In very rare circumstances the catalogue handling code may detect an inconsistency in the catalogue's data and automatically sets about a corrective action on this and repairs the problem. This involves a process of recreating catalogue chains and whilst this is happening exclusive access of the catalogue is required. In the majority of cases this is invisible to the user as the time to perform this task is insignificant.

However, only on sites with large catalogues and only where the inconsistencies found require extensive rechaining action, this action can take longer than normal and in the worst case could be hours rather than minutes. This catalogue corruption must be fixed and it is essential that this process completes. A message is output to the OPER, but in some circumstances this will not be displayed until this catalogue repair has been completed. To the users this will appear to be a creeping death as more and more VMs will eventually queue waiting on access to the catalogue.

Inevitably a cab key dump would eventually be taken.

It is essential that this process completes and the cab key avoided as stopping the catalogue repair mid way could result in a worse catalogue state. In order for the operator to be aware of the situation and avoid an unnecessary system break, REP CA/K/0491 has been raised. This repair displays "5EC999" on the nodal lights whilst the catalogue is held for reconstruction. When it is complete and usable again the system time is displayed on the lights.

With this repair applied the operators are advised to check the nodal lights if they suspect the system is idling. If "5EC999" is displayed the system is not idling and no action is required as this process will complete.

The above scenario is extremely rare.