ACID Compliance and CAP Theorem

Question 1

Explain the A in ACID Compliance

Answer 1

Atomicity: The entire transaction must either succeed or fail, no in between. E.g., you can’t successfully write a portion of data and have the other portion fail

Question 2

Explain the C in ACID Compliance

Answer 2

Consistency: All the database rules are enforced. Self-explanatory, but a part nonetheless

Question 3

Explain the I in ACID Compliance

Answer 3

Isolation: No transaction is affected by any other transaction that is still in progress

Question 4

Explain the D in ACID Compliance

Answer 4

Durability: Once a transaction is committed, it stays, even if the system crashes immediately after

Question 5

What is the CAP Theorem?

Answer 5

There are three features in databases: Consistency, Availability, and Partition-Tolerance. You can pick 2, but you can’t have all 3

Question 6

Explain the C in the CAP Theorem

Answer 6

Consistency: Every read receives the most recent write or an error

Question 7

Explain the A in the CAP Theorem

Answer 7

Availability: Every request receives a non-error response, without the guarantee that it’s the most up-to-date

Question 8

Explain the P in the CAP Theorem

Answer 8

Partition Tolerance: The (distributed) system continues to operate despite an arbitrary number of messages dropped/delayed by network between nodes

A “partition” in this case is a comms break within a distributed system

Question 9

When a network failure happens, what do you do when you want to ensure consistency?

Answer 9

Cancel the operation, thus decreasing availability but ensuring consistency

Question 10

When a network failure happens, what do you do when you want to ensure availability?

Answer 10

Proceed with the operation, thus risking inconsistency but providing availability

Question 11

Which of the three CAP Theorem features is typically given up in modern databases?

Answer 11

Availability, since scalability and data consistency are typically more important for customers