chapter 6

Question 1

how do we keep replicas consistent?

Answer 1

• ensure that all conflicting operations are done in the the same order everywhere
• when one copy is updated we need to ensure that the other copies are updated as well.

Question 2

give an example of a Conflicting operations.

Answer 2

• concurrent Read–write conflict
• concurrent Write–write conflict

Question 3

Reasons for Replication?

Answer 3

1) Replication for reliability
- possible to continue working after one replica crashes by simply switching to one of the other replicas.
- to provide better protection against corrupted data

2) Replication for performance
- when number of processes need to access data managed by a single server. by replicating the server and subsequently dividing the work.
- by placing a copy of data in the proximity of the process using them, the time to access the data decreases.

Question 4

Guaranteeing global ordering on conflicting operations may be a costly operation, downgrading scalability. Solution?

Answer 4

weaken consistency requirements so that hopefully global synchronization can be avoided

Question 5

describe a Data-centric consistency models.

Answer 5

is a contract between a distributed data store [ distributed collection of storages ] and processes in which, the data store will specify the result of a read and write operation during concurrency.

Question 6

describe a Continuous Consistency.

Answer 6

• allows us to measure the degree of consistency.
• replicas may have:
• inconsistent numerical values
• inconsistent staleness [ time of ops ]
• inconsistent number or order of update ops

Question 7

describe a conit.

Answer 7

specifies the data unit, whose consistency we are measuring
- stack market [ conit = stock value ]

Question 8

describe weak consistency

Answer 8

Shared data can be counted on to be consistent only after a synchronization is done.

Question 9

Properties Weak Consistency:

Answer 9

1. Accesses to synchronization variables associated with a data-store are sequentially consistent.
2. No operation on a synchronization variable is allowed to be performed until all previous writes have been completed everywhere.
3. No read or write operation on data items are allowed to be performed until all previous operations to synchronization variables have been performed.

Question 10

describe Causal consistency

Answer 10

Writes that are potentially causally related must be seen by all processes in the same order. Concurrent writes may be seen in a different order by different processes.

Question 11

describe Sequential consistency.

Answer 11

The result of any transaction is the same as if all operations contained within the transaction were executed in a sequential order specified by the program.

Question 12

list types of data centric Consistency models.

Answer 12

• sequential
• strict
• casual
• weak

Question 13

what is the goal of a Client-centric consistency models?

Answer 13

Aims to achieve the consistency that the client wants instead of the consistency that the server should maintain.

Question 14

what are the techniques to find the best place fore replica placement?

Answer 14

1. Select best location out of N − K for which the average distance to clients is minimal. Then choose the next best server. EXPENSIVE
2. Select the K-th largest autonomous system and place a server at the best-connected host. COMPUTATIONALLY EXPENSIVE
3. Position nodes in a d-dimensional geometric space, where distance reflects latency. Identify the K regions with highest density and place a server in every one. COMPUTATIONALLY CHEAP

Question 15

what are the types of replicas?

Answer 15

• permanent
• server initiated
• client initiated

Question 16

describe Permanent replicas.

Answer 16

describes a process or a machine who always has a replica

Question 17

describe Server-initiated replicas

Answer 17

To enhance performance, a data store dynamically places a replica in a certain geographical location to meet the unexpected burst of request coming from a server in that location.

Question 18

describe a Client-initiated replica

Answer 18

A type of replica that is dynamically hosted [ created ] at the request of client
- same machine, LAN, WAN

Question 19

draw The logical organization of replicas.

Answer 19

inner to outer circle
permanent -> server Initiated -> client initiated -> client

Question 20

explain how a Server-initiated replicas works

Answer 20

• Specific files on a server can be migrated or replicated to servers placed in the proximity of clients that issue many requests for those files.

** count access [ requests ] to file F.
- if access count is below the deletion threshold = drop file
- if access count exceeds the replication threshold = replicate file
- if access count is b/n the deletion and replication threshold = migrate file

Question 21

what are the 3 models of Content distribution?

Answer 21

1. propagate notification of an update
2. transfer data from 1 copy to another
3. Propagate the update operation to other copies

Question 22

describe propagate notification of an update model

Answer 22

• Performed by an invalidation protocol
• other copies are informed that an update has taken place and that the data they contain are no longer valid.
• used for caching
• adv - less bandwidth

Question 23

describe Transfer data from one copy to another model.

Answer 23

• passive replication
• useful when the read-to-write ratio is relatively high
• Log the changes and transfer only those logs to save bandwidth.

Question 24

describe Propagate the update operation to other copies model.

Answer 24

• active replication
• Tell each replica which update operation it should perform and send only the parameter values that those operations need

Question 25

which Content distribution approach is the best?

Answer 25

but depends highly on available bandwidth and read-to-write ratio at replicas.

Question 26

explain push protocols in updating

Answer 26

** Pushing updates: server-initiated approach
- updates are propagated to other replicas without those replicas requesting updates.
- Used between permanent and server-initiated replicas and can used to push updates to client caches.
- Used when replicas need to maintain a relatively high degree of consistency.

Question 27

explain pull protocols in updating

Answer 27

** Pull-based - client-based protocols
- a client requests another server to send the updates it has at that moment.
- Used by client caches.

Question 28

compare and contrast b/n Push-based Pull-based in terms of:
1. State at server
2: Messages to be exchanged
3: Response time at the client

Answer 28

1. push [ list of client ] , pull [ None ]
2. push [ update , maybe fetch updates ] , pull [ poll and fetch ]
3. push [ immediate or fetch update time ], pull [ fut ]

Question 29

describe a lease.

Answer 29

a lease is a contract b/n a client and a server, where the server will make a promise that it will continue to push updates until the lease expires.
- allows dynamic switching between pulling and pushing

Question 30

what happens When a lease expires?

Answer 30

• the client is forced to poll the server for updates and pull in the modified data if necessary.
• Or a client requests a new lease

Question 31

describe an age-based lease.

Answer 31

An object [ a file ] that hasn’t changed for a long time, will not change in the near future, so provide a long-lasting lease
- cheaper for me

Question 32

describe a Renewal-frequency based lease?

Answer 32

The more often a client requests a specific object, the longer the expiration time for that client (for that object) will be

• you’re bugging me, go away. I will send you updates when I get them. stop nagging
• cheaper for me to …

Question 33

describe a State-based lease

Answer 33

The more loaded a server is, the shorter the expiration times become