System models for DS (2)

Question 1

What are the different types of system models?

Answer 1

Physical models
(hardware composition)

Architectural models
(Software/ system architecture)

Fundamental models
(formal description of common properties in architectural models)

Question 2

What are common elements in architectural models?

Answer 2

Communicating entities

Communication paradigms

Roles and responsibilities

Placement strategies

Question 3

What are examples of communicating entities in architectural models?

Answer 3

(distributed) Objects
(distributed) Components
web services

Question 4

What are examples of communication paradigms in architectural models?

Answer 4

IPC
Remote invocation
Indirect communication

Question 5

How does caching in placement strategies affect the system?

Answer 5

Moving the resource closer to the user by having proxy server, reduces load on main server

Server cache reduces latency if fetching from the DB is not needed.

Disadvantage is that cached data might be outdated.

Question 6

What are the two types of interaction model in distributed systems? How do they differ?

Answer 6

Asyncrhonous and synchonous systems.

In synchronous systems there is known lower and upper bound to the execution and transmission time as well as amount of clock drift.
In async systems it could be arbitrarily long.

Question 7

What are some regular architectural patterns?

Answer 7

Layered architecture

Tiered architecture

Microservices

Thin clients

Question 8

What are microservices?

Answer 8

A service is deployed on a simple VM/ execution environment.

Each microservice is:

• Responsible for a simple feature.
• Reusable and standalone.
• Autonomous

Question 9

What are thin clients?

Answer 9

An architectural pattern where you try to move complexity from client to server. Server does the processing and sends results to client.

Question 10

What types of middleware solutions are there in layered architecture models?

Answer 10

Distributed objects
Distributed components
Pub/sub systems
Message queues
Web services
P2P

Question 11

What are the most common types of fundamental models?

Answer 11

Interaction model.

Failure model.

Security model.

Question 12

What is the purpose of fundamental models?

Answer 12

Make all relevant assumptions about the system explicit and find out what is feasible under the given assumptions.

Question 13

What is an interaction model?

Answer 13

A model focused on the processes, messages and coordination of the system. It must reflect that messages could be delayed, and that delay limits coordination and maintenance of global time.

Question 14

Why are failure models useful?

Answer 14

The define in how a failure might occur in the system. This gives a basis for understanding the effect of failures and can be used to construct a new service that hides the faulty behaviour.

Question 15

What are the three main types of failure types?

Answer 15

Omission failures
(Process or channel fails perform actions it's supposed to do)

Arbitrary failures
(Process or channel may exhibit arbitrary behaviour when failing)

Timing failures
(Relevant in synchronous systems. e.g. responses not available to clients in specified timing interval)

Question 16

What is a fail-stop failure?

Answer 16

Process halts and remains halted. Other processes may detect this state.

Question 17

What is a crash failure?

Answer 17

Process halts and remains halted. Other processes may not be able to detect this state.

Question 18

What is an omission failure?

Answer 18

A message inserted in an outgoing message buffer never arrives in the other end’s incoming buffer.

Question 19

what is a send omission failure?

Answer 19

A process completes a send- operation, but the message is not put into the outgoing message buffer.

Question 20

what is receive omission failure?

Answer 20

A message is put into a process’s incoming message buffer, but the process does not receive it

Question 21

For what kind of systems would using an arbitrary failure model be useful.

Answer 21

Systems that we want to be “ultra- reliable” e.g. systems in air planes, systems in nuclear power plants.

Question 22

For what kind of systems would using a timing failure model be useful?

Answer 22

Synchronous distributed systems. e.g. control and monitoring systems, multimedia systems.