Exam AWS Certified Solutions Architect - Associate SAA-C02 topic 1 question 127 discussion

Should be A

For all those answering A can anyone tell what is the purpose of cache here when the questions states clearly " Users would like new messages to be read with as little latency as possible" .. Read "new messages" , answer should be C .

Consider between A, C: little latency as possible -> A is the answer.

Use caching: If your traffic is read heavy, consider using a caching service, such as Amazon DynamoDB Accelerator (DAX). DAX is a fully managed, highly available, in-memory cache for DynamoDB that delivers up to a 10x performance improvement, from milliseconds to microseconds, even at millions of requests per second.

A has wrong solution, because: "You do not need to modify application logic because DAX is compatible with existing DynamoDB API calls. " https://aws.amazon.com/dynamodb/dax/

I don't get it. Why C? I understand minimal changes to application and increasing the RCU could help but what do you do after that? It says "Double the RCU" and that's it. What if it's still not enough when the traffic increases? Plus it's manual work.

IMHO The key point is - fewest possible application modifications Lets eliminate wrong answers one by one: D - not sure why we need to use Amazon ElastiCache if we have better option DynamoDB Accelerator (DAX). But anyway it requires application modification B - Add DynamoDB read replicas. DynamoDB uses leaderless approach to replicate data among nodes. This means that there is no thing like replica in the similar way like for RDS - they do it through global tables instead to distribute load among the regions. A. - Configure Amazon DynamoDB Accelerator (DAX) . It's the most one hard to skip, but taking into consideration business logic of application (it's chat application so I won't expect that caching for small part of persons in chat wouldn't provide huge benefit) + it anyway require application change. This why C is a best candidate - it will allow to improve number of read without changing application codebase.

A. Configure Amazon DynamoDB Accelerator (DAX) for the new messages table. Update the code to use the DAX endpoint.

From ops perspective, any new msg must first be created and saved to the DDB by the sender client app. Then followed by the recipient client app to retrieve the "freshly" created msg from the DDB. Thus, DB cache will help in this use case.

- C - A and D are wrong. Adding a cache will NOT speed up reading of NEW messages. Cache only speeds up the reading of OLD / SEEN messages - Between adding read replica (B) and increasing RCU (C), C will have a more direct impact (say from 1K reads/sec to 10K reads/sec)

I'm pretty confident they are wanting C as the answer. In reading up on the "read capacity units" setting (user settable in DynamoDB Provisioned Mode), its clear that the DB cannot keep up sub-millisecond responses if this setting is too low in Provisioned Mode. The fact that they are requesting a fix that requires minimal application changes suggest this C. I'm sure the setting can be changed on the fly. If you run in On-Demand mode, DynamoDB adjusts this setting automatically, but you may risk getting a bigger bill than you expected.

It's A because no changes to the application are required

Must be A. DAX has a Write-Through mode where any new data that is written through to the DB will be cached so the next read after the write will be reading from the Cache. Quite similar to storage based caching concept. read - https://aws.amazon.com/blogs/database/amazon-dynamodb-accelerator-dax-a-read-throughwrite-through-cache-for-dynamodb/

Ans is A, One thing to note here is little latency is acceptable and therefore A is ans reason is given below: For read-heavy or bursty workloads, DAX provides increased throughput and potential operational cost savings by reducing the need to overprovision read capacity units. This is especially beneficial for applications that require repeated reads for individual keys. If DAX does not have the item available (a cache miss), DAX passes the request through to DynamoDB. When it receives the response from DynamoDB, DAX returns the results to the application. But it also writes the results to the cache on the primary node. Remember for strong consistency dax does not cache data and simply pass request and dynamoDB and response to client.

I would go with A Explanation: "DAX does all the heavy lifting required to add in-memory acceleration to your DynamoDB tables, without requiring developers to manage cache invalidation, data population, or cluster management." Source: https://aws.amazon.com/dynamodb/dax/

Its DAX. Dynamo DB is already Serverless so why care about provisioning read capacities. Moreover DAX always refer back to the Dynamo DB and still provide response blazingly fast for new reads. Remember DAX is a write-through cache. Don't compare it with other in-cache services. Have a look : https://aws.amazon.com/blogs/aws/amazon-dynamodb-accelerator-dax-in-memory-caching-for-read-intensive-workloads/

Answer is A. DAX is a fully managed caching service for DynamoDB, i.e. it makes DynamoDB even faster.