How to monitor RPC block responses for token transfers?

Question

I am wondering how to effectively monitor for some given address for FA 1.2 and FA 2 token balance updates. Given that the entrypoints to the contract are defined, I'd like to write something which reads in each "block" response from the Tezos node, iterates over all executed transactions, and analyzes the entrypoints to track when one of my addresses had a transaction occur.

This would of course only be feasible if all potential token transfers appear as "Transaction" operations in the block response, with the appropriate endpoints.

An example of a token I may want to track is tzBTC, which does call transfer internally - and it does show up in the RPC response.

The other method of doing this would be to look inside the storage of each contract, but that doesn't seem particularly scalable.

Answer 1

An example of a token I may want to track is tzBTC, which does call transfer internally - and it does show up in the RPC response.

That is the answer, isn't that? :)

---

So, you need to iterate over transactions in a block and check for those where destination is {tzBTC} and parameters.entrypoint is "transfer". Then you need to analyze the parameters. They look like this:

"parameters": {
  "entrypoint": "transfer",
  "value": {
    "prim": "Pair",
    "args": [
      {
        "string": "tz1Y77S4EcSQwyUWzAwtYGU6NTtKXbH3VQLg"      <--- from
      },
      {
        "prim": "Pair",
        "args": [
          {
            "string": "tz1ZbokEB5tekbPxyWdo28ugn3UUkbQjguK3"  <--- to
          },
          {
            "int": "119380000"                                <--- amount
          }
        ]
      }
    ]
  }
}

As you can see, you can easily extract tokens amount, sender and receiver, and check whether it's you.

However, I highly recommend you use existing solutions, like BCD API, because there are so many pitfalls. Otherwise you will likely shoot yourself in the foot.