Minimum, maximum, and desired capacity in AWS - Step-by-Step Execution

Process Flow - Minimum, maximum, and desired capacity

Start Auto Scaling Group

↓

Check Desired Capacity

↓

Is Desired < Minimum?

Yes→Set Desired = Minimum

↓

Proceed

↓

Is Desired > Maximum?

Yes→Set Desired = Maximum

↓

Proceed

↓

Launch or Terminate Instances to Match Desired

↓

End

The system checks desired capacity against minimum and maximum limits, adjusts if needed, then launches or terminates instances to match desired capacity.

Execution Sample

AWS

MinCapacity=2
MaxCapacity=5
DesiredCapacity=6

if DesiredCapacity < MinCapacity:
    DesiredCapacity = MinCapacity
elif DesiredCapacity > MaxCapacity:
    DesiredCapacity = MaxCapacity

LaunchInstances(DesiredCapacity)

This code ensures desired capacity stays within min and max limits before launching instances.

Process Table

Step	Variable Checked	Condition	Action Taken	Resulting DesiredCapacity
1	DesiredCapacity=6	6 < 2?	No change	6
2	DesiredCapacity=6	6 > 5?	Set DesiredCapacity = 5	5
3	LaunchInstances	Launch 5 instances	Instances launched	5

💡 DesiredCapacity adjusted to max 5; 5 instances launched to match desired capacity.

Status Tracker

Variable	Start	After Step 1	After Step 2	Final
MinCapacity	2	2	2	2
MaxCapacity	5	5	5	5
DesiredCapacity	6	6	5	5

Key Moments - 2 Insights

Why does DesiredCapacity change from 6 to 5?

Because DesiredCapacity exceeded MaxCapacity (step 2 in execution_table), it was set down to MaxCapacity to respect the limit.

What happens if DesiredCapacity is less than MinCapacity?

DesiredCapacity is raised to MinCapacity to ensure minimum instances are running, similar to how it was capped at max in step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is DesiredCapacity after step 1?

A 2

B 5

C 6

D Undefined

💡 Hint

Check the 'Resulting DesiredCapacity' column in row for step 1.

At which step does DesiredCapacity get adjusted to respect limits?

A Step 1

B Step 2

C Step 3

D No adjustment

💡 Hint

Look for the step where 'Action Taken' changes DesiredCapacity.

If MinCapacity was 7 instead of 2, what would DesiredCapacity be after adjustments?

A 7

B 5

C 6

D Cannot determine

💡 Hint

Consider that DesiredCapacity must be at least MinCapacity, referencing variable_tracker logic.

Concept Snapshot

Minimum, maximum, and desired capacity control how many instances run.
Desired capacity is adjusted to be no less than minimum and no more than maximum.
AWS Auto Scaling launches or terminates instances to match the desired capacity.
This ensures resources are within set limits for cost and performance.

Full Transcript

In AWS Auto Scaling, you set three key numbers: minimum capacity, maximum capacity, and desired capacity. The system checks if the desired capacity is below the minimum or above the maximum. If it is, it adjusts the desired capacity to fit within those limits. Then it launches or terminates instances to match the adjusted desired capacity. For example, if desired capacity is 6 but maximum is 5, it lowers desired capacity to 5 and launches 5 instances. This keeps your resources balanced and controlled.

Practice

1. What does the MinSize setting control in an AWS Auto Scaling group?

easy

A. The exact number of servers to run at all times

B. The largest number of servers allowed to run

C. The smallest number of servers that must always be running

D. The number of servers to add during scaling

Solution

1. Step 1: Understand MinSize meaning

   MinSize defines the minimum number of servers that the Auto Scaling group keeps running to handle load.

2. Step 2: Compare with other settings

   MaxSize is the maximum allowed, DesiredCapacity is the target number, so MinSize is the smallest number always running.

3. Final Answer:

   The smallest number of servers that must always be running -> Option C

4. Quick Check:

   MinSize = smallest server count [OK]

Hint: MinSize = minimum servers always running [OK]

Common Mistakes:

• Confusing MinSize with DesiredCapacity
• Thinking MinSize is maximum limit
• Assuming MinSize is the exact number to run

2. Which of the following is the correct way to set the desired capacity to 5 in an AWS Auto Scaling group configuration snippet?

easy

A. DesiredCapacity => 5

B. "DesiredCapacity" = 5

C. DesiredCapacity: "5"

D. "DesiredCapacity": 5

Solution

1. Step 1: Identify correct JSON syntax

   In JSON, keys and string values use double quotes and key-value pairs use colon (:).

2. Step 2: Check each option

   "DesiredCapacity": 5 uses correct JSON syntax: key in quotes, colon, numeric value without quotes.

3. Final Answer:

   "DesiredCapacity": 5 -> Option D

4. Quick Check:

   JSON key-value uses colon and quotes [OK]

Hint: JSON uses colon and quotes for keys [OK]

Common Mistakes:

• Using equals sign instead of colon
• Putting numbers in quotes unnecessarily
• Using arrow symbols instead of colon

3. Given an Auto Scaling group with MinSize=2, MaxSize=6, and DesiredCapacity=4, what happens if the current number of running servers is 5 and one server fails?

medium

A. The group does nothing because the current number of running servers equals DesiredCapacity

B. The group launches a new server to maintain DesiredCapacity of 4

C. The group terminates another server to reach DesiredCapacity

D. The group launches two new servers to reach MaxSize

Solution

1. Step 1: Understand DesiredCapacity role

   DesiredCapacity is the target number of servers to keep running, here 4.

2. Step 2: Analyze failure impact

   One server fails, so running servers drop from 5 to 4, which matches DesiredCapacity.

3. Step 3: Check if scaling needed

   Since running servers dropped below DesiredCapacity, the group launches a new server to maintain DesiredCapacity.

4. Final Answer:

   The group launches a new server to maintain DesiredCapacity of 4 -> Option B

5. Quick Check:

   Running servers < DesiredCapacity triggers scaling [OK]

Hint: Scaling triggers only if running servers < DesiredCapacity [OK]

Common Mistakes:

• Assuming scaling happens immediately on failure
• Confusing MinSize with DesiredCapacity
• Thinking MaxSize limits minimum servers

4. You configured an Auto Scaling group with MinSize=3, MaxSize=5, and DesiredCapacity=6. What is the problem?

medium

A. DesiredCapacity cannot be greater than MaxSize

B. MinSize cannot be less than DesiredCapacity

C. MaxSize cannot be less than MinSize

D. No problem, this configuration is valid

Solution

1. Step 1: Check relationship between capacities

   DesiredCapacity must be between MinSize and MaxSize inclusive.

2. Step 2: Compare values

   DesiredCapacity is 6, but MaxSize is 5, which is less than DesiredCapacity.

3. Step 3: Identify error

   This violates the rule that DesiredCapacity cannot exceed MaxSize.

4. Final Answer:

   DesiredCapacity cannot be greater than MaxSize -> Option A

5. Quick Check:

   DesiredCapacity ≤ MaxSize [OK]

Hint: DesiredCapacity must be ≤ MaxSize [OK]

Common Mistakes:

• Setting DesiredCapacity higher than MaxSize
• Confusing MinSize and MaxSize roles
• Assuming any values are allowed

5. You want an Auto Scaling group to always run at least 2 servers, never more than 8, and normally keep 5 running. If traffic suddenly spikes, which setting controls the maximum servers it can scale up to, and what should it be set to?

hard

A. MaxSize controls maximum servers; set to 8

B. DesiredCapacity controls maximum servers; set to 8

C. MinSize controls maximum servers; set to 5

D. DesiredCapacity controls maximum servers; set to 2

Solution

1. Step 1: Identify which setting limits maximum servers

   MaxSize sets the upper limit of servers allowed in the group.

2. Step 2: Match requirement to setting

   To never exceed 8 servers, MaxSize must be set to 8.

3. Step 3: Understand DesiredCapacity and MinSize roles

   DesiredCapacity is the target number (5), MinSize is the minimum (2), neither limits maximum scaling.

4. Final Answer:

   MaxSize controls maximum servers; set to 8 -> Option A

5. Quick Check:

   MaxSize = max servers allowed [OK]

Hint: MaxSize limits max servers; DesiredCapacity is target [OK]

Common Mistakes:

• Confusing DesiredCapacity with MaxSize
• Setting MinSize as maximum limit
• Not setting MaxSize high enough for spikes