Redundancy - Yedeklilik

Giriş
Türkçesi yedeklilik. Redundancy Fault Tolerance- Arızaya Dayanıklılık için kullanılabilecek yöntemlerden bir tanesi.

1. Failover - Yedeğe Geçiş
Ana ve yedek olmak üzere iki sistem bulunur. Ana sistem arızalanınca yedeğe geçiş (failover) gerçekleşir. Failover'ı tesbit ederken Split Brain Scenario problemine dikkat etmek gerekir.

SSS maddesi şöyle olabilir.

System shall run on two identical servers for redundancy.

2. Triple Modular Redundancy
Açıklaması şöyle.

Also known as Triple Mode Redundancy (or TMR), this is a form of redundancy where three systems perform the same process and their results are checked by a majority voting system that in turn, produces a single output (see next image). If one of the three systems fails, then the other two will correct it by providing the accurate output to the voting system.

3. Aynı Yazılımın Farklı Donanım İle Yedeklenmesi
Aviyonik Dünyasında şöyle bir yaklaşım var.

Redundancy is not only achieved by multiplying the computers, but also by diversifying them. On Airbus airliners, two different computers are used (one with Intel chips, the other with Motorola chips in case of the A320) and software is written twice, one for control, the other for monitoring, by two teams which are not allowed to interact.

Airbus'ta hayata geçirilmesi şöyle

Despite the nonrecurring costs induced by dissimilarity, it is fundamental that the five computers all be of different natures to avoid common mode failures. These failures could lead to the total loss of the electrical flight control system. Consequently, two types of computers may be distinguished:

2 ELAC (elevator and aileron computers) and 3 SEC (spoiler and elevator computers) on A320/A321 and,

3 FCPC (flight control primary computers) and 2 FCSC (flight control secondary computers) on A330/A340.

Taking the 320 as an example, the ELACs are produced by Thomson-CSF around 68010 microprocessors and the SECs are produced in cooperation by SFENA/Aerospatiale with a hardware based on the 80186 microprocessor. We therefore have two different design and manufacturing teams with different microprocessors (and associated circuits), different computer architectures, and different functional specifications. At the software level, the architecture of the system leads to the use of four software packages (ELAC control channel, ELAC monitor channel, SEC control channel, and SEC monitor channel) when, functionally, one would suffice.

4. Forward Error Correction
Mesajlaşma protokollerinde kullanılır. Açıklaması şöyle.

Forward Error Correction (or FEC) adds redundancy into the message itself, that way the receiver can verify the actual data and correct a limited amount of errors detected due to noisy or unstable channels.

Redundancy Olarak Çift Motor
Açıklaması şöyle.

In aeronautics, redundancy is of primary concern. Having at least two engines is common for civilian aircraft.

Redundancy Olarak RAID Disk

RAID Disk yazısına taşıdım