Scheduled scaling in AWS - Commands & Configuration

Introduction

Scheduled scaling lets you automatically adjust the number of servers running your app at specific times. This helps save money by running fewer servers when traffic is low and adding more when traffic is high.

When you know your app gets more visitors every weekday morning and less at night.

When you want to reduce costs by lowering server count during weekends.

When you expect a big event and want to prepare by increasing servers beforehand.

When you want to automate scaling without waiting for traffic to trigger it.

When you want predictable server capacity changes based on time, not load.

Config File - scheduled-scaling.json

scheduled-scaling.json

{
  "AutoScalingGroupName": "my-app-asg",
  "ScheduledActionName": "scale-up-morning",
  "StartTime": "2024-07-01T08:00:00Z",
  "DesiredCapacity": 5,
  "MinSize": 2,
  "MaxSize": 10
}

This JSON file defines a scheduled scaling action for an AWS Auto Scaling group named my-app-asg. It sets the desired number of servers to 5 starting at 8 AM UTC on July 1, 2024. The minimum and maximum sizes ensure the group stays between 2 and 10 servers.

Commands

This command creates or updates a scheduled scaling action using the JSON file. It tells AWS when and how to change the number of servers.

Terminal

aws autoscaling put-scheduled-update-group-action --cli-input-json file://scheduled-scaling.json

Expected Output

No output (command runs silently)

→

--cli-input-json - Specifies the JSON file with the scheduled scaling details

This command shows all scheduled scaling actions for the Auto Scaling group to confirm the schedule was set.

Terminal

aws autoscaling describe-scheduled-actions --auto-scaling-group-name my-app-asg

Expected Output

{

    "ScheduledUpdateGroupActions": [

        {

            "AutoScalingGroupName": "my-app-asg",

            "ScheduledActionName": "scale-up-morning",

            "StartTime": "2024-07-01T08:00:00Z",

            "DesiredCapacity": 5,

            "MinSize": 2,

            "MaxSize": 10

        }

    ]

}

→

--auto-scaling-group-name - Specifies the Auto Scaling group to list scheduled actions for

Key Concept

If you remember nothing else from this pattern, remember: scheduled scaling lets you plan server count changes ahead of time based on clock, not traffic.

Common Mistakes

Setting the StartTime in local time instead of UTC.

AWS expects StartTime in UTC, so using local time causes the action to run at the wrong moment.

Always convert your local time to UTC before setting StartTime.

Not specifying MinSize and MaxSize in the scheduled action.

Without these, the Auto Scaling group might scale outside desired limits, causing unexpected costs or outages.

Always include MinSize and MaxSize to keep scaling within safe boundaries.

Forgetting to verify the scheduled action after creation.

If the schedule is incorrect or missing, your app won't scale as planned.

Run describe-scheduled-actions to confirm your schedule is set correctly.

Summary

Create a JSON file defining when and how to scale your Auto Scaling group.

Use the AWS CLI command to apply the scheduled scaling action from the JSON file.

Verify the scheduled action exists and is correct with a describe command.

Practice

1. What is the main purpose of scheduled scaling in AWS?

easy

A. To automatically adjust resources at specific times

B. To manually increase server capacity when needed

C. To monitor security threats in real-time

D. To backup data at scheduled intervals

Solution

1. Step 1: Understand scheduled scaling concept

   Scheduled scaling is designed to change resource capacity automatically based on a set schedule.

2. Step 2: Compare options with the concept

   Only To automatically adjust resources at specific times describes automatic adjustment at specific times, matching scheduled scaling.

3. Final Answer:

   To automatically adjust resources at specific times -> Option A

4. Quick Check:

   Scheduled scaling = automatic timed resource adjustment [OK]

Hint: Scheduled scaling means automatic resource changes on a timer [OK]

Common Mistakes:

• Confusing scheduled scaling with manual scaling
• Thinking it handles security or backups
• Assuming it reacts only to traffic, not time

2. Which of the following is a valid cron expression for scheduled scaling in AWS?

easy

A. cron(0 0 0 0 ? *)

B. cron(60 24 * * ? *)

C. cron(* * * * *)

D. cron(0 12 * * ? *)

Solution

1. Step 1: Review cron expression format in AWS

   AWS cron expressions have six fields: minutes, hours, day-of-month, month, day-of-week, year.

2. Step 2: Validate each option

   cron(0 12 * * ? *) is valid: minute=0, hour=12, day-of-month=*, month=*, day-of-week=?, year=*. cron(0 0 0 0 ? *) has invalid zero day-of-month and month values. cron(60 24 * * ? *) has invalid 60 minutes and 24 hours. cron(* * * * *) has only five fields, missing year.

3. Final Answer:

   cron(0 12 * * ? *) -> Option D

4. Quick Check:

   Valid AWS cron = 6 fields, correct ranges [OK]

Hint: AWS cron needs 6 fields with valid ranges [OK]

Common Mistakes:

• Using 5-field cron instead of 6-field
• Setting minutes or hours out of range
• Confusing day-of-week and day-of-month

3. Given this scheduled scaling configuration snippet:

{"ScheduledActionName": "ScaleUpMorning", "StartTime": "2024-07-01T08:00:00Z", "DesiredCapacity": 5}

What will happen at 8 AM UTC on July 1, 2024?

medium

A. The resource capacity will be set to 5

B. The resource capacity will decrease to 0

C. The scaling action will be ignored due to missing EndTime

D. The resource capacity will remain unchanged

Solution

1. Step 1: Understand the scheduled action parameters

   The action named "ScaleUpMorning" is set to start at 8 AM UTC on July 1, 2024, with DesiredCapacity 5.

2. Step 2: Interpret the effect of StartTime and DesiredCapacity

   At the StartTime, the capacity will be set to 5. EndTime is optional and does not block the action.

3. Final Answer:

   The resource capacity will be set to 5 -> Option A

4. Quick Check:

   StartTime triggers capacity change to DesiredCapacity [OK]

Hint: StartTime sets capacity; EndTime is optional [OK]

Common Mistakes:

• Thinking EndTime is required to trigger scaling
• Assuming capacity decreases without explicit setting
• Ignoring the StartTime effect

4. You created a scheduled scaling action with this cron expression:
cron(0 25 * * ? *)
Why does the scaling not trigger as expected?

medium

A. The day-of-week field is missing

B. The minute value 0 is invalid

C. The hour value 25 is invalid in cron expressions

D. The cron expression needs only 5 fields

Solution

1. Step 1: Check the hour field in the cron expression

   The hour field is set to 25, but valid hour values are 0 to 23.

2. Step 2: Understand cron validation rules in AWS

   AWS rejects cron expressions with invalid field values, so the scaling action won't trigger.

3. Final Answer:

   The hour value 25 is invalid in cron expressions -> Option C

4. Quick Check:

   Hour must be 0-23 in AWS cron [OK]

Hint: Check cron field ranges; hours max 23 [OK]

Common Mistakes:

• Assuming minute 0 is invalid
• Thinking day-of-week is mandatory
• Using 5 fields instead of 6

5. You want to reduce your AWS Auto Scaling group capacity to 2 instances every night at 11 PM UTC and increase it to 10 instances every weekday morning at 7 AM UTC. Which setup correctly uses scheduled scaling to achieve this?

hard

A. Create one scheduled action with cron(0 23,7 * * ? *) setting DesiredCapacity=2 and DesiredCapacity=10

B. Create two scheduled actions: one with cron(0 23 * * ? *) setting DesiredCapacity=2, another with cron(0 7 ? * MON-FRI *) setting DesiredCapacity=10

C. Create two scheduled actions: one with rate(1 day) setting DesiredCapacity=2, another with rate(1 hour) setting DesiredCapacity=10

D. Create one scheduled action with cron(0 7 * * ? *) setting DesiredCapacity=10 only

Solution

1. Step 1: Understand the scheduling requirements

   Capacity should be 2 at 11 PM daily and 10 at 7 AM on weekdays only.

2. Step 2: Match cron expressions to requirements

   cron(0 23 * * ? *) runs daily at 11 PM; cron(0 7 ? * MON-FRI *) runs at 7 AM Monday to Friday. Two separate scheduled actions are needed with correct DesiredCapacity values.

3. Final Answer:

   Create two scheduled actions: one with cron(0 23 * * ? *) setting DesiredCapacity=2, another with cron(0 7 ? * MON-FRI *) setting DesiredCapacity=10 -> Option B

4. Quick Check:

   Separate cron for each time and capacity [OK]

Hint: Use separate scheduled actions with correct cron and capacity [OK]

Common Mistakes:

• Trying to set two capacities in one scheduled action
• Using rate expressions instead of cron for weekdays
• Missing weekday restriction for morning scale-up