Replication (6)

Question 1

What are the benefits of replication?

Answer 1

Better performance(parallel processing of client requests)

Less delay and network traffic because servers can be located closer to clients.

Better availability, if a replicated server fails we can just use a different while fixing it.

Question 2

How can updates be propagated among replicas?

Answer 2

Push- based

Pull- based

Hybrid push- pull(lease- based, or pushing invalidations)

Question 3

What is push- based propagation?

Answer 3

A replica pushes updates to everyone else.

Could push new data or parameters of update operation.

Question 4

What is pull- based propagation?

Answer 4

A replica requests that another send their newest data. They first poll for new data.

Question 5

What is ideal consistency?

Answer 5

If a system has ideal consistency it means that its behaviour is identical to a non- replicated system.

Question 6

What is sequential consistency?

Answer 6

That for every possible global history produced by a system execution, there should exist a linearisation that fulfils the specification of each object (the order of operations should make sense basically)

Question 7

What is active replication? (aka replicated state machine)

Answer 7

Every replica sees the exact same set of update messages in the same order and will process them in that order.

Question 8

What are the advantages of using active replication?

Answer 8

Each server can respond to client queries with updated data.

If a server crashes, a different server can immediately complete its queries.

Question 9

What are the limitations of using active replication?

Answer 9

Waste of resources since all replicas do the same operation.

It has update propagation only which demands determinism. (I think this means that every replica has to see the same set of update messages in same order)

Question 10

what is passive replication(primary- backup replication)?

Answer 10

One server plays a primary role. The primary completes all updates and can propagate them to backups eagerly or lazily.

Question 11

What are some advantages with passive replication compared to active?

Answer 11

It can be implemented without deterministic operations.

Easier to implement.

Creates less network traffic during normal operation.

Question 12

What types passive replication have we gone through?

Answer 12

Cold backup

Warm backup

Hot standby

Local -write scheme

Question 13

What is cold backup?

Answer 13

Only primary server is active. It checkpoints its state to backup periodically.
If primary falters, a backup is initiated.

Question 14

What is warm backup?

Answer 14

It is similar to cold backup, but in this type the backup server is partially live so that recovery is quicker.

Question 15

What is hot standby?

Answer 15

There is a primary server, but the backup server is also active and is constantly updated about changes in primary’s condition.

Question 16

What is local- write scheme?

Answer 16

A type of passive replication where the role of primary migrates between server, for instance if a lot of traffic is expected. If a copy wants to write, it localizes the primary and requests to write.

Question 17

What is quorum- based replication?

Answer 17

The updates to state are executed on the majority of replicas. The idea is that clients must request and receive permission from the majority of servers to complete a (write) operation.

Question 18

What are the two rules in the generalized version of quorum- based replication and why do they exist?

Answer 18

number of nodes in read quorum: NR
number of nodes in write quorum: WR
Number of nodes in total: N

1. NR + NW > N
   (prevents read- write conflict)
2. NW > N / 2
   (prevents write- write conflicts)

Question 19

What does a group membership service do?

Answer 19

It does dynamic maintenance of the group. It discovers failures, distributes info about membership changes and does address expansion.

Question 20

What is a view in group communication?

Answer 20

A view is the time period between two consecutive membership changes.(node join/leave)
The evolution of the system can be seen as a global sequence of views.

Question 21

What is view delivery/ installation?

Answer 21

A member delivering a view happens when a membership occurs and the application is notified of the new membership.

Question 22

What are the basic requirements for view delivery in a membership service?

Answer 22

Order: If a process p delivers view v(g) and then v(g’) then no other process can deliver v(g’) before v(g)

Integrity: If process p delivers view v(g) then p is an element in v(g)

Non- triviality: If process q joins a group and becomes reachable from a different process p then q will always be in views delivered by p.