ElastiCache and AWS CLI

Question 1

When should you not cache data

Answer 1

When you deal with anti-patterns, where data is changing rapidy

Question 2

What is the lazy loading caching pattern

Answer 2

If application cache miss in elasticache, it reads from D.B and writes back to cache

Question 3

What are the pros and cons of the lazy loading caching pattern

Answer 3

Pros: Only requested data is cached (the cache isn’t filled up with unsued data). Node failures are not fatal.
Cons: Cache miss penalty that results in 3 round trips, noticeable delay for that request.
Stale data: Data can be updated in database but not in cache

Question 4

What is the WriteThrough caching pattern

Answer 4

If application cache miss, it writes to the D.B, then writes to teh cache

Question 5

What are the pros and cons of the WriteThrough caching pattern

Answer 5

Pros: Data in cache is never stale, reeads are quick. Write penaly vs. Read penalty(each write requires 2 calls.
Cons:
Missing data until it is added/updated in the D.B.
A lot of data will never be read.

Question 6

What are the three ways of cache evictions?

Answer 6

You delete the item explictily in the cache
Item is evicted becasue the memory is full and it’s not recently used (LRU)
You set an item TTL

Question 7

When should you not set a TTL

Answer 7

When you’re using WriteThrough

Question 8

Do read replicas require us to change our SQL connection string?

Answer 8

Yes, they add new endpoints for databases to read from

Question 9

How does your organization enforce SSL connections on your MySQL database?

Answer 9

Apply a ‘Require SSL’ statement to all your users in your SQL database

Question 10

If you get an AWS command not found:

Answer 10

The aws executable is not in the PATH environmental variable

Question 11

EC2 instance metadata and url:

Answer 11

• Allows Ec2 instances to “learn about themselves” without using an IAM role for that purpose.
• IP: 169.254.169.254/latest/meadata
• Can retrieve IAM role name but not policy.

Question 12

If you want to perform actions on AWS directly from your application code?

Answer 12

Use an AWS Sdk

Question 13

API Rate limits

Answer 13

Describe Instances API for Ec2 has a limit of 100 calls per second.
GetObject on S3 has a limit of 5,500 GET per second per prefix

Question 14

For Interment Errors:

Answer 14

Implement exponential back off

Question 15

For consistent errors

Answer 15

Request an API throttling limit increase

Question 16

How can you request a service limit increase for running on demand standard instances?

Answer 16

By opening a ticket

Question 17

How can you request a service quota increase?

Answer 17

By using the Service Quota API

Question 18

If you get a ThrollingException intermittently use

Answer 18

Exponential backoff

Question 19

“An application deployed on an Ec2 instance is using environmental variables w//credentials from an IAM user to call the Amazon S3 API, the IAM instance profile was assigned to the Ec2 instance, but it still had access to all S3 buckets. Why?

Answer 19

Credential chain is still giving priorities to the environment variables

Question 20

When you call the use SDK or CLI, HTTP requests

Answer 20

Are signed for, if not using it, need to sue Sigv4