Bird
Raised Fist0
AWScloud~5 mins

Launching an EC2 instance in AWS - Time & Space Complexity

Choose your learning style10 modes available
LearnWhyDeepVisualTryChallengeProjectRecallTime

Start learning this pattern below

Jump into concepts and practice - no test required

or
Recommended
Test this pattern10 questions across easy, medium, and hard to know if this pattern is strong
Time Complexity: Launching an EC2 instance
O(n)
Understanding Time Complexity

When launching an EC2 instance, it is important to understand how the time to complete the launch changes as you launch more instances.

We want to know how the number of instances affects the total time and operations needed.

Scenario Under Consideration

Analyze the time complexity of the following operation sequence.


import boto3
ec2 = boto3.client('ec2')

for i in range(n):
    ec2.run_instances(
        ImageId='ami-12345678',
        MinCount=1,
        MaxCount=1,
        InstanceType='t2.micro'
    )

This code launches n EC2 instances one by one using the AWS SDK.

Identify Repeating Operations

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

  • Primary operation: The run_instances API call to launch one EC2 instance.
  • How many times: This call is made once for each instance, so n times.
How Execution Grows With Input

Each instance requires one API call to launch. So if you launch more instances, the total calls increase directly with the number of instances.

Input Size (n)Approx. Api Calls/Operations
1010
100100
10001000

Pattern observation: The number of API calls grows linearly as you increase the number of instances.

Final Time Complexity

Time Complexity: O(n)

This means the time and operations needed grow directly in proportion to the number of instances you launch.

Common Mistake

[X] Wrong: "Launching multiple instances at once will take the same time as launching one instance."

[OK] Correct: Each instance requires its own API call and provisioning, so total time grows with the number of instances.

Interview Connect

Understanding how launching resources scales helps you design efficient cloud operations and answer questions about system behavior as load grows.

Self-Check

"What if we used a single API call to launch multiple instances at once? How would the time complexity change?"

Practice

(1/5)
1. What does launching an EC2 instance in AWS create?
easy
A. A storage bucket
B. A virtual server in the cloud
C. A database instance
D. A physical server in your office

Solution

  1. Step 1: Understand EC2 purpose

    EC2 stands for Elastic Compute Cloud, which provides virtual servers.

  2. Step 2: Identify what launching means

    Launching an EC2 instance means creating a virtual server in AWS cloud.

  3. Final Answer:

    A virtual server in the cloud -> Option B

  4. Quick Check:

    EC2 instance = virtual server [OK]
Hint: EC2 = virtual server, not physical hardware [OK]
Common Mistakes:
  • Confusing EC2 with physical hardware
  • Thinking EC2 creates storage or database
  • Mixing EC2 with S3 storage
2. Which AWS CLI command is used to launch a new EC2 instance?
easy
A. aws ec2 run-instances
B. aws ec2 start-instance
C. aws ec2 create-instance
D. aws ec2 deploy-instance

Solution

  1. Step 1: Recall AWS CLI commands for EC2

    The correct command to launch EC2 instances is 'run-instances'.

  2. Step 2: Eliminate incorrect commands

    Commands like 'create-instance', 'start-instance', and 'deploy-instance' do not exist or are incorrect.

  3. Final Answer:

    aws ec2 run-instances -> Option A

  4. Quick Check:

    Launch EC2 = run-instances command [OK]
Hint: Remember 'run-instances' to start EC2 instances [OK]
Common Mistakes:
  • Using 'create-instance' which is invalid
  • Confusing 'start-instance' with launching
  • Assuming 'deploy-instance' is a valid command
3. What will happen if you run this command?
aws ec2 run-instances --image-id ami-12345678 --count 1 --instance-type t2.micro --key-name MyKeyPair --security-groups MySecurityGroup
medium
A. Deletes the specified AMI image
B. Creates a new security group named MySecurityGroup
C. Launches one t2.micro EC2 instance with specified settings
D. Starts an existing stopped EC2 instance

Solution

  1. Step 1: Analyze the command parameters

    The command uses 'run-instances' with image ID, count=1, instance type, key pair, and security group.

  2. Step 2: Understand the command effect

    This command launches one new EC2 instance of type t2.micro with the given AMI and security settings.

  3. Final Answer:

    Launches one t2.micro EC2 instance with specified settings -> Option C

  4. Quick Check:

    run-instances with parameters = launch instance [OK]
Hint: run-instances with --count 1 launches one instance [OK]
Common Mistakes:
  • Thinking it deletes or modifies AMI
  • Confusing security group creation with usage
  • Assuming it starts an existing instance
4. You run this command but get an error: InvalidKeyPair.NotFound. What is the likely cause?
aws ec2 run-instances --image-id ami-87654321 --count 1 --instance-type t3.small --key-name WrongKey --security-groups Default
medium
A. The key pair named 'WrongKey' does not exist in your AWS account
B. The AMI ID is invalid
C. The instance type t3.small is not supported
D. The security group 'Default' is missing

Solution

  1. Step 1: Understand the error message

    'InvalidKeyPair.NotFound' means the specified key pair is not found in your AWS account.

  2. Step 2: Match error to command parameters

    The key-name 'WrongKey' likely does not exist or is misspelled, causing the error.

  3. Final Answer:

    The key pair named 'WrongKey' does not exist in your AWS account -> Option A

  4. Quick Check:

    InvalidKeyPair error = missing key pair [OK]
Hint: Check key pair name spelling if InvalidKeyPair error occurs [OK]
Common Mistakes:
  • Assuming AMI or instance type caused the error
  • Thinking security group 'Default' is missing
  • Ignoring key pair existence in the region
5. You want to launch 3 EC2 instances with the same settings but different subnet IDs for high availability. Which approach is best?
hard
A. Use aws ec2 create-subnet to create three subnets and launch one instance
B. Run one aws ec2 run-instances command with --count 3 and a single --subnet-id
C. Launch one instance and then clone it twice manually
D. Run three separate aws ec2 run-instances commands, each with a different --subnet-id

Solution

  1. Step 1: Understand subnet and instance launch

    Each EC2 instance can be launched in only one subnet at a time.

  2. Step 2: Plan for multiple subnets

    To launch instances in different subnets, run separate commands specifying each subnet ID.

  3. Step 3: Evaluate options

    Run three separate aws ec2 run-instances commands, each with a different --subnet-id. This runs three commands with different subnet IDs, ensuring instances spread across subnets.

  4. Final Answer:

    Run three separate aws ec2 run-instances commands, each with a different --subnet-id -> Option D

  5. Quick Check:

    One subnet per instance, multiple commands for multiple subnets [OK]
Hint: One subnet per instance, use multiple commands for multiple subnets [OK]
Common Mistakes:
  • Trying to launch multiple subnets in one command
  • Assuming cloning instances copies subnet settings
  • Confusing subnet creation with instance launch