Provided are techniques for increasing transaction processing throughput. A transaction item with a message identifier and a session identifier is obtained. The transaction item is added to an earliest aggregated transaction in a list of aggregated transactions in which no other transaction item as the same session identifier. A first aggregated transaction in the list of aggregated transactions that has met execution criteria is executed. In response to determining that the aggregated transaction is not committing, the aggregated transaction is broken up into multiple smaller aggregated transactions and a target size of each aggregated transaction is adjusted based on measurements of system throughput.