How can consistency between mainServer and hotBackup be maintained?

Question

In Section 12.3, we examined an issue related to access control and notification in the CTC system. Select a
similar issue that could occur in the development and CTC and populate it with relevant proposals, criteria,
arguments, and justify a resolution. Examples of such issues include:
This exercise is best done in groups of three or more. The goal is not to test the student’s knowledge of the CTC
systems and its related issues, but rather, to test their ability to structure the debate resulting in this knowledge using
rationale techniques.

Answer 1

I[1]: Maintaining consistency between mainServer and hotBackup.
P[1]: The mainServer receives all the requests and forwards them to the hotBackup server after
they have been processed. Replies from hotBackup are simply ignored.
A[1] against P[1]: If the mainServer crashes after successfully precessing a request but
before forwarding it to the hotBackup, the states of both servers will be inconsistent.
P[2]: Requests to the mainServer are concurrently forwarded to the hotBackup (e.g., by a proxy
server standing between the clients and the servers). The replies from both servers are
received by the proxy but those from hotBackup are simply ignored.
A[2] for P[2]: This solution can ensure consistency if requests are processed sequentially
by both servers and if no processing occurs between requests.
A[3] against P[2]: This adds a component in the architecture (the proxy) which represents
a single point of failure.
P[3]: Requests and replies from the main server are first logged and then forwarded to the
mainServer or the requesting client, respectively. When switching servers, the hotBackup
examines the log of requests to determine the state of the server at the crash time.
A[4] against P[3]: This solution introduces a delay during switching. Given a reliable
log, restarting the mainServer may be simpler.
R[1]: P[2] since P[1] does not solve the problem and P[3] is inefficient.