Dynamo DB

Question 1

What are NoSQL databases?

Answer 1

NoSQL database are non-relational databases and are distributed.

Question 2

What is DynamoDB?

Answer 2

A fully-managed, highly available, NoSQL database that scales to massive workloads.

Question 3

What are the data types supported by DynamoDB?

Answer 3

• Scalar Types: String, Number, Binary, Boolean, Null
• Document Types: List, Map
• Set Types: String Set, Number Set, Binary Set

Question 4

How many items (rows) can a DynamoDB table have?

Answer 4

An infinite number of rows.

Question 5

What is the maximum size of a item (row) in DynamoDB?

Answer 5

400KB

Question 6

What are the options for DynamoDB Primary Keys?

Answer 6

• Partition Key (HASH)
• Partition Key + Sort Key (HASH + RANGE)

Question 7

What are the read/write capacity modes for DynamoDB? How often can you switch between these modes?

Answer 7

• Provisioned Mode (default): Plan for capacity beforehand
• On-Demand Mode: Read/writes scale with the workload

You can switch between these modes every 24 hours.

Question 8

If the Burst Capacity throughput is exceeded what exception will you get? How should you retry on this exception?

Answer 8

You’ll get a ProvisionedThroughputExceededException. You should you use an exponential backoff when retrying.

Question 9

What is a WCU?

Answer 9

A Write Capacity Unit (WCU) represents one write per second for an item up to 1KB in size.

Question 10

How many WCUs are used when we write 10 items per second with an item size of 2KB?

Answer 10

We need 10*(2KB/1KB) = 20 WCUs

Question 11

How many WCUs are used when we write 6 items per second with an item size of 4.5KB?

Answer 11

6*(5KB/1KB) = 30 WCUs (4.5 gets rounded to the upper KB)

Question 12

What are the two read modes for DynamoDB?

Answer 12

• Strongly Consistent Read: If we read just after a write we will get the correct data. Consumes twice the RCU.
• Eventually Consistent Read (default): Possible to get stale data before replication

Question 13

What is an RCU?

Answer 13

A Read Capacity Unit (RCU) represents one Strongly Consistent Read per second or two Eventually Consistent Reads per second for an item up to 4KB.

Question 14

How many RCUs are used when we have 10 Strongly Consistent Reads per second with an item size 4KB.

Answer 14

10*(4KB/4KB) = 10 RCUs

Question 15

How many RCUs are used when we have 16 Eventually Consistent Reads per second with an item size of 12KB?

Answer 15

(16/2)*(12KB/4KB) = 24 RCUs

Question 16

How many RCUs are used when we have 10 Strongly Consistent Reads per second with an item size of 6KB?

Answer 16

10 * (8KB/4KB) = 20 RCUs (We must round up 6KB to 8KB)

Question 17

How is data stored in DynamoDB?

Answer 17

Data is stored in partitions. Partition Keys go through a hashing algorithm to know which partition they go to.

Question 18

How much more expensive is R/W On-Demand Capacity than Provisioned Capacity?

Answer 18

2.5x more expensive.

Question 19

What is a Local Secondary Index?

Answer 19

A Local Secondary Index (LSI) is an alternative sort key for your table, which consists of a scalar attribute (String, Number or Binary). Up to 5 LSI per table defined at table creation time. You must provision RCUs and WCUs for the index.

Question 20

What is a Global Secondary Index?

Answer 20

The Global Secondary Index (GSI) is an alternative primary key from the base table. The Index Key consists of scalar attributes. Must provision RCUs and WCUs for the index. Can be added/modified after table creation.

Question 21

What is PartiQL?

Answer 21

Use a SQL-like syntax to manipulate DynamoDB tables. Supports some but not all statements as well as batch operations.

Question 22

What is Optimistic Locking?

Answer 22

DynamoDB supports Conditional Writes. A strategy to ensure that an item hadn’t changed before you update/delete it using an attribute that each item has as a version number.

Question 23

What is DAX?

Answer 23

The DynamoDB Accelerator (DAX) is a fully-managed, highly available, seamless in-memory cache for DynamoDB. It has microseconds latency for cached reads and queries. Doesn’t require application logic modification.

Question 24

What are DynamoDB Streams?

Answer 24

Ordered stream of item-level modifications (create/update/delete) in a table.

Question 25

What is a DynamoDB TTL attribute used for?

Answer 25

It’s used to automatically delete an item after an expiry timestamp. Doesn’t consume any WCUs. The TTL attribute must be a Number data type with Unix Epoch timestamp value. Expired items are deleted within 48 hours of expiration.

Question 26

Is Transactional a Read Mode?

Answer 26

Yes

Question 27

How many WCUs and RCUs do DynamoDB Transactions consume?

Answer 27

2x WCU and RCU.

Question 28

How many WCUs are used when we have 3 Transactional reads per second, with item size 5KB?

Answer 28

3 * (5KB/1KB) * 2 (Transactional Cost) = 30 WCUs

Question 29

How many RCUs are used when we have 5 Transactional reads per second, with an item size of 5KB?

Answer 29

5 * (8KB/4KB) * 2 (Transactional Cost) = 20 RCUs

Question 30

How many write types are there in DynamoDB?

Answer 30

• Concurrent Writes: Writes can overwrite each other
• Conditional Writes: Write conditionally on some previous value
• Atomic Writes
• Batch Writes

Question 31

What are the options to complete a Table Cleanup?

Answer 31

• Option 1: Scan + Delete Item - Very slow, consumes RCU and WCU, expensive
• Option 2: Drop Table + Recreate Table - Fast, efficient, cheap

Question 32

What are the options to copy a DynamoDB table?

Answer 32

• Option 1: Using AWS Data Pipeline
• Option 2: Backup and restore into a new table - takes some time
• Option 3: Scan + PutItem or BatchItem