A redundancy scheme that builds a system from a primary (active) system that does the processing and a standby system kept ready for failures, and switches to the standby to continue processing when the primary fails. Depending on how ready the standby is, it is classified as hot standby, warm standby or cold standby.
Japanese name: デュプレックスシステム
Duplex system appears in 2 practice questions across 1 topic, including System Architecture and Reliability.
A dual system runs the same processing on two systems at the same time and compares the results. In a duplex system, normally only the primary does the processing, and the standby waits in case the primary fails. A dual system excels at error detection and fast switchover but costs more because both systems always run. A duplex system keeps costs down more easily, but processing stops during the switchover, and processing just before the switchover may be lost.
In a hot standby, the standby system is kept running and the primary's data is always reflected to it, so switchover takes a short time. In a cold standby, the standby is kept stopped or used for other work and is started, with its data restored, only after a failure; switchover takes longer, but costs are lower. A warm standby is in between: the standby is running, but preparation such as starting applications or applying data is needed before it can take over. The choice depends on the allowed downtime and the cost.
A failure of the primary is detected by mechanisms such as a heartbeat, a signal exchanged regularly between the primary and the standby, and processing is moved to the standby automatically (failover). To prevent a state in which a communication failure makes each side decide the other has stopped and both run as primary (split brain), a third monitoring mechanism is sometimes provided. Whether the standby can take over correctly must also be confirmed by regular switchover tests.
Answer: A. The standby system is kept running and the primary's data is always reflected to it, so processing can switch over in a short time when the primary fails
A hot standby keeps the standby system running and always reflects the primary's data to it, so it can take over processing in a short time when the primary fails. If the MES stops, the fab can no longer direct how lots move, specify recipes to equipment or record manufacturing history, so a configuration that can switch over in a short time is generally chosen. Keeping the standby stopped and starting it after a failure is a cold standby, and the intermediate case (running, but needing preparation before taking over processing) is called a warm standby. Performing and comparing the same processing on two systems at the same time is a dual system.
In a cold standby, the standby may be stopped or used for other work, such as non-urgent batch processing. When the primary fails, that work is stopped and the standby takes over the primary's processing. In a hot standby, the standby keeps receiving the primary's data and waits ready to take over immediately.
If processing can continue as long as either the primary or the standby is running, it can be estimated with the parallel formula 1 − (1 − a)². In practice, however, the switchover equipment, the time needed to switch over and causes of failure common to both (such as the power supply or the installation site) mean it is not as high as this formula gives.
Dividing into a primary and a standby and switching over on failure is a duplex system.
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Dual system Hot standby Differential backup Fail-safe Fail-soft Fault tolerance Fool-proof Incremental backup Load balancer RAID