API Gateway & Dynamo DB

Question 1

Can you use mapping templates when you have the proxy setting enabled?

Answer 1

No

Question 2

What is the difference between provisioned and on-demand mode for DynamoDB?

Answer 2

Provisioned (default) - you specify the number of reads/writes per second, plan capacity beforehand, pay for provisioned units.

On-demand - read/writes automatically scale with your workloads. No planning needed but more expensive.

Question 3

What is one write capacity unit (WCU) in DynamoDB?

Answer 3

Represents one write per second for an item up to 1KB in size (rounded up to the nearest whole KB)

Question 4

Work out the number of write capacity units needed to write 6 items per second into DynamoDB each with a size of 1.5KB

Answer 4

6 x 2 = 12 1.5KB would be rounded up to 2KB (because amazon are money greedy bastards)

Question 5

Work out the number of write capacity units needed to write 120 items per minute into DynamoDB each with a size of 2KB

Answer 5

120/60 x 2 = 4

Question 6

What are the two “modes” for reading items from DynamoDB (in the context of consistency)

Answer 6

Eventually consistent read (default) - if we read straight after a write we might get stale data as replication may not complete in time.

Strongly consistent read - reading always up-to-date data, this is done via a parameter in the api call, consumes twice the RCU (read capacity units).

Question 7

What does one read capacity unit represent in DynamoDB?

Answer 7

Represents one strongly consistent read per second or two eventually consistent reads per second, for an item up to 4KB in size.

Question 8

Work out the number of read capacity units for 10 strongly consistent reads per second, with an item size of 4KB

Answer 8

10 RCUs

Question 9

Work out the number of read capacity units for 16 eventually consistent reads per second with an item size of 12KB

Answer 9

We need (16/2) x (12kb / 4kb) = 24 RCUs

Question 10

What is the difference between the PutItem and UpdateItem api calls to DynamoDB?

Answer 10

PutItem creates or replaces an item (based on primary key) whereas UpdateItem edits a subset of an existing items attributes.

Question 11

What is the most efficient way to delete all the items in a DynamoDB table?

Answer 11

Use the DeleteTable api call, do not use the DeleteItem with a scan

Question 12

In DynamoDB when using batch operations what happens if one operation in a batch fails?

Answer 12

The whole batch does not roll back, just the parts that failed are returned as failed, the client can then retry just those failed operations.

Question 13

What is PartiQL?

Answer 13

SQL compatible query language for DynamoDB (without joins)

Question 14

What is DynamoDB Accelerator (DAX)?

Answer 14

In memory cache for DynamoDB

Question 15

What can be cached in a DAX?

Answer 15

Individual objects, queries / scans

Question 16

When might you use ElastiCache instead of DAX?

Answer 16

Storing the result of computed data

Question 17

What are DynamoDB Streams?

Answer 17

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

Question 18

Where can DynamoDB Streams be sent?

Answer 18

Kinesis Data Streams, Lambda, Kinesis Client Library apps

Question 19

When DynamoDB automatically deletes an item that had a TTL attribute set are any WCUs consumed?

Answer 19

No, no extra cost

Question 20

How quickly do expired DynamoDB items (using TTL) get deleted?

Answer 20

Within 48 hours

Question 21

When using transactions in DynamoDB how many more capacity units are used?

Answer 21

Twice as many

Question 22

Work out how many WCUs are needed for 3 transactional writes per second, with an item size of 5Kb

Answer 22

3 x (5Kb / 1Kb) x 2 (transactional cost) = 30WCUs

Question 23

Work out how many RCUs are needed for 5 transactional reads per second, with an item size of 8Kb

Answer 23

5 x (8Kb / 4Kb) x 2 (transactional cost) = 20RCUs

Question 24

What is the difference between using DynamoDB vs ElastiCache as session state cache (user logins)

Answer 24

ElastiCache is in memory. DynamoDB is serverless and auto scales. Both are key/value stores.

Question 25

What two ways can you copy a DynamoDB table?

Answer 25

Use an AWS data pipeline (via an S3 bucket) or backup and restore into a new table

Question 26

What is the maximum size of an item in a DynamoDB table?

Answer 26

400KB

Question 27

You would like to query a DynamoDB table using an attribute that is not part of your tables primary key. What should you do to make this query efficient?

Answer 27

Create a global secondary index

Question 28

You are designing a new DynamoDB table where you want to make a query using an attribute that is not part of your tables primary key. You need to use the >= predicate while keeping the same partition key. What should you do to make this query efficient? Nothing, create a local secondary index or create a global secondary index?

Answer 28

Create a local secondary index

Question 29

What CLI option should you use to restrict the number of entries returned from a DynamoDB get call?

Answer 29

–max-items (and –starting-token if paganating)