NonStop Systems Introduction for H-Series RVUs

Requirements of Real-Time Solutions
NonStop Systems Introduction for H-Series RVUs—540083-001
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Data Integrity
The solution must be able to update operational data continuously in real time; this
requires continuous availability of the data store. Critical business applications cannot
afford even a second of data store downtime.
Without fault tolerance, computer systems can experience downtime for these reasons:
•
A hardware failure such as a processor failure or communications line failure can
halt transactions in progress.
•
A power failure can halt transactions in progress
•
A software failure such as the failure of an I/O process that manages a disk drive
can halt transactions in progress.
•
The entire system can be taken out of commission while customer engineers repair
a faulty component.
•
A particular application can be taken down so that operators can configure and
install more disk drives, communications lines, and processors for the application
or start additional copies of the application.
Real-time solutions cannot be susceptible to any of these causes of downtime. The
solution must be designed to keep applications up and running by eliminating these
causes. Furthermore, the solution must continue to provide service even when existing
applications are brought offline for maintenance. It must continue to offer data store
access even during data store reorganizations and backups, system reconfiguration
operations, and so on.
Data Integrity
Data integrity means that neither the failure of a particular transaction nor the failure of
a hardware component such as a processor or disk drive can corrupt the data in the
database.
One of the most important roles of a real-time solution is to keep all of the operational
data bases in synch. Consider a sales order transaction. Suppose that for some
reason (such as a program error) the transaction stops after it has added a new order
to the order database and updated the data store with information about the order, but
before the hub can push the updated the information out to the inventory and
accounts-receivable databases to reflect the order. The result is that all three
databases have lost data integrity.
The order database contains a record of the new order, but the new order data is
inconsistent with the current inventory and accounts-receivable data. If the new order
record states that customer A has ordered five boxes of stationery, but the inventory
record for stationery does not show that five boxes have been deducted from the
quantity of stationery on hand in the warehouse, then either the order record is wrong
or the inventory record is wrong. But there is no way of knowing for sure.
For another example, suppose a customer calls a company’s credit card call center to
charge a purchase. The call center credit application then queries the data store to