introduction (1)

Question 1

What are the consequences of the way a distributed is defined?

Answer 1

concurrency.

No global clock.

Independent failure of different components.

Unreliable communication.

Insecure communication.

Expensive communication.

Scalability.

Question 2

What is distribution transparency and what kind of types are there?

Answer 2

Hide from the user that processes and resources are distributed across different machines and related aspects.

Types:

Access
Location
Migration
Relocation
Replication
Concurrency
Failure

Question 3

What is access transparency?

Answer 3

Hide differences in data representation and how resources are accessed.

Question 4

What is location transparency?

Answer 4

Hide that resources are located across different locations.

Question 5

What is migration transparency?

Answer 5

Hide that a resource is moved from one location to another.

Question 6

What is relocation transparency?

Answer 6

Hide that a e.g. a server decides to move a file from one location to another while in use.

Question 7

What is concurrency transparency?

Answer 7

Hide that a resource is replicated across several locations.

Question 8

What is concurrency transparency?

Answer 8

Hide that the resource is used by several users at the same time.-

Question 9

What is failure transparency?

Answer 9

Hide the failure and recovery of a resource.

Question 10

Why is it not always a good idea to implement the highest degree of transparency?

Answer 10

It gives little room for optimization, many extra layers also increases the complexity of the system.

Question 11

What is a scalable system?

Answer 11

A scalable system remains effective even though the number of users or amount of data grows a lot.

Question 12

Which dimension are there to scalability?

Answer 12

Size:
Users and resources can easily be added.

Geographic:
Users and resources can be located far apart geographically.

Administratively:
If the system is still easy to manage even though it spans across many independent organization and groups of people.

Question 13

What are some techniques used to improve scalability?

Answer 13

Replication

Sharding (split and spread resource across system)

Hiding communication latencies (async communication etc.)

Question 14

What does it mean that a distributed system has a high degree of openness?

Answer 14

That the system has components that can easily be used by or integrated with other systems.

Question 15

How does replication affect the scalability of a system?

Answer 15

It helps with load balancing and increases availability.

The disadvantage that could decrease scalability is that the replicated data must be kept consistent.

Question 16

What does hiding communication latencies mean and how does it affect system scalability?

Answer 16

Avoiding that the user has to wait for a response by using async communication or reducing the number of remote requests.

Helps with geographical scalability if the resource is located far away.

Question 17

What are the goals of a distributed system?

Answer 17

Resource sharing
Distribution transparency 
Openness
Scalability
Fault tolerance
Allowing heterogeneity ( interoperability)

Question 18

What are common techniques for handling failures?

Answer 18

Detecting failures (checksum)
Masking failures (retransmission in protocols)
Tolerating failures (as in web-browsers, 404)
Recovery from failures (roll back)
Redundancy (replicate servers in failure-independent ways)