Default vs custom metrics in AWS - Performance Comparison

Time Complexity: Default vs custom metrics

O(n)

Understanding Time Complexity

When working with AWS monitoring, it is important to understand how the time to collect and process metrics changes as you add more metrics.

We want to know how the number of metrics affects the work AWS does behind the scenes.

Scenario Under Consideration

Analyze the time complexity of the following operation sequence.

# Fetch default metrics for EC2 instances
aws cloudwatch get-metric-data --metric-data-queries '[{"Id":"m1","MetricStat":{"Metric":{"Namespace":"AWS/EC2","MetricName":"CPUUtilization","Dimensions":[{"Name":"InstanceId","Value":"i-1234567890abcdef0"}]},"Period":300,"Stat":"Average"},"ReturnData":true},{"Id":"m2","MetricStat":{"Metric":{"Namespace":"AWS/EC2","MetricName":"NetworkIn","Dimensions":[{"Name":"InstanceId","Value":"i-1234567890abcdef0"}]},"Period":300,"Stat":"Sum"},"ReturnData":true},{"Id":"m3","MetricStat":{"Metric":{"Namespace":"AWS/EC2","MetricName":"NetworkOut","Dimensions":[{"Name":"InstanceId","Value":"i-1234567890abcdef0"}]},"Period":300,"Stat":"Sum"},"ReturnData":true}]'

# Publish custom metrics for application
aws cloudwatch put-metric-data --namespace MyApp --metric-name RequestCount --value 1
aws cloudwatch put-metric-data --namespace MyApp --metric-name ErrorCount --value 0
    

This sequence fetches default metrics automatically provided by AWS and sends custom metrics created by the user.

Identify Repeating Operations

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

• Primary operation: Fetching or sending each metric data point via CloudWatch API calls.
• How many times: Once per metric per time interval (e.g., per minute).

How Execution Grows With Input

As the number of metrics increases, the number of API calls grows roughly in direct proportion.

Input Size (n metrics)	Approx. Api Calls/Operations
10	10 calls
100	100 calls
1000	1000 calls

Pattern observation: Doubling the number of metrics doubles the number of API calls needed.

Final Time Complexity

Time Complexity: O(n)

This means the work grows linearly with the number of metrics you handle.

Common Mistake

[X] Wrong: "Adding more custom metrics won't affect performance much because AWS handles it automatically."

[OK] Correct: Each metric requires separate API calls and processing, so more metrics mean more work and longer processing times.

Interview Connect

Understanding how monitoring scales with the number of metrics helps you design efficient cloud systems and shows you can think about cost and performance together.

Self-Check

"What if we batch multiple custom metrics into a single API call? How would the time complexity change?"

Practice

1. Which statement best describes AWS default metrics in CloudWatch?

easy

A. They are automatically collected and free within certain limits.

B. They require manual setup and extra charges.

C. They only track custom application data.

D. They are disabled by default and need activation.

Solution

1. Step 1: Understand default metrics collection

   AWS CloudWatch automatically collects default metrics for many services without extra setup.

2. Step 2: Recognize cost and availability

   These default metrics are free within certain usage limits, making them easy to use.

3. Final Answer:

   They are automatically collected and free within certain limits. -> Option A

4. Quick Check:

   Default metrics = automatic + free [OK]

Hint: Default metrics are automatic and free within limits [OK]

Common Mistakes:

• Thinking default metrics need manual setup
• Assuming default metrics track custom data
• Believing default metrics are disabled by default

2. Which of the following is the correct way to send a custom metric to AWS CloudWatch using the AWS CLI?

easy

A. aws cloudwatch send-metric --name CPUUtilization --value 70

B. aws cloudwatch add-metric-data --metric-name CPUUtilization --value 70

C. aws cloudwatch create-metric --metric CPUUtilization --value 70

D. aws cloudwatch put-metric-data --metric-name CPUUtilization --namespace AWS/EC2 --value 70

Solution

1. Step 1: Identify the correct AWS CLI command

   The correct command to send custom metric data is put-metric-data under aws cloudwatch.

2. Step 2: Check the syntax and parameters

   The command requires --metric-name, --namespace, and --value parameters as shown in aws cloudwatch put-metric-data --metric-name CPUUtilization --namespace AWS/EC2 --value 70.

3. Final Answer:

   aws cloudwatch put-metric-data --metric-name CPUUtilization --namespace AWS/EC2 --value 70 -> Option D

4. Quick Check:

   Use put-metric-data with metric-name and namespace [OK]

Hint: Use 'put-metric-data' with metric-name and namespace [OK]

Common Mistakes:

• Using incorrect command names like send-metric or create-metric
• Omitting the namespace parameter
• Confusing metric creation with data sending

3. Given the following AWS CloudWatch custom metric data sent every minute:

aws cloudwatch put-metric-data --metric-name PageViews --namespace MyApp --value 5

What will be the average value shown in CloudWatch after 3 minutes if the command is run 3 times with values 5, 7, and 8 respectively?

medium

A. 6.67

B. 7

C. 20

D. 5

Solution

1. Step 1: Sum the metric values sent

   The values sent are 5, 7, and 8. Their sum is 5 + 7 + 8 = 20.

2. Step 2: Calculate the average over 3 data points

   Average = Total sum / Number of data points = 20 / 3 ≈ 6.67.

3. Final Answer:

   6.67 -> Option A

4. Quick Check:

   Average = sum/3 = 6.67 [OK]

Hint: Average = sum of values divided by count [OK]

Common Mistakes:

• Adding values but not dividing by count
• Taking the last value as average
• Multiplying values instead of averaging

4. You tried to send a custom metric to CloudWatch but it does not appear in the console. Which of the following is the most likely cause?

medium

A. CloudWatch only shows metrics after 24 hours delay.

B. You forgot to specify the metric namespace in the command.

C. Custom metrics are automatically visible without sending data.

D. Default metrics do not require namespaces, so this is normal.

Solution

1. Step 1: Check required parameters for custom metrics

   Custom metrics require a namespace to organize and display them properly in CloudWatch.

2. Step 2: Understand impact of missing namespace

   If the namespace is missing, CloudWatch cannot categorize the metric, so it won't appear in the console.

3. Final Answer:

   You forgot to specify the metric namespace in the command. -> Option B

4. Quick Check:

   Missing namespace hides custom metric [OK]

Hint: Always specify namespace when sending custom metrics [OK]

Common Mistakes:

• Assuming default metrics need namespaces
• Expecting immediate visibility without data sent
• Believing CloudWatch delays metric display by 24 hours

5. You want to monitor a custom application metric that tracks the number of user logins per hour. The default AWS metrics do not provide this data. What is the best approach to achieve this?

hard

A. Enable detailed monitoring on EC2 instances to get user login data.

B. Use the default CPUUtilization metric to estimate user logins.

C. Send custom metric data for user logins to CloudWatch using a unique namespace and metric name.

D. Create an alarm on default network metrics to infer user logins.

Solution

1. Step 1: Identify the need for custom data

   User logins per hour is specific to the application and not tracked by default AWS metrics.

2. Step 2: Use custom metrics for application-specific data

   Sending custom metric data with a unique namespace and metric name allows tracking exactly this information in CloudWatch.

3. Final Answer:

   Send custom metric data for user logins to CloudWatch using a unique namespace and metric name. -> Option C

4. Quick Check:

   Custom metrics track app-specific data [OK]

Hint: Use custom metrics for app-specific tracking [OK]

Common Mistakes:

• Trying to use unrelated default metrics for app data
• Assuming detailed monitoring tracks application events
• Using alarms on unrelated metrics to guess user activity