(1) Natural - e.g., hurricane, tornado, flood, and fire.
(2) Human - e.g., operator error, sabotage, implant of malicious code, and terrorist attacks.
(3) Environmental - e.g., equipment failure, software error, telecommunications network outage, and
electric power failure.
In contrast to the physical layout of data items in a linked list, where pointers connect each element together and
specifying their order, a sequential list, or ordered list, is configured as shown below—in the order that the
items are read. If an action is logged, but it is not completed before the system failed, the actions can be read
from the log and completed. If a log entry of an action is not completely written to the hard drive, then any
actions that were part of the incomplete transaction can be removed. If an action was requested, but not yet
logged, then no changes were made to the database, and the action will have to be resubmitted.