A system-initiated alter for an eligible structure may begin when structure full monitoring determines that a structure contains monitored objects (e.g. entries, elements, or EMCs) that are at or above structure full threshold specified by FULLTHRESHOLD. For cache structures, structure full monitoring will only consider changed or locked-for-castout objects, since unchanged/unlocked objects are eligible for reclaiming, and therefore do not contribute to a possible structure full condition. For all structure types, each object type will be considered independently within that structure. Therefore a structure can be above the structure full threshold in terms of one structure object type and be under structure full threshold for another structure object type.

After issuing message IXC585E to externalize the structure full threshold condition, the system may issue an alter request against the affected structure (see Conditions preventing a system-initiated alter for the list of Conditions that could prevent a system-initiated alter from occurring). The objective of the alter request will be to reapportion the monitored structure objects, expand the size of the structure, or both, so as to attempt to relieve the observed structure object resource shortage. The attempt to relieve the structure object resource shortage will hopefully avoid the occurrence of a structure full condition which could cause structure updates to fail, which could possibly cause the exploiter that depends on the updates to fail or invoke recovery procedures to handle the structure full condition.