Module 13 - CAP principle

Question 1

What does “Consistency” mean in the context of CAP?

Answer 1

Consistency:

Clients agree on the latest state of the data

Question 2

What does “Availability” mean in the context of CAP?

Answer 2

Availability:

Clients are able to execute both read-only queries and updates

Question 3

What does “Partition tolerance” mean in the context of CAP?

Answer 3

Partition Tolerance:

System continues to function if the network fails and nodes are separated into disjoint sets

Question 4

What is the CAP principle?

Answer 4

In the event of a partition (P), the system must choose either consistency (C) or availability (A), and cannot provide both simultaneously

Question 5

In the event of no partition, what does the CAP principle state about availability and consistency?

Answer 5

During failure-free (partition free), an operation on a system may be simultaneously highly available and strongly consistent

Question 6

What is a CP system?

Answer 6

In the event of a partition, choose C (consistency) over A (availability)

Question 7

What is a AP system?

Answer 7

In the event of partition choose A (availability) over C (consistency)

Question 8

What are AP systems in the real world appropriate for?

Answer 8

AP systems are appropriate for latency-sensitive, inconsistency-tolerant applications

Question 9

In AP systems, are there transactions?

Answer 9

No transactions in AP systems

Data accessed mostly using get/put operations

Question 10

What are characteristics of consistency in CP systems?

Answer 10

• Serializability
• Linearizability
• Sequential consistency
• N_r + N_w > N

Question 11

What are characteristics of consistency in AP systems?

Note that they are much less so than in CP systems

Answer 11

It depends on the type of AP, but some of them can have either of these, or none:

• Eventual consistency
• Causal consistency

Question 12

What does PACELC mean?

Answer 12

If there is a network Partition, then choose between Availability and Consistency. Else choose between low Latency and Consistency

Question 13

What is strong consistency in the context of key-value storage systems?

Answer 13

When clients read and write overlapping sets of replicas, then every read is guaranteed to observe the effects of all writes that finished before the read started

Question 14

Whats the idea behind Tunable consistency?

What are characteristics of tunable consistency systems?

Answer 14

The idea that a system is neither CP or AP, but rather different configurations of them.

These systems use quorum-based replication, and N_r and N_w can be tuned to adjust to behave more like AP to more like CP system

Question 15

In a distributed system, if there’s no network partition, and we’d like to achieve low latency, what must be give up?

Answer 15

If there’s no partition in the network, then we must give up consistency to achieve low latency (and vice versa)

Question 16

In a tunable consistency distributed system, how do you ensure that the system is strongly consistent in the context of CAP?

Note: strongly consistent in the context of CAP, means that it is a CP system

Answer 16

Make N_r + N_w > N, this ensures that we can detect read-write conflicts

Question 17

For a distributed system, if you give up consistency, do you get availability?

Answer 17

Not always. In the event of a network partition in which all replicas of a data object are on 1 side of the partition, then a user which has access to a replica on the other side will not have availability to the system

Question 18

What are sloppy quorums?

Hinted handoff is an example of sloppy quorums. How does it work?

What assumptions does this idea make? In what case is hinted handoff not necessary?

Answer 18

In Sloppy Quorums, the set of replicas (partial quorums) can change dynamically.

Hinted handoffs allows an arbitrary node to accept an update for a given key, and hold the update until one of the write replicas becomes available.

This assumes that there is partial replication. In a quorum based system with full-replication, then hinted handoff is not necessary.

Question 19

Hinted handoff ensures _____ availability, but not _____ availability

Answer 19

write

read

Question 20

Which client-side consistency level is used in Apache Cassandra to enable full write availability in the presence of network partitions?
Does Apache Cassandra support tunable consistency?

Answer 20

Partial quorums

Apache Cassandra supports tunable consistency with (optional) full write availability

Question 21

In Apache Cassandra, what do the terms:
1. ONE
2. ANY
3. TWO
4. THREE
5. QUORUM
6. ALL 
mean in terms of replication strategy?

Answer 21

1. N_r or N_w = 1
2. For writes only, use any number of read/write quorums with hinted handoff
3. N_r or N_w = 2
4. N_r or N_w = 3
5. N_r or N_w = ceiling{(N+1)/2}
6. N_r or N_w = N

Question 22

In Apache Cassandra, what does the coordinator do?

Answer 22

Executes puts and get requests for the client. It is not a centralized service - the coordinator is simply an implementation on each node