06 - MongoDB Sharding and Replication

Question 1

Define

shard

Answer 1

subset of your data.

Question 2

Each shard should be deployed in a ____.

Answer 2

replica set

Question 3

Define

Config Servers

Answer 3

Store metadata and configuration settings for your cluster

Question 4

Define

mongos

Answer 4

Query router to interface with client applications

Question 5

When does MongoDB split chunks?
How does this get triggered?

Answer 5

• splits chunks when they grow beyond the configured chunk size.
• Both inserts and updates can trigger a chunk split.

Question 6

Can a chunk with 1 shard key value be split?

Answer 6

no

Question 7

What happens when you shard a collection?

Answer 7

• Distributing a single dataset across multiple databases, which then allows for storage on multiple machines
• Larger dataset -> smaller chunks

Question 8

How does MongoDB partition a collection?

Answer 8

MongoDB partitions a collection of documents based on the shard key

• Data is divided into chunks and chunks are allocated to each server.

Question 9

Why should we choose a shard key carefully?

Answer 9

• shard key has a direct impact on the cluster’s performance
• suboptimal shard key performance or scaling issues due to uneven chunk distribution
• You can always change your data distribution strategy by changing your shard key.

Question 10

What are the benefits of sharding a collection?

Answer 10

Can now handle more requests
Horizontal-scaling or scale out

Question 11

Differentiate

vertical vs horizontal scaling

Answer 11

Vertical: increasing power of a single machine
Horizontal: distributing across diff machines

Question 12

Ranged/Dynamic Sharding

Answer 12

takes a field/attribute as an input and creates predefined ranges

Question 13

List

Key attributes of an effective shard key

2

Answer 13

high cardinality
well-distributed frequency

Question 14

Define

cardinality

Answer 14

number of possible values of that key

• Ex. If a shard key only has 3 possible values, then there can only be 3 shards max

Question 15

Define

frequency

Answer 15

distribution of the data along the possible values

• Ex. If 95% of records occur with a single shard key value then, due to this hotspot, 95% of the records will be allocated to a single shard.

Question 16

Hashing

Answer 16

set of fields determines the allocation of the record to a given shard.
* allows more even distribution across shards even when there is not a suitable shard key
* no lookup table needs to be maintained.

Question 17

List

Drawbacks of hashing

Answer 17

1. query operations for multiple records are more likely to get distributed across multiple shards. Whereas ranged sharding reflects the natural structure of the data across shards, hashed sharding typically disregards the meaning of the data. This is reflected in increased broadcast operation occurrence.
2. expensive

Question 18

LIst

Advantages - Sharded & replicated cluster

3

Answer 18

Increased read/write throughput
Increased storage capacity
High availability

Question 19

List

Disadvantages - Sharded & replicated cluster

3

Answer 19

Query Overhead
* Additional latency on every operation, need to query each shard and merge results

Complexity of administration
* Increased upkeep and maintenance

Increased infrastructure costs
* Additional compute and machines

Question 20

List

Alternatives to Sharded & Replicated Cluster

2

Answer 20

Vertical Scaling
Replication

Question 21

Define

MongoDB replication

Answer 21

process of creating a copy of the same data set in more than one MongoDB server.

• One drawback of all shards having secondary server for each shard is the cost.