Customize the structure of a transaction

Question

I'm still a newbie with ethereum and I want to know if it is possible to customize the structure of a transaction in order to add extra fields like a message or an integer or anything else.

If the answer is yes, can you please give me some good hints to guide my researchs.

Thank you!

Answer 1

In Ethereum, payments are normal functions with value attached. That means anything you can pass to a function you can include in a payment. It's fairly common to have some data about what was bought together with the funds to pay for it.

Simple example:

function checkOut(bytes32 itemId, uint qty) payable returns(bool success) {
   // do stuff like check the math
   if(items[itemId].price * qty != msg.value) throw;
   // track inventory, status, etc.
   return true;
}

Example assumes a struct full of item information and checks the price * qty matches the ETH sent with the transaction (msg.value). Throw reverts state changes and gives the money back in case that check failed (error or mischief on the front-end).

This example has a payable function: http://solidity.readthedocs.io/en/develop/solidity-by-example.html#safe-remote-purchase

payable is used to explicitly declare that receiving ETH is allowed, otherwise they autotmatically reject unexpected funds for security reasons. It's a relatively new feature for non-payable functions to throw when they receive funds.

Hope it helps.