tool but, only one of them searched for it on the Internet and recommended it to the rest of the team. I know the same thing happened at various workplaces where some of my friends are working.

So I guess word of mouth is pretty powerful. And the only way I know to maximize it is to provide the best experience and create the best product!

How are you doing today and what does the future look like?

Mockoon is still a side-project for me, but a successfully growing one. The monthly active users of the application recently crossed 10,000 and keep growing. In parallel, the amount of work is increasing, and the question of working full-time on this project is more pressing now. Each added feature increases the work that needs to be done by 10: tutorials to update, new documentation to create, posting on social networks, interviewing users, etc.

Users since the launch. It took one full year to really start seeing some decent traffic

2021 will surely be the year where we focus on content. There is a lot of room to grow by publishing tutorials, blog posts, customer stories, etc. It all contributes to reinforcing our brand and capitalizing on search engines’ long tail.

Launch early but gather feedback earlier. Contrarily to the adage, nobody will come, even if you build it. You have to find your users. Go where they gather, talk to them, but most of all, listen to them!

As an example, we started creating API mock samples to allow developers to mock their favorite API, like Paypal or Stripe, in one click. We think that there are a lot of secondary tools and products to create that will ultimately serve the user's growth.

We also continue interviewing users to learn how they use the tool, what they need, and what their pain-points are. It will also allow us to publish nice “customer stories.”

Being an open-source application, Mockoon is still 100% free to use. During next year, we may slowly start working on optional paid services while keeping the core application open-source. We already know some users and companies that would be interested in such features. However, we feel that we should prioritize growth over monetization in the short term.

Through starting the business, have you learned anything particularly helpful or advantageous?

I discovered that having too much time on your hands can be detrimental to your productivity. I have two kids, and I realized that I have never been as productive as now that I can only spend two to three hours on the project during the evenings.

So, I would say, don’t work too hard on your project to avoid filing the time you have with bogus tasks and limit them to what serves your growth. Employ the extra time to learn, read, exercise, or spend time with your family.

Another tip. Everybody says it, everybody knows it, but we all forget about it most of the time we are starting a new venture: marketing/sales will be at least 80% of the work. It was hard to grasp for me as a developer. So, don’t spend one year or more developing full time in a cave. You should get your project out there, talk about it, write about it, find clients, find users, etc.

What platform/tools do you use for your business?

Our backbone is GitHub. We use everything from them: code hosting, issues, discussions, releases, continuous integration pipelines (Actions), etc. We are lucky to have developers as users, so it speaks to them, and we don’t need to use any other tool to handle support requests like Intercom.

We couldn’t live without Firebase from Google. We use it for hosting, remote configuration, authentication, etc. It’s cheap and makes everything so much easier.

We also manage our domains and DNS with Cloudflare. Put in front of Firebase Hosting, it reduces to zero all the hosting costs for our website.

On the SEO side, we use a combination of Google Search Console and Ahrefs. They are both great for analyzing organic traffic and planning our content strategy.

What have been the most influential books, podcasts, or other resources?

Even if Mockoon is still a side project, I try to read a lot and learn from successful entrepreneur stories.

The following books are the one which resonated with me the most:

  • Getting Real: It’s a must-read if you are building an application. But the ideas developed in this book can also apply to a lot of other domains. You can read all of Basecamp's books (It doesn’t need to be crazy at work, Rework, Shape up, etc.) and you will be equipped to tackle the most complex project with a completely different mindset.
  • The lean startup: It's one of the most insightful books on entrepreneurship I have read. You can easily take away a lot of advice that applies to non-tech business, or solopreneurs.
  • Microconf’s videos are a must to learn from successful entrepreneurs. One thing I learned and won’t forget: always, always, raise your price!

Advice for other entrepreneurs who want to get started or are just starting?

Know why you are doing things. Depending on the answer, you may have to stop right now. The best example for me is the following one: you are creating this great app, and when choosing the framework you will be using, you follow the hype and choose the one you do not know at all. I see this all the time.

If you are creating this app to learn, enrich your portfolio, it’s ok, continue. If you are starting a business that needs to pay the bills, you must stop immediately and choose the tool you know best, even if it is a basic PHP script. In this regard, I find Peter Levels’ story very inspiring.

Asking yourself "why" for everything you do or content you create also helps you assess if the tasks you are currently doing serve your business in its current state, contribute to its growth, or are utter procrastination.

Also, no magic wand here, but read startup books, blog posts, and watch videos from successful entrepreneurs. You can learn how to avoid most of the mistakes by listening to all the great advice available out there for free.

One last piece of advice: launch early but gather feedback earlier. Contrarily to the adage, nobody will come, even if you build it. You have to find your users. Go where they gather, talk to them, but most of all, listen to them!

Where can we go to learn more?

If you have any questions or comments, drop a comment below!