1. A method for programming a computer to process a batch of data messages using a computing device having at least a processor, a memory, a display device and an input device, the method comprising: storing in said memory a function arranged to process said batch of data messages; retrieving the function from the memory and arranging a flow diagram on the display device in response to user input to specify a process, the data flow diagram including a function icon corresponding to the selected function; allowing a developer to select error handling attributes associated with said function icon in order to define error handling modes for said function, wherein said error handling modes include an Optimistic Mode which includes: processing said batch of data messages as a group of data messages so that said processing is faster than if said data messages were processed individually; determining that an error has occurred in using the function when processing a data message from said batch of data messages; rolling back said processing of said batch of data messages; marking the data message that gave rise to said error in processing by said function; and reprocessing the batch of data messages excluding the marked data message.