1. A coordination method for writing shared data in a computer sysplex having a plurality of central processor complexes (CPCs) having local storages containing local caches, direct access storage devices (sysplex DASDs) connected to the CPCs for storing data items sharable among the CPCs in the sysplex, and at least one shared electronic storage (SES) connected to some or all CPCs in the sysplex, the write coordination method comprising the steps of: command signalling a write command by a CPC to a SES to access a SES directory entry associated with a new data version of a data item contained in the CPC, writing an unchanged state indication in the SES directory entry for the new version in response to the write command, writinq the new version of the data item from the CPC to the sysplex DASD in coordination with the SES directory entry, and optionally writing the data item in a SES data area in the SES if indicated by the write command, SES requesting CPC invalidation of each copy (complement copy) of the data item in the CPCs for coordinating the execution of the write command, except not invalidating a copy associated with the signalling Step in the CPC sending the write command, setting into the SES directory entry a write-for-castout indication and a lock indication for identifying the CPC sending the write command and to lock the SES directory entry, executing by the CPC an I/O transfer of the data item from the CPC to the sysplex DASD in response to the CPC receiving a command acceptance signal from the SES, lock-release signalling by the CPC to the SES in response to the CPC receiving a successful I/O write completion signal from the sysplex DASD, and resetting by the SES to an unlocked state the write-for-castout indication and the lock indication in the SES directory entry in response to the lock-release signalling step to allow other CPC commands to access the SES directory entry and to control accessing of any copy of the data item in the SES which may or may not be storing a copy of the data item.