Multiplex transactions that modify shared objects behave according to certain rules.
A local transaction does not modify a shared object.
A local transaction may be read-only or read-write but modify data in local objects only (SA tables or temp tables). Local transactions may be performed on any multiplex node, and the effects of the transaction are only visible to that node.
All multiplex transactions begin as local transactions.
A global transaction 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.
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 transaction-level versioning (TLV) log.