Transactions

A transaction method gets exclusive, atomic access to state in order to update it (similar to a writer). But unlike a writer, a transaction can also call the reader, writer, and transaction methods of other states. Those reads and writes all occur as a (distributed) ACID transaction.

A transaction method gets passed a context of type TransactionContext. A TransactionContext can be used to make calls to reader's, writer's, and other transaction methods (although currently nested calls to a transaction method may only read/write mutually exclusive state.)

note

Just like with writer's, to replace an entire state you'll need to do state.CopyFrom(...) in Python and state.copyFrom(...) in Node.js.

Here's an example of a transaction method called Transfer on our Bank state that deposits money in one account and withdraws it from another:

Python

async def Transfer(
    self,
    context: TransactionContext,
    state: Bank.State,
    request: TransferRequest,
) -> TransferResponse:
    from_account = Account.lookup(request.from_account_id)
    to_account = Account.lookup(request.to_account_id)

    await from_account.Withdraw(context, amount=request.amount)
    await to_account.Deposit(context, amount=request.amount)

    return TransferResponse()

TypeScript

async transfer(
  context: TransactionContext,
  state: Bank.State,
  request: TransferRequest
): Promise<PartialMessage<TransferResponse>> {
  const fromAccount = Account.lookup(request.fromAccountId);
  const toAccount = Account.lookup(request.toAccountId);

  await fromAccount.withdraw(context, { amount: request.amount });
  await toAccount.deposit(context, { amount: request.amount });

  return {};
}

Again, like writers, state can be modified directly in a transaction method, for example here is an example from the SignUp method for Bank that stores the account IDs:

Python

# Transactions like writers can alter state directly.
state.account_ids.append(account.state_id)

TypeScript

// Transactions like writers can alter state directly.
state.accountIds.push(newAccountId);