Готовлюсь к SCEA и наткнулся на такой вопрос
авторWhich of the following are true about Active Replication?


A It is a fault-tolerance mechanism
B It is achieved by taking cold back ups
C Each replica attempts to process each request but extra calls may be intercepted via an interceptor
D State changes are logged and periodically flushed to the replicas
E State is synchronized only when the primary service fails
F Each replica is identical to the main service.
Choices A, C and F are correct.

Active Replication is a fault tolerance mechanism where each replica is identical to the main service and attempts to service each request. An interceptor is used to block extra responses. Active replication is similar in concept to Hot Backups. Hence choices A, C and F are correct.

Active Replication is not achieved through cold backups. In cold backups, the replicas are not constantly updated. Their state is synchronized with the primary service in regular intervals. In the event that the primary service fails, one of the replicas has to be synchronized (perhaps with data from a storage device) before it can service requests. Hence choice B is incorrect.

Choice D refers to the concept of Warm Backus. In warm backups, only the primary service responds to requests. However all activity is constantly logged. Periodically these logs may also get flushed to the replicas. When the primary service fails, a replica takes over after its state is synchronized with the logs. Hence choice D is incorrect.

With active replication, as mentioned before, the state of all replicas is constantly synchronized. Hence choice E is incorrect.


и в книге
авторThe factors that affect the system availability include the following:

System downtime—The system downtime can be caused by a
failure in hardware, network, server software, and application
component.

Long response time—If a component does not produce a
response quick enough, the system can be perceived to be
unavailable.


Discussion
The most common practice to improve the system availability is through
one of the following types of replication, in which redundant hardware
and software components are introduced and deployed:

Active replication—The request is sent to all the redundant
components, which operate in parallel, and only one of the generated
responses is used. Because all the redundant components
receive the same request and perform the same computation, they
are automatically synchronized. In active replication, the downtime
can be short because it involves only component switching.

Passive replication—Only one of the replicated components
(the primary component) responds to the requests. The states of
other components (secondary) are synchronized with the primary
component. In the event of a failure, the service can be resumed
if a secondary component has a sufficiently fresh state.

В википедии
авторПри репликации изменения, сделанные в одной копии объекта, могут быть распространены в другие копии.

А в книге
авторActive replication—The request is sent to all the redundant
components, which operate in parallel, and only one of the generated
responses is used.
и далее
авторBecause all the redundant components
receive the same request and perform the same computation, they
are automatically synchronized.
а если они имеют разную вычислительную мощь?

И чтото очень мутная суть из всего этого. Можете ктонибудь объяснить на примере чтоли?

Всётаки запутанно тут както
авторActive Replication is a fault tolerance mechanism where each replica is identical to the main service and attempts....
далее
авторF Each replica is identical to the main service. - incorrect

пардон