Azure Reserved Instances - Time & Space Complexity

Time Complexity: Azure Reserved Instances

O(n)

Understanding Time Complexity

We want to understand how the time to manage Azure Reserved Instances changes as the number of instances grows.

Specifically, how does the effort to purchase or apply reserved instances scale with more resources?

Scenario Under Consideration

Analyze the time complexity of purchasing and applying reserved instances to virtual machines.

// Pseudocode for applying reserved instances
for each vm in virtualMachines {
  check if vm matches reservedInstance
  if match {
    apply reservedInstance discount
  }
}

This sequence checks each virtual machine against reserved instances and applies discounts where applicable.

Identify Repeating Operations

Look for repeated actions in the process.

• Primary operation: Checking each virtual machine against reserved instances.
• How many times: Once per virtual machine in the list.

How Execution Grows With Input

As the number of virtual machines increases, the number of checks grows proportionally.

Input Size (n)	Approx. API Calls/Operations
10	10 checks
100	100 checks
1000	1000 checks

Pattern observation: The number of operations grows directly with the number of virtual machines.

Final Time Complexity

Time Complexity: O(n)

This means the time to apply reserved instances grows in a straight line as you add more virtual machines.

Common Mistake

[X] Wrong: "Applying reserved instances happens instantly no matter how many virtual machines there are."

[OK] Correct: Each virtual machine must be checked and matched, so more machines mean more work and time.

Interview Connect

Understanding how operations scale with resource count helps you design efficient cloud cost management strategies.

Self-Check

"What if reserved instances were applied in bulk to groups of virtual machines instead of individually? How would the time complexity change?"

Practice

1. What is the main benefit of using Azure Reserved Instances?

easy

A. Automatically scale virtual machines up and down

B. Save money by pre-paying for virtual machines

C. Create virtual machines without any cost

D. Backup virtual machines to Azure Storage

Solution

1. Step 1: Understand the purpose of Azure Reserved Instances

   Azure Reserved Instances allow you to pre-pay for virtual machines to get a discount compared to pay-as-you-go pricing.

2. Step 2: Compare options with this purpose

   Only Save money by pre-paying for virtual machines mentions saving money by pre-paying, which matches the main benefit.

3. Final Answer:

   Save money by pre-paying for virtual machines -> Option B

4. Quick Check:

   Reserved Instances = Pre-pay savings [OK]

Hint: Reserved Instances = pre-pay to save money [OK]

Common Mistakes:

• Confusing Reserved Instances with auto-scaling
• Thinking Reserved Instances are free
• Mixing backup features with Reserved Instances

2. Which Azure CLI command is used to purchase a Reserved Instance?

easy

A. az vm create

B. az vm start

C. az storage account create

D. az reservations reservation-order create

Solution

1. Step 1: Identify the CLI command for Reserved Instances

   The Azure CLI command to purchase Reserved Instances is under the 'az reservations' group, specifically 'reservation-order create'.

2. Step 2: Eliminate unrelated commands

   Commands like 'az vm create' create virtual machines, not reservations. 'az vm start' starts VMs, and 'az storage account create' creates storage accounts.

3. Final Answer:

   az reservations reservation-order create -> Option D

4. Quick Check:

   Purchase Reserved Instance = az reservations reservation-order create [OK]

Hint: Reservations commands start with az reservations [OK]

Common Mistakes:

• Using VM creation commands to buy reservations
• Confusing storage commands with reservations
• Trying to start VMs to create reservations

3. Given this Azure CLI command output snippet after buying a Reserved Instance:

{
  "id": "/subscriptions/123/resourceGroups/myRG/providers/Microsoft.Capacity/reservationOrders/abc123",
  "properties": {
    "term": "P1Y",
    "appliedScopeType": "Shared",
    "skuName": "Standard_D2s_v3"
  }
}

What does the "term": "P1Y" mean?

medium

A. The reservation lasts for 1 year

B. The reservation is for 1 month

C. The reservation is for 1 day

D. The reservation is permanent

Solution

1. Step 1: Interpret the term format

   The term 'P1Y' follows ISO 8601 duration format, where 'P' means period and '1Y' means 1 year.

2. Step 2: Match the term to options

   Only The reservation lasts for 1 year correctly states the reservation lasts for 1 year.

3. Final Answer:

   The reservation lasts for 1 year -> Option A

4. Quick Check:

   Term P1Y = 1 year reservation [OK]

Hint: P1Y means period of 1 year in ISO duration [OK]

Common Mistakes:

• Thinking P1Y means 1 month or 1 day
• Assuming term means permanent
• Ignoring ISO 8601 duration format

4. You tried to purchase a Reserved Instance but received an error: "Invalid scope type". What is the likely cause?

medium

A. You specified an unsupported scope type for the reservation

B. You forgot to specify the VM size

C. You used the wrong Azure CLI command

D. Your subscription has no active VMs

Solution

1. Step 1: Understand the error message

   The error 'Invalid scope type' means the scope type parameter for the reservation is not valid or supported.

2. Step 2: Analyze other options

   Missing VM size or wrong CLI command would cause different errors. Having no active VMs does not cause this error.

3. Final Answer:

   You specified an unsupported scope type for the reservation -> Option A

4. Quick Check:

   Invalid scope type error = wrong scope parameter [OK]

Hint: Check scope type parameter when 'Invalid scope type' error appears [OK]

Common Mistakes:

• Assuming missing VM size causes this error
• Using wrong CLI command without checking scope
• Thinking no active VMs cause scope errors

5. You have a steady workload running 4 Standard_D2s_v3 VMs continuously. You want to save costs using Azure Reserved Instances. Which approach is best?

hard

A. Purchase 4 Reserved Instances for different VM sizes

B. Purchase 1 three-year Reserved Instance for Standard_D2s_v3 VM

C. Purchase 4 one-year Reserved Instances for Standard_D2s_v3 VMs

D. Use pay-as-you-go pricing and scale VMs manually

Solution

1. Step 1: Match workload to reservation quantity and term

   You have 4 steady VMs of the same size. Buying 4 Reserved Instances matching the VM size and quantity saves costs.

2. Step 2: Choose appropriate term length

   One-year term is common for steady workloads. Three-year is longer commitment but not required here. Different VM sizes do not match your workload.

3. Final Answer:

   Purchase 4 one-year Reserved Instances for Standard_D2s_v3 VMs -> Option C

4. Quick Check:

   Match VM count and size with Reserved Instances [OK]

Hint: Match Reserved Instances count and size to steady VMs [OK]

Common Mistakes:

• Buying fewer Reserved Instances than VMs
• Choosing wrong VM sizes for reservations
• Ignoring steady workload benefits