AWS Cost Explorer basics - Time & Space Complexity

Time Complexity: AWS Cost Explorer basics

O(1)

Understanding Time Complexity

We want to understand how the time to get cost data changes as we ask for more information in AWS Cost Explorer.

Specifically, how does the number of API calls grow when we request cost details for many resources or time periods?

Scenario Under Consideration

Analyze the time complexity of the following operation sequence.

# Using AWS CLI to get cost and usage data
aws ce get-cost-and-usage \
  --time-period Start=2024-01-01,End=2024-01-31 \
  --granularity DAILY \
  --metrics "BlendedCost" "UsageQuantity" \
  --group-by Type=DIMENSION,Key=SERVICE
    

This command fetches daily cost and usage data grouped by AWS service for one month.

Identify Repeating Operations

Identify the API calls, resource provisioning, data transfers that repeat.

• Primary operation: The single API call to get-cost-and-usage requesting grouped daily data.
• How many times: One call per request, but the data returned grows with the number of days and services.

How Execution Grows With Input

As you ask for more days or more services, the amount of data returned grows roughly by multiplying these factors.

Input Size (n)	Approx. Api Calls/Operations
10 days, 5 services	1 call, data for 50 items
30 days, 10 services	1 call, data for 300 items
90 days, 20 services	1 call, data for 1800 items

Pattern observation: The number of data points grows with days times services, but the API call count stays the same.

Final Time Complexity

Time Complexity: O(1)

This means the number of API calls stays the same no matter how much data you request, but the data size returned grows with input.

Common Mistake

[X] Wrong: "Requesting more days or services means more API calls will be made automatically."

[OK] Correct: Actually, one API call returns all requested data; the call count does not increase with data size.

Interview Connect

Understanding how API calls scale with data requests helps you design efficient cloud cost monitoring tools and shows you think about resource use carefully.

Self-Check

"What if we changed the grouping from SERVICE to TAG? How would the time complexity change?"

Practice

1. What is the main purpose of AWS Cost Explorer?

easy

A. To monitor network traffic

B. To create AWS virtual machines

C. To help you track and understand your AWS spending

D. To manage user permissions

Solution

1. Step 1: Understand AWS Cost Explorer's function

   AWS Cost Explorer is designed to show your cloud spending clearly and help manage costs.

2. Step 2: Compare options with this function

   Options A, B, and D describe other AWS services or features unrelated to cost tracking.

3. Final Answer:

   To help you track and understand your AWS spending -> Option C

4. Quick Check:

   Cost tracking = C [OK]

Hint: Cost Explorer is for spending, not resources or permissions [OK]

Common Mistakes:

• Confusing Cost Explorer with EC2 or IAM services
• Thinking it manages network or security settings

2. Which of the following is the correct way to start AWS Cost Explorer from the AWS Console?

easy

A. Go to Services > S3 > Cost Explorer

B. Go to Services > EC2 > Cost Explorer

C. Go to Services > IAM > Cost Explorer

D. Go to Services > Billing > Cost Explorer

Solution

1. Step 1: Identify where billing tools are located

   Cost Explorer is found under Billing in the AWS Console, not under EC2, IAM, or S3.

2. Step 2: Match the correct navigation path

   Only Go to Services > Billing > Cost Explorer correctly shows Services > Billing > Cost Explorer.

3. Final Answer:

   Go to Services > Billing > Cost Explorer -> Option D

4. Quick Check:

   Billing section = A [OK]

Hint: Cost Explorer is under Billing, not compute or storage [OK]

Common Mistakes:

• Looking for Cost Explorer under EC2 or S3
• Confusing IAM with billing tools

3. You use AWS Cost Explorer to view your monthly AWS costs grouped by service. If your report shows $100 for EC2 and $50 for S3, what does this mean?

medium

A. Your EC2 and S3 usage is free this month

B. You spent $100 on EC2 and $50 on S3 this month

C. You reserved EC2 instances worth $100 and S3 storage worth $50

D. You have $100 credit for EC2 and $50 credit for S3

Solution

1. Step 1: Understand grouping by service in Cost Explorer

   Grouping by service shows how much money you spent on each AWS service in the selected time.

2. Step 2: Interpret the amounts shown

   $100 for EC2 and $50 for S3 means those are your costs, not credits or reserved amounts.

3. Final Answer:

   You spent $100 on EC2 and $50 on S3 this month -> Option B

4. Quick Check:

   Costs shown = spending, not credits or free usage [OK]

Hint: Costs mean spending, not credits or reservations [OK]

Common Mistakes:

• Thinking amounts are credits or free usage
• Confusing cost with reserved capacity

4. You tried to filter your AWS Cost Explorer report by a tag, but no data appears. What is the most likely reason?

medium

A. The tag is not activated for cost allocation in AWS Billing settings

B. You have no AWS account

C. Cost Explorer does not support filtering by tags

D. You need to restart your AWS instance

Solution

1. Step 1: Understand tag filtering in Cost Explorer

   To filter by tags, the tags must be activated for cost allocation in Billing preferences.

2. Step 2: Identify why no data appears

   If the tag is not activated, Cost Explorer cannot use it to filter, so no data shows.

3. Final Answer:

   The tag is not activated for cost allocation in AWS Billing settings -> Option A

4. Quick Check:

   Tag activation needed for filtering = A [OK]

Hint: Activate tags in billing before filtering by them [OK]

Common Mistakes:

• Assuming Cost Explorer can't filter by tags
• Thinking AWS instance restart fixes this
• Believing no AWS account is the cause

5. You want to analyze your AWS costs for the last 3 months, grouped by linked accounts and filtered to only show costs for EC2 and S3. Which steps should you take in AWS Cost Explorer?

hard

A. Set the time range to last 3 months, group by linked accounts, and filter services to EC2 and S3

B. Set the time range to last 3 months, group by service, and filter accounts to EC2 and S3

C. Set the time range to last month, group by linked accounts, and filter services to all except EC2 and S3

D. Set the time range to last 3 months, group by tags, and filter services to EC2 only

Solution

1. Step 1: Choose correct time range and grouping

   Set the time range to last 3 months and group by linked accounts to see costs per account.

2. Step 2: Apply service filters correctly

   Filter services to only EC2 and S3 to focus on those costs.

3. Step 3: Verify options

   Set the time range to last 3 months, group by linked accounts, and filter services to EC2 and S3 matches all requirements. Other options have wrong grouping, filtering, or time range.

4. Final Answer:

   Set the time range to last 3 months, group by linked accounts, and filter services to EC2 and S3 -> Option A

5. Quick Check:

   Time + grouping + filter = B [OK]

Hint: Match time, grouping, and filters exactly for correct report [OK]

Common Mistakes:

• Mixing grouping by service vs linked accounts
• Filtering accounts instead of services
• Choosing wrong time range