CheckCheetah: A Rails Web App for Ordering and Paying at a Bar
Filed under: Projects — posted by Rob on December 13, 2013
In a previous entry, I mentioned that much of my personal free time had been devoted to a closed-source programming effort for a startup I’m involved with. After we spent several months trying to sell the completed MVP and concept to prospective users, we didn’t have much luck. This idea of business and marketing and sales is new to us, so we didn’t expect to succeed right away. Even if we are never able to sell this, I think we learned several valuable lessons along the way. I’d like to share some of the technical insights learned while building the app.
CheckCheetah is a mobile web app which allows a customer at a bar or restaurant to see the menu, place an order, and then pay for the items & tip using their Amazon.com accounts. We knew the market was rather saturated, but we felt that by utilizing the Amazon payment method, we could offer a much quicker ordering process than competitors. The reason is because almost everyone already has their credit card details saved in Amazon, not to mention they trust Amazon with such financial details. On the business administrative end, the app allows the business to customize their menus in real time and even setup specials that would change the price of an item at a given time of day or day of the week.
Obviously, the site requires the capability to receive payments. We chose to use Amazon Flexible Payment Services (FPS) API for this. If the state of the documentation is any indication, I feel like FPS is not one of the most well-supported or used services offered by Amazon, but it was the only payment system we found which offered the types of complex payment handling we wanted to do (more on this in Lessons Learned). It had the bonus side-effect of using the payment information most people already have stored in Amazon, while also freeing us from the need to be PCI compliant or worry about securing credit card numbers.
We decided to use Heroku for the obvious reasons: we did all this work in our free time with full-time jobs, so we didn’t have precious hours to configure AWS instances and worry about how to scale them if we had hundreds of users on a Friday night. The benefit of Heroku is that it works magically with Rails. For this particular application without much asset pipeline complications, deployments were quick and easy. Never once did a deployment fail, and we never had downtime issues. Being able to develop everything for free with 1 dyno is fairly hard to beat. The tooling around Heroku’s PostgreSQL is also amazing and we were confident it would scale with us, with things like Follower Databases.
Amazon FPS API
For this app, we had some unique payment needs. The goal was to allow a customer to pay a business directly, but for us as the SaaS provider to take a percentage cut. This essentially meant that financial transactions involved three parties, instead of the usual two. This quickly excluded many of the common APIs from places like Square or Paypal. We also had extra requirements. For example, we didn’t want someone to have to go through the Amazon sign in and pick their credit card every time, if they were a repeat customer. This required the ability to “remember” past financial details. It turns out that Amazon FPS (Flexible Payment Services) allows both of these unique cases. By using the “Marketplace” preset, we could accomplish our three party system. By creating multi-use recipient tokens, we could save a customer –> business “session” for later use, by asking the user to authorize $500 (an arbitrary number we picked) ahead of time. It turns out that even for both of these somewhat complex use cases, FPS had a “preset” for us. I was highly impressed by the flexibility (it really lives up to its name), and realize that we only scratched the surface: FPS allows writing custom payment instructions in an XML-based DSL for even more advanced transactions.
FPS uses statuses for everything, and the state transitions were somewhat cumbersome. The API was fully asynchronous, so placing an order would return immediately with a status of “Pending”. This caused some challenges as we would have to poll for success, but Amazon has a solution to this. They allow you to setup an API endpoint where they will POST a request to you when the order reaches a terminal state. This was extremely useful, since we didn’t have to worry about managing the interim states, simply letting Amazon feed us a “success” or “error” at some time in the future. Designing our internal statuses around this led to quite a few business requirements discussions, but from a technical point of view it was fairly simple and avoided a lot of the complications most payment APIs might lead to.
We never reached the need to scale or use FPS heavily in production, so I can’t vouch for that side of it. However, I can say that from my experience the API was easy to use (there are some Ruby gems to make it easier, but even doing it manually is a fairly straight-forward API call with HMAC signing) and has an impressive array of features.
You can check out a working version on our staging server here. Note that the Faye server is currently not working on staging so the “push” updates won’t be working. The system gracefully supports manual refreshes in this case. It is also tied to the sandbox for Amazon FPS, so feel free to place an order using your Amazon account – it will come pre-baked with a fake credit card number and billing address.
Below are some screenshots of the system in action from customer viewpoint (on a phone) and business viewpoint (on desktop).