2 - Modelling

Question 1

Explain what a distributed system is?

Answer 1

As a set of processes. Each process does local computations, and they’re interconnected with commincation channels. (links) A network is assumed to be at least connected (there is a path between each two nodes)

Question 2

What components have to be defined for your network model?

Answer 2

• Type of Network
• Type of Process
• Type of Timing

Question 3

What is a correct node?

Answer 3

A node that always follows the protocol

Question 4

Explain the fault-model of crash-stop

Answer 4

A node is faulty if it crashes. After crashing, it stops executing forever.

Question 5

Explain the fault-model of crash-recovery

Answer 5

A node might crash, lose its internal state, and might resume executing sometime later.

Question 6

Explain the fault-model of byzantine

Answer 6

A node can deviate arbitrarily from a protocol

Question 7

What is another name for the byzantine fault-model?

Answer 7

the fail-arbitrary fault model

Question 8

What is the defenition of a complete network?

Answer 8

A direct link between any two processes.

Question 9

The is the definition of the state of a process

Answer 9

the set of values of all its variables (includes initial and terminal states)

Question 10

The is the definition of the state of a channel

Answer 10

messages in transmission on the channel (aka not received, initially empty)

Question 11

The is the definition of the global state

Answer 11

set of the process states + set of the channel states

Question 12

What is another term for global state?

Answer 12

Configuration of a Distributed System

Question 13

What is a transition?

Answer 13

A state change in a DS

Question 14

What triggers a transition?

Answer 14

events in processes (e₁)

Question 15

Explain what reliable links are.

Answer 15

A message is received iff it is sent. Messages
may be reordered.

Question 16

Explain what Fair-loss links are.

Answer 16

Messages may be lost, duplicated or reordered. If you keep trying, eventually messages arrive.

Question 17

Explain what Arbitrary links are.

Answer 17

a malicious adversary may interfere with messages. (eavesdrop, modify, drop, replay)

Question 18

What is an synonym for reliable links?

Answer 18

perfect link

Question 19

Explain how a system with Async. Com. can be simulated using a system with Sync. Com.

Answer 19

introduce for every link a process with a buffer that may delay messages

Question 20

Explain how a system with Sync. Com. can be simulated using a system with Async. Com.

Answer 20

Use explicit acknowledgements to force the sender to wait untill the message is received.

Question 21

Explain the basic properties of an Asynchronous network.

Answer 21

1. message delays are finite but arbitrary
2. the relative processor speeds may be unbounded
3. there is no common clock

Question 22

Explain the basic properties of an Synchronous network.

Answer 22

1. message delays are bounded
2. the relative processor speeds are bounded
3. there is a common clock

Question 23

What is the difference in the driver between sync and async networks?

Answer 23

Async networks are event driven. (sending receiving internal events trigger different code parts).
Sync networks are time driven; they loop over their tasks a finite number of time(s).

Question 24

Explain what partial synchrony is.

Answer 24

The system is asynchronous for some unknown but finite amount
of time, and synchronous afterwards.

Question 25

explain the expected message delay for partial synchronous systems.

Answer 25

There exist some time bound Δ and a special event GST (Global Stabilization Time) such that:
- The adversary must cause the GST event to eventually happen after some unknown finite time.
- Any message sent at time x must be delivered by time Δ + max(x,GST).

Question 26

skip

Answer 26

yessirr