Hey, great package.
I'm having trouble getting started because I'm not 100% sure what is correct to write within an activity (as far as idempotency and determinism is concerned).
Would you mind adding some real code examples to the documentation in the form of Do's & Don'ts & Fix-This-Don'ts?

For example, I am making a workflow for generating a quickbooks entry for a successful payment.
The activities will likely be something like:

PrepareCustomer // find an existing quickbooks customer for this email, or create a new one
AddSalesReceipt // create a new sales receipt in quick books
SaveReceipt // save a copy of the receipt to the local disk for caching purposes, and insert a row in the db with its path for retrieval/clean-up
EmailReceipt // send a copy of the receipt via email to the customer

Now, if I've understood the documentation correctly, the execute method of any of these activities may be run multiple times for the same workflow, i.e. "replayed". So, in the first two activities at least I should always check whether a customer/sales-receipt has already been created for this order/customer.
And in the third activity, SaveReceipt, I'd do a similar thing where I either keep a workflow_id or order_id field in the row I insert in the DB to check if I've not already cached the receipt.
Is this correct?