SOA-C03 practice question: Caching with CloudFront and ElastiCache (1 of 2)

Reliability and Business Continuity A free question from our original SOA-C03 practice exam. Try it, then open the answer to see why each option is right or wrong.

A pet insurer's quote website runs on EC2 instances that query an Amazon RDS for MySQL database.

Query analysis shows that most database load comes from the same few hundred plan-lookup queries, whose results change only once a day.

Database CPU reaches 90 percent at peak, and the team wants sub-millisecond reads for these lookups.

What should a CloudOps engineer recommend?

Choose one answer.

A. Turn on Multi-AZ for the DB instance so that the standby instance serves part of the read traffic.
B. Put Amazon DynamoDB Accelerator (DAX) in front of the RDS database so that repeated queries are answered from memory.
C. Move the DB instance to a larger instance class so that it has more CPU to process the repeated lookups at peak.
D. Add an ElastiCache cluster and cache the plan-lookup results in the application with a TTL of a few hours.

Show the answer and explanation

Answer: D

A. Wrong: In a Multi-AZ DB instance deployment the standby does not serve read traffic. It exists for failover, so database load would not drop. Learn more: Multi-AZ DB instance deployments (opens in a new tab)
B. Wrong: DAX is an in-memory cache built only for DynamoDB tables. It cannot cache queries sent to an RDS for MySQL database. Learn more: In-memory acceleration with DynamoDB Accelerator (DAX) (opens in a new tab)
C. Wrong: A larger instance relieves CPU, but each lookup still runs against the database and takes milliseconds. It costs more and does not give sub-millisecond reads. Learn more: DB instance classes (opens in a new tab)
D. Right: Correct. Repeated reads of data that rarely changes are ideal for an in-memory cache. ElastiCache returns results in sub-millisecond time and removes most of these queries from the database. Learn more: Caching strategies for ElastiCache (opens in a new tab)

Explanation

When a small set of queries with slowly changing results drives database load, caching them in memory is the standard fix. ElastiCache serves them in sub-millisecond time and frees the database for other work. Multi-AZ standbys do not serve reads, DAX works only with DynamoDB, and scaling up keeps every query on the database.

SOA-C03 practice question: Caching with CloudFront and ElastiCache (1 of 2)

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