In this series of articles, I’m doing a walkthrough of a hobby project I’ve worked on, a payment system for the Ethereum blockchain. You can read the intro article for more context about the project.
For this article, I will assume that you have read part 1 about authentication and wallet management, and part 2 about the data models used in the system. Therefore, I will start from the assumption that you’re aware of the basic foundation on how to let users connect their crypto wallets to as website, how to display a list of available digital products that they can buy from, and that you are also somewhat familiar with the data models presented in part 2.
In this part, I will cover how payments are processed and validated. This includes the following steps, in order:
- Server-side prevalidation: validating, before even processing anything, that the wallet is authorized to buy a particular digital product, and that this product is available.
- Purchase: sending a payment transaction to the Ethereum blockchain for the required price, and getting a transaction identifier in return (also called a “transaction hash”)
- Polling the blockchain: using the transaction hash to check if the transaction has succeeded or failed.
- Triggering post-payments actions: dealing with the aftermath of the transaction status, which means showing an error message in case the transaction failed, or starting further steps by allowing access to resources in case the transaction succeeded.
The Ethereum blockchain makes it very easy to process payments, because every transaction is given a unique transaction hash, which means that once you have submitted a transaction to the Ethereum network for validation, all you need is this transaction hash, and you’ll be able to track the status of the transaction by polling the Ethereum blockchain.
In reality, for a small application such as the hobby project I’m presenting here, there is no need to poll the Ethereum blockchain directly. Instead, my program uses a third-party provider that makes polling the blockchain easier via an API. Many medium to large applications nowadays are running their own Ethereum nodes so that they can poll the blockchain directly. It costs them extra teams and complexity to maintain that infrastructure, but it comes with the benefits of higher trust and control of their data, and also processing speed. For them, the tradeoff is worth it.
Below is a systems diagram showing how I have implemented the payment processing part. It assumes that the user has connected his wallet and already obtained a JWT that identifies him uniquely.
The diagram contains a description of each step, and the rest of this section gives further details as to what happens in each step.