Distributed Databases

Question 1

What are the three key topics of DD?

Answer 1

• Replication: Keep a copy of the same data on several different nodes
• Partitioning: Split the database into smaller subsets and distribute the partitions to different nodes
• Transactions: Units of work that groups several reads and writes to be performed together in the database (NOT EXAM MAT!!)

Question 2

Why is Replication important?

Answer 2

• Data scalability: increase read throughput by allowing several machines to serve read only requests
• Geo-scalability: to have the data close to clients
• Fault tolerance: to allow the system to work even if parts are down

Question 3

What are the roles of Replication nodes?

Answer 3

• Leader: nodes that accept write queries from clients
• Follower: nodes tha tprovide read-only access to data

Question 4

What are the usual paradigms for implementing replication?

Answer 4

• Single-leader: a single leader accepts writes, which are distributed to followers
• Multi-leader: multiple leaders accept writes, keep themselves in syunc, and update follwers
• Leaderless: all nodes are peers in the replication network

Question 5

What is the main idea of Write Ahead Logs (WAL)?

Answer 5

WAL replication writes all changes to the leader and follower. Then, the followers apply the WAL entries to get consistent data.

Question 6

Who uses WAL?

Answer 6

PostgreSQL and Oracle

Question 7

What is logical based replication?

Answer 7

It generates a stream of logical updates for each update to the WAL

Question 8

What are some examples of Logical updates?

Answer 8

• For new records, the insertion value
• For deleted records, the ID
• For updates records, the ID and updated value

Used by MongoDB and MySQL

Question 9

What is the main problem with WAL?

Answer 9

It is bound to the implementation of the data strcutre. If it changes in the leader, it stops working

Question 10

How does replication work when using Logical based replication?

Answer 10

• Take a snapshot from leader
• Ship it to replica
• Get an ID to the state of the leader’s replication log at the time the snapshot was created
• Initialize the replication function to the latest leader ID
• Retrieve and apply the replication log until it catches up

Question 11

What is the main point of Synchronous replication?

Answer 11

The writes need to be confirmed by a configurable number of followers before the leader reports success.

Question 12

What is the main point of Asynchronous replication?

Answer 12

The leader reports success as soon as the write was confirmed to disk, followers apply their own changes.

Question 13

Name some characteristics of Synchronous replication.

Answer 13

• A follower is guaranteed to have up to date information with the leader
• The data is available even if the leader fails, on the followers
• If not enough followers respond, the operation cannot be processed
• All writes are blocked by the leader until enough follower writes are confirmed
• Impractical in real life

Question 14

Name some characteristics of Asynchronous replication

Answer 14

• It has higher availability since writes are not blocked as much
• A follower is never guaranteed to have up to date copy
• Writes are not guaranteed to be durable in case of leader failure

Question 14

Describe Synchronous replication as it relates to consistency and availability.

Answer 14

SR is very consistent, but not so available since it blocks writes until the current one is reported a success.

Question 14

Describe Asynchronous replication as it relates to consistency and availability.

Answer 14

AR is not so consistent, but it is widely available, since it does not block writes for long.

Question 15

What is the CAP Theorem

Answer 15

CAP comes from Consistency, Availability, and Partition Tolerance. The theorem states that there is no real life system that can have all three, and thus one has to choose two of these attributes when designing a system.

Question 16

What is eventual consistency?

Answer 16

All updates eventually deliver to all replicas.

Question 17

What is causal consistency?

Answer 17

Causally related operations are delivered to other replicas inthe correct order. Thus, it maintains a partial order of events based on causality.

Question 18

What is client-centric consistency?

Answer 18

A system that guarantees the correct order of operations only for a single client process

Question 19

What is sequential consistency?

Answer 19

The operations appear to take place in a total order that is consistent with the order of operations on each replica. All replicas observe the same order of operations.

Question 20

What is linearizability?

Answer 20

Sequential consistency + the total order or operation conforms to the real time ordering. Once a write is completed, the value is replicated to all nodes. At any point in time, all nodes read the same value.