How to get a new transaction without closing the earlier transaction?

I need to get a new transaction without calling the item_done for the previous transaction. How can I achieve this??

Since the sequencer can only send one transaction at a time, you can’t get get a new transaction without calling item_done for the previous transaction. However, you can implement a pipeline mechanism to get multiple items and operate on them in parallel.

Take a look at the pipelined article and see if it meets your needs.