A global transaction is any transaction that modifies data in shared objects or changes the schema of any persistent object. Global transactions may only be performed on the coordinator node or a writer node. The effects of a global transaction are visible on all nodes of the multiplex.
All multiplex transactions begin as local transactions. A transaction only becomes global when the first read-write operation (such as an insert) is initiated on a shared IQ object. When a global transaction is initiated from a secondary writer node, the coordinator node must be running, because the writer node must communicate with the coordinator node in order to coordinate the global aspects of the transaction.
In a writer-initiated global transaction, the writer node CPU and local disks are used to do the work of the read-write operation, while the coordinator node handles the global bookkeeping for the transaction, such as the assignment of global transaction IDs, global lock management, and writing the TLV log.