Reserved capacity in DynamoDB - Step-by-Step Execution

Concept Flow - Reserved capacity

Start

↓

Purchase Reserved Capacity

↓

Reserved Capacity Active

↓

Use Reserved Capacity for Read/Write

↓

Billing Applies Discount

↓

Reserved Capacity Period Ends

↓

End or Renew Reserved Capacity

This flow shows how you buy reserved capacity, use it for your database operations, get billing discounts, and then the reserved period ends or renews.

Execution Sample

DynamoDB

1. Purchase 1000 RCUs for 1 year
2. Use 800 RCUs for reads
3. Use 1200 WCUs for writes
4. Billing applies reserved discount
5. Reserved capacity expires after 1 year

This example traces buying reserved capacity units, using them, and how billing applies discounts during the reserved period.

Execution Table

Step	Action	Reserved Capacity State	Usage	Billing Effect
1	Purchase 1000 RCUs reserved capacity	1000 RCUs reserved active	0 used	No billing yet
2	Use 800 RCUs for reads	1000 RCUs reserved active	800 RCUs used	Usage covered by reserved capacity
3	Use 1200 WCUs for writes	1000 RCUs reserved active	1200 WCUs used	Standard provisioned rates apply for WCUs
4	Billing cycle processes	1000 RCUs reserved active	800 RCUs used, 1200 WCUs used	Discount applied to RCU usage, standard provisioned rates for WCUs
5	Reserved capacity period ends	Reserved capacity expires	Usage continues at standard provisioned rates	No discount after expiration

💡 Reserved capacity period ends, usage billed at standard provisioned rates unless renewed

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
Reserved RCUs	0	1000 active	1000 active	1000 active	1000 active	0 expired
Used RCUs	0	0	800	800	800	800
Used WCUs	0	0	0	1200	1200	1200
Billing Discount	None	None	Applied to RCUs	Applied to RCUs	Applied to RCUs	None

Key Moments - 2 Insights

Why does usage of WCUs not get a reserved capacity discount in step 3?

Because reserved capacity was only purchased for RCUs, not WCUs. Step 3 shows 1200 WCUs used with no reserved capacity, so billing is at standard provisioned rates.

What happens to reserved capacity after the period ends in step 5?

Reserved capacity expires and no longer applies. Usage after this is billed at standard provisioned rates, as shown in the exit note and step 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the reserved RCUs state after step 2?

A 1000 RCUs reserved active

B 800 RCUs reserved active

C 0 reserved capacity

D Reserved capacity expired

💡 Hint

Check the 'Reserved Capacity State' column at step 2 in the execution table.

At which step does billing start applying discounts for reserved capacity?

A Step 1

B Step 2

C Step 4

D Step 5

💡 Hint

Look at the 'Billing Effect' column in the execution table to see when discounts apply.

If you also purchased 1000 WCUs reserved capacity at step 1, how would billing change at step 3?

A RCUs usage would be billed at standard provisioned rates

B WCUs usage would be covered by reserved capacity discount

C WCUs usage would still be billed at standard provisioned rates

D No billing discount applies at all

💡 Hint

Refer to the 'Usage' and 'Billing Effect' columns in the execution table for step 3.

Concept Snapshot

Reserved Capacity in DynamoDB:
- Buy reserved RCUs or WCUs for a fixed period
- Usage within reserved limits gets billing discounts
- Usage beyond reserved capacity is billed at standard provisioned rates
- Reserved capacity expires after the term
- Renew to continue discounts

Full Transcript

Reserved capacity in DynamoDB means you pay upfront for a set amount of read or write capacity units (RCUs or WCUs) for a period, usually one year. When you use capacity within this reserved amount, your billing is discounted compared to standard provisioned capacity pricing. If you use more than reserved, the extra is billed at standard provisioned capacity rates. After the reserved period ends, the discount no longer applies unless you renew. This example showed purchasing 1000 RCUs reserved capacity, using 800 RCUs and 1200 WCUs, and how billing discounts apply only to the reserved RCUs. It also showed that after the reserved period ends, usage is billed at standard provisioned capacity rates.

Practice

1. What is the main benefit of using Reserved Capacity in DynamoDB?

easy

A. It automatically scales the database up and down.

B. It backs up data to another region.

C. It saves money by pre-paying for database capacity.

D. It encrypts data at rest.

Solution

1. Step 1: Understand Reserved Capacity purpose

   Reserved Capacity allows you to pay upfront for a set amount of database throughput, which reduces costs compared to on-demand pricing.

2. Step 2: Compare with other options

   Scaling, backup, and encryption are separate features and not related to Reserved Capacity.

3. Final Answer:

   It saves money by pre-paying for database capacity. -> Option C

4. Quick Check:

   Reserved Capacity = Cost saving by pre-payment [OK]

Hint: Reserved Capacity means paying upfront to save money [OK]

Common Mistakes:

• Confusing Reserved Capacity with auto-scaling
• Thinking it manages backups or encryption
• Assuming it is a query or API feature

2. Which of the following is the correct way to purchase Reserved Capacity for DynamoDB?

easy

A. Through AWS Management Console or AWS CLI

B. Using a SQL query in DynamoDB console

C. By calling a DynamoDB API operation

D. By enabling a setting in the table properties

Solution

1. Step 1: Identify how Reserved Capacity is bought

   Reserved Capacity is purchased via the AWS Management Console or AWS CLI, not through database queries or API calls.

2. Step 2: Eliminate incorrect methods

   SQL queries and table property settings do not handle Reserved Capacity purchases.

3. Final Answer:

   Through AWS Management Console or AWS CLI -> Option A

4. Quick Check:

   Reserved Capacity purchase = Console or CLI [OK]

Hint: Buy Reserved Capacity only via AWS Console or CLI [OK]

Common Mistakes:

• Trying to buy Reserved Capacity with API calls
• Assuming it can be set in table properties
• Confusing it with query commands

3. If a company has a steady workload of 1000 read capacity units and 500 write capacity units, which option best describes the cost benefit of Reserved Capacity?

medium

A. Reserved Capacity saves money by pre-paying for steady usage.

B. Reserved Capacity will cost more than on-demand pricing.

C. Reserved Capacity only applies to burst workloads.

D. Reserved Capacity automatically adjusts capacity units.

Solution

1. Step 1: Understand workload type

   The workload is steady and predictable, which is ideal for Reserved Capacity.

2. Step 2: Match Reserved Capacity benefits

   Reserved Capacity saves money by pre-paying for steady usage, unlike on-demand which is more expensive for constant loads.

3. Final Answer:

   Reserved Capacity saves money by pre-paying for steady usage. -> Option A

4. Quick Check:

   Steady workload + Reserved Capacity = Cost saving [OK]

Hint: Reserved Capacity suits steady workloads to save money [OK]

Common Mistakes:

• Thinking Reserved Capacity is for bursty or variable workloads
• Assuming it auto-scales capacity units
• Believing it costs more than on-demand for steady use

4. A developer tries to buy Reserved Capacity using a DynamoDB API call but gets an error. What is the likely cause?

medium

A. Reserved Capacity requires table to be in on-demand mode.

B. Reserved Capacity cannot be purchased via API calls.

C. The API call syntax is incorrect.

D. Reserved Capacity is only for global tables.

Solution

1. Step 1: Check how Reserved Capacity is purchased

   Reserved Capacity is bought only through AWS Console or CLI, not through DynamoDB API calls.

2. Step 2: Understand error cause

   Trying to buy it via API results in error because the API does not support this operation.

3. Final Answer:

   Reserved Capacity cannot be purchased via API calls. -> Option B

4. Quick Check:

   Reserved Capacity purchase method = Console/CLI only [OK]

Hint: Reserved Capacity purchase via API causes error [OK]

Common Mistakes:

• Assuming API calls can buy Reserved Capacity
• Blaming syntax errors without checking purchase method
• Confusing Reserved Capacity with table modes

5. A company has unpredictable spikes in traffic but wants to save costs using Reserved Capacity. What is the best approach?

hard

A. Reserved Capacity automatically adjusts to spikes, so buy any amount.

B. Buy Reserved Capacity for the peak traffic to cover all spikes.

C. Use only on-demand capacity to handle spikes efficiently.

D. Buy Reserved Capacity for the average baseline and use on-demand for spikes.

Solution

1. Step 1: Analyze workload pattern

   The workload has unpredictable spikes, so buying Reserved Capacity for peak would be costly and wasteful.

2. Step 2: Combine Reserved and on-demand

   Buying Reserved Capacity for the steady baseline and using on-demand for spikes balances cost and flexibility.

3. Final Answer:

   Buy Reserved Capacity for the average baseline and use on-demand for spikes. -> Option D

4. Quick Check:

   Baseline Reserved + on-demand spikes = Cost efficient [OK]

Hint: Reserve baseline, use on-demand for spikes to save money [OK]

Common Mistakes:

• Buying Reserved Capacity for peak traffic wastes money
• Relying only on on-demand increases cost
• Thinking Reserved Capacity auto-scales with spikes