How DAML handle double spending problem

Questions
1

Double-spending is a problem that arises when transacting digital currency that involves the same tender being spent multiple times. How does DAML handle double spending problem?

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2

Hi @Rocky

Tender can be represented as a Daml contract, by default, each Daml contract is unique. In addition, there is the concept of the consumption of a contract, meaning the original contract is destroyed and can no longer be accessed. (A user cannot perform a “spend” or “transfer” action twice), and a “spend” or “transfer” action will essentially archive that amount belonging to the user performing the action.

For example the below snippet,

template CBDCv4
  with
    issuer: Party
    owner: Party
    amount: Decimal
    currency: Text
  where
    signatory issuer, owner
 
    controller owner can
      Transfer    -- Creates a new TransferRequest 
      ...
 
template TransferRequest
  with
    receiver: Party
    digitalCash: CBDCv4 -- This contains data required to create a new 
                         -- CBDCv4, i.e., issuer, amount, currency
  where
    signatory digitalCash.issuer, digitalCash.owner
 
    controller receiver can
      Accept    -- Creates a new CBDCv4 with the new owner
      ...

Once you propose a certain amount to be transferred, the contract that belongs you will you will archived, and a new contract for the recipient will be created. The archival of the contract that belongs to you prevents double spend, because you will not be able to create a transfer from that instance of the contract anymore.

You can read this article that gives more details “Why Daml is Great to represent digital currency”

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