Not all systems are running in the same configuration.
Some systems are stopped by regular maintenance, in-house systems that are not used for a certain time, and systems that continue to operate 24 hours a day, 365 days a year.
A dual system is used to operate a system that cannot be stopped.
In recent years, this method has been used in systems that require particularly high reliability.
This time, we will introduce the basic knowledge of the dual system, the advantages and disadvantages of applying it, and the differences from the duplex system.
What is a dual system?
The Dual System is one of the highly reliable systems. “Dual” means double in Japanese, and it is a method of performing the same processing on two sets of systems and systems, and performing the processing while checking the execution result with a collator. Conversely, performing one process on only one system is called a “simplex system”.
The dual system performs the same processing on two sets of systems at the same time, and in the unlikely event of a failure, disconnects the problematic system and keeps the entire system running. Since the problematic system is separated from the main processing, the processing can be continued without stopping the processing of the entire system.
You can also continue processing on the remaining system while recovering by investigating the cause of the failed system and taking recovery measures. The DS is said to be a reliable system because it can continue processing without stopping the system completely.
The dual system is a system configuration that was widely used in the days when the reliability of the processor was low and has been adopted only for special purposes in recent years. For example, the DS is used when high reliability is required, such as financial institutions such as banks and securities that cannot tolerate incorrect execution results or suspension.
Advantages and disadvantages of the dual system
The highly reliable dual system is used only for special purposes because of its disadvantages. Here, we will explain the advantages and disadvantages of the DS.
● Merit
The advantage of the dual system is that it ensures high reliability. By collating the two processing results against each other, the correctness of the processing is confirmed at any time, so it is unlikely that an error will occur in the result, and even if it is incorrect, it is possible to discover where the error occurred.
Another big advantage is that you don’t have to worry about processing or system outages. If two sets of systems are running at the same time and the main system fails, the other can continue. The entire system will not stop unless an event occurs that causes two sets of systems to stop at the same time. Because of these benefits, dual systems are used in critical systems that cannot be stopped.
● Disadvantages
It is a dual system that can achieve high reliability, but of course, there are disadvantages. System operation with a DS is costly. Since the same process is performed twice, the cost of hardware and software, the cost of electricity to operate the system, the upgrade work, etc. are twice as much as the simple comparison.
Difference between dual system and duplex system
A system configuration similar to a dual system is the “duplex system”. I will explain the difference between the DS and the duplex system.
● What is the Duplex System?
The duplex system is a method in which two sets of systems are prepared for one process, and one is kept on standby as a spare machine in the event of a failure. Normally, the processing is executed on the main system side, and when a system stops due to a failure or an error in the result is detected, the system is switched to the standby system that has been on standby to continue processing.
● Three types of a duplex system
<Hot standby>
Hot standby is a method of keeping the spare machine ready to start. Since it is ready to operate, there is no downtime when switching to a spare machine due to a failure on the main system side, or switching can be completed in an extremely short time.
However, it is the most costly duplex system because it keeps two systems running at the same time like a dual system.
<Warm standby>
Warm standby is a method of keeping a spare machine on standby with some preparations such as OS and application installation. It is located between hot standby and cold standby, which will be described later, and in the event of a failure, it is possible to switch the system with just a certain operation.
Warm standby requires some operation when switching systems. As a result, system downtime will occur for a certain time.
<Cold standby>
Cold standby is the cheapest of the three. In cold standby, reserve the spare machine so that it can be used only by the system without preparing for the operation. Therefore, if a failure occurs on the main system side, it must be executed from the start of the spare machine, and the downtime at the time of failure is the longest in the duplex system.
However, compared to a simplex system where one system is operated by only one system, recovery work such as procuring hardware and installing the software does not have to be performed from scratch. There is no need to worry about “there is no prospect of system recovery”.
● Difference between the two
Both the dual system and the duplex system are the same in that two sets of systems are prepared. However, there is a difference in that the duplex system is prepared for standby compared to the DS where the two systems are always in production.
Even though it is an important system, there is a risk that the cost will increase if the DS is introduced without planning. If your system allows for a certain amount of downtime, you may choose a duplex system instead of a dual system.
Both dual and duplex systems have the same purpose of ensuring high reliability, so choose which is more appropriate based on the downtime and operating costs that can be tolerated in the event of a failure.
The dual system is for systems that cannot be stopped
Due to the bloated installation and operation costs, dual systems have not been adopted much in recent years.
However, there are cases where dual systems are selected for critical systems that are the core of the business that cannot be stopped 24 hours a day, 365 days a year.
Once the system is developed, it is not complete.
It is important to maintain reliable operations to keep the planned processing on time.
For systems that cannot be stopped, why not consider using a DS or duplex system?
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