Writers

A writer method gets exclusive, atomic access to state in order to update it. Any updates a writer makes to the passed in state argument are persisted after the method returns, but before returning to the caller. If the method fails (or it is part of a transaction which aborts), its changes to state are not persisted.

A writer method gets passed a context of type WriterContext. A WriterContext can only be used to make calls to reader methods (that is not a typo, writer's can only call other reader's).

note

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 writer method called Deposit on our Account state that increments the account's current balance by some amount in the request:

Python

async def Deposit(
    self,
    context: WriterContext,
    state: Account.State,
    request: DepositRequest,
) -> DepositResponse:
    state.balance += request.amount
    return DepositResponse(updated_balance=state.balance)

TypeScript

async deposit(
  context: WriterContext,
  state: Account.State,
  request: DepositRequest
): Promise<PartialMessage<DepositResponse>> {
  state.balance += request.amount;

  return { updatedBalance: state.balance };
}

In addition to returning a response and updating state, a writer can also schedule async tasks, which are atomically started (or enqueued) if and only if the writer completes successfully.