3-Databases

Question 1

How is data on DynamoDB stored?

Answer 1

On SSDs that are replicated in 3 geographically distinct data centres

Question 2

What are the conceptual elements of DynamoDB?

Answer 2

Tables are broad collections of data, they consist of items which each have attributes

It supports both the key-value and document data models

Question 3

How can access to DynamoDB be restricted?

Answer 3

IAM provides very fine-grained control.

The dynamodb:LeadingKeys and dynamodb:attributes conditions restrict access to items and attributes respectively

Question 4

How are items in DynamoDB reference?

Answer 4

With a primary key

Question 5

How do primary keys work in DyanmoDB?

Answer 5

There are two kinds: a partition key is based on a unique attribute i.e. user ID

If the partition key is not unique, a composite key is used instead. It consists of a partition key and a sort key

Question 6

What is the purpose of database indexes?

Answer 6

They allow fast queries to be performed on specific columns/attributes

Question 7

What indexes does DynamoDb support?

Answer 7

Local Secondary Index - same partition key as the original table but different sort key, Can only be created when the table is created

Global Secondary Index - can be added later. Hase a differetn partition key and a different sort key as the original table

Question 8

What API calls are used to get data out of DynamoDB?

Answer 8

Query, Scan, Get and BatchGetItem

Question 9

How does Query work in DynamoDB?

Answer 9

It finds items based on the primary key of a table or index.

It is eventually consistent by default but can be set to strongly consistent

Question 10

How can Query be refined?

Answer 10

It usually returns all attributes but the ProjectionExpression parameter can control this

Usually results are ascending by sort key but setting ScanIndexForward to false reverses this

Question 11

How does Scan work in DynamoDB?

Answer 11

It dumps all items in a table and filters those of interest

The attributes it returns can be controlled with the ProjectionExpression parameter

Question 12

How can the performance impact of scans be minimised?

Answer 12

Performing parallel scans - by default data is processed sequentially in 1 MB blocks

Set a lower-page size as this means fewer items are returned per API call

Question 13

What are the consistency models of Scan and Query?

Answer 13

Scan is eventually consistent

Query is eventually consistent by default but can be made strongly consistent

Question 14

How is DyanmoDB capacity managed?

Answer 14

Manually with capacity units, or automatically with the On-Demand Capacity option

Question 15

How is DynamoDB throughput calculated?

Answer 15

Each write capacity unit allows for 1 x 1 KB write per second

Each read capacity unit allows for either 1 x 4 KB strong consistent read per second or 2 x 4 KB eventually consistent reads per second

Question 16

How can DynamoDB be made even more performant?

Answer 16

With DAX, an in-memory caching service

Question 17

What caching strategy does DAX use?

Answer 17

Write-through

Question 18

What operations does DAX accelerate?

Answer 18

Only read operations that can be eventually-consistent

Question 19

What happens if the request rate for a table is too high? How can this be managed?

Answer 19

The client will trigger a ProvisionThroughputExceed exception.

This is managed by retrying with exponential backoff

Question 20

What is the consistency model of DAX?

Answer 20

It is eventually consistent

Question 21

What database model do DynamoDB Transactions enable?

Answer 21

ACID (Atomic, Consistent, Isolated, Durable)

Question 22

What is ACID?

Answer 22

A requirement that database operations are:

Atomic - all or nothing
Consistent - always leave the database in a valid state
Isolated - no cross-transaction dependency
Durable - successfully committed changes remain if the system goes down

Question 23

How can old items in DynamoDB be managed?

Answer 23

The ExpirationTime attribute sets them to expire when that time is reached

Expired items are deleted within 24 hours

Question 24

What are DynamoDB streams?

Answer 24

Logs consisting of item level events to that DyanmoDB table in time-order

They are encrypted and retained for 24 hours

By default, only the primary key is recorded. If more is needed, use Before and After Images

Question 25

What is ElastiCache?

Answer 25

An in-memory cache that optimises read-heavy applications or those that have repeated compute-intensive queries

Question 26

What cache engines does ElastiCache support? How do they differ?

Answer 26

Memcached - isn’t clustered so no multi-AZ support

Redis - Supports complex data structures such as sorted lists and sets. It supports master / slave replication and mult-AZ

Question 27

What cache strategies does ElastiCache support?

Answer 27

Both engines support Lazy loading and write-through

Question 28

How does ElastiCache prevent caches from becoming stale?

Answer 28

With TTL