AWS Cost Saving Tip 17: Add Amazon RDS Read Replica's on need basis and save costs

Many online applications are usually Read intensive in nature. Though the usage of Cache, NoSQL, CDN are becoming part of the core architecture in many such applications, still most of them are heavily dependent upon Traditional RDBMS (like MySQL, Oracle) for serving their requests.
Architects usually design their DB tier with Master + Read Slaves model to cater the read intensive needs of the application. Writes and Immediate Write + Reads are directed to Master and all other Read queries are balanced between Read Slaves. Translating this model to AWS infrastructure means, using RDS MySQL Master + Multiple Read Replica Slaves.

To know more about Load balancing Amazon RDS Read Replica's Click here

Scenario: Weekly load volatility impacts: 

Online Ticketing and Travel companies are usually prone to weekly fluctuations in their load pattern. Imagine during peak load they need five M2.4XLarge RDS MySQL Read Replica's to serve their read intensive site.

As an Infra Architect you have two options in the Elastic AWS world: 

Option 1: Run your DB tier with peak capacity all days of the week. ( May be suitable for Traditional infra environments, but a bad idea in AWS infra)

Option 2: Elastically increase the number of RDS Read Replica instances during the peak period and decrease the number of instances once the peak period is over. (AWS is built for leveraging such cases)

Number of EC2 Instances	Number of days in a week	EC2 Instance Type	Weekly($)	Monthly($)
5	7	M2.4XLarge	1860	7442
			1860	7442
2	4	M2.4XLarge	425	1701
5	3	M2.4XLarge	797	3189
			1222	4890

Cost for Option 1: Running all 5 of them for a Week costs : $1860
Cost for Option 2: Running with 2 RR normally for 4 days a week and elastically increasing to Five M2.4Xlarge during the peak days(i.e. three days a week) will cost : $1222

Savings: From the above analysis you can observe that elastically increasing/Decreasing Amazon RDS RR based on your load you can achieve ~30 % cost savings in a week for the mentioned use case.

To know more about "How Amazon Auto Scaling can save costs on your Web/App Tier ? Click here "

Scenario: Impact during Campaigns 

Imagine your online business demands big campaigns once every quarter. The campaign usually runs for 2 weeks and traffic increases 4-5X during these times. During normal period you run 1 RDS Read replica and during campaigns you need 5 RDS Read replicas. 

If the campaign is planned once a year during holiday season, companies lease hardware, but if the campaign is every quarter or more frequent it becomes irrelevant to lease-build-maintain peak capacity.

Number of EC2 Instances	Number of Months	EC2 Instance Type		Monthly($)
5	10	M2.4XLarge	All times	98877
				98877
1	10	M2.4XLarge	Other times	16479
5	2*	M2.4XLarge	During campaigns	16479
				32959

Instance Hours per day: 744

*Campaign weeks are aggregated to months

US-Oregon region - $2.215 (M2.4Xlarge RDS RR pricing as on posting)

Cost for Option 1: Running all 5 of them for a year costs :  98877 USD

Cost for Option 2: Running with 1 RR on regular weeks and elastically increasing to Five M2.4Xlarge during the peak weeks will cost : 32959 USD a year

Savings: From the above analysis you can observe that elastically increasing/Decreasing Amazon RDS RR you can achieve ~55 % cost savings in a year.

For in depth analysis on Amazon RDS Second tier Read Replica's Click here 

Other Cost Saving Tips

Cost Saving Tip 1: Amazon SQS Long Polling and Batch requests
Cost Saving Tip 2: How right search technology choice saves cost in AWS ?
Cost Saving Tip 3: Using Amazon CloudFront Price Class to minimize costs
Cost Saving Tip 4 : Right Sizing Amazon ElastiCache Cluster
Cost Saving Tip 5: How Amazon Auto Scaling can save costs ?
Cost Saving Tip 6: Amazon Auto Scaling Termination policy and savings
Cost Saving Tip 7: Use Amazon S3 Object Expiration
Cost Saving Tip 8: Use Amazon S3 Reduced Redundancy Storage  
Cost Saving Tip 9: Have efficient EBS Snapshots Retention strategy in place 
Cost Saving Tip 10: Make right choice between PIOPS vs Std EBS volumes and save costs 
Cost Saving Tip 11: How elastic thinking saves cost in Amazon EMR Clusters ? 
Cost Saving Tip 12: Add Spot Instances with Amazon EMR 
Cost Saving Tip 13: Use Amazon Glacier for archive data and save costs 
Cost Saving Tip 14: Plan your deletion in Amazon Glacier and avoid cost leakage 

Cost Saving Tip 15: Plan your Amazon Glacier Retrieval strategy and avoid cost leakage 

Cost Saving Tip 16: Periodically remove your unwanted AWS resources and save costs (new)