WeightWagers

This is a proof of concept app that allows users to bet money on whether they can lose weight.

Wager Creation

Users create a wager by sending Ether to the WeightWagers contract. When a user creates a wager, he also sends

1. How much time he needs in order to lose the weight
2. How much weight he bets he can lose
3. His smart scale credentials (we will oraclize calls to his smart scale's API to verify his starting and ending weights)

Now, because this is a proof of concept, time is measured in seconds instead of days. This is for ease of testing. If this were to go to production, it would be days, and I'd have to introduce some extra tooling for controlling time on a local blockchain in order to facilitate testing.

Also, since it's a proof of concept, the "smart scale credentials" are simply a string that tell the Google Cloud App that mocks the scale data what data to return. I had in mind the Naked scale (https://nakedlabs.com/) when I made this app, but as that scale doesn't have an API yet, I went with this very simplified approach.

Wager Verification

Users can attempt to "win" their wagers at any time. There are three possible outcomes for each of the user's wagers.

1. The wager can be expired. This happens if the user didn't lose enough weight in the amount of time he gave himself when creating the wager. In this case, the wager is deleted and the contract keeps the user's funds.
2. The wager is unchanged. This happens if the user didn't lose enough weight, but the wager still hasn't expired. In this case, the contract keeps the user's funds and doesn't delete the wager, so that the user can try to verify again in the future.
3. The wager is verified. This happens if the user did lose enough weight before the wager expires. In this case, the user gets his original funds back plus an additional 3.1% of his original wager.

Admin functionality

The owner of the contract (the second address that ganache supplies) has access to additional functionality in the app. They can perform an emergency stop on the contract, which will prevent wager creation (but still allow wager verification).

How to install

Install global dependencies

1. Make sure ganache-cli is installed globally. You can run npm install -g ganache-cli to install it globally.
2. Make sure truffle is installed globally. You can run npm install -g truffle to install it globally.

Install the app

1. Clone the WeightWagers app from github with git clone https://github.com/djshapiro/WeightWagers.git.
2. Navigate into this app's directory with cd WeightWagers.
3. Run npm install
4. Run truffle install oraclize-api. Run this command from the top level of the WeightWagers directory. If you run truffle commands from a subdirectory of WeightWagers, it will fail.

Install ethereum-bridge

1. Install ethereum-bridge in its own directory (not inside of WeightWagers) for testing oraclize locally by running the following four commands.

• mkdir ethereum-bridge
• git clone https://github.com/oraclize/ethereum-bridge ethereum-bridge
• cd ethereum-bridge
• npm install

How to run

1. In the WeightWagers directory, run ganache-cli -m "$(cat ganacheMnemonic)". This will start ganache with a specific mnemonic.
2. Copy the ganache mnemonic so you can paste it into metamask later. Remember that the first account is a regular user while the second account is the admin.
3. In the ethereum-bridge directory, run node bridge -a 9. This will start ethereum-bridge.
4. After about a minute, ethereum-bridge will say something that looks like "OAR = OraclizeAddrResolverI(0x6f485C8BF6fc43eA212E93BBF8ce046C7f1cb475);". Copy this line and paste it into contracts/WeightWagers.sol in the constructor at around line 117. Delete the line in the contract that looks like that line.
5. Run truffle compile; truffle migrate from the top level of the WeightWagers directory. If you run truffle commands from a subdirectory of WeightWagers, it will fail.
6. Run npm run start
7. Navigate to localhost:3000 in your browser.

Notes on using the app

Oraclizing is expensive and time-consuming. Expect your wager creation and verification calls to take about 30 seconds.

Further reading

• Notes on design choices I made can be found in design_pattern_decisions.md.
• Notes on security-related choices I made can be found in avoiding_common_attacks.md.
• User stories can be found in user_stories.md

Contact me

Please let me know if you have any problems installing or running the app.