ECS service auto scaling in AWS - Step-by-Step Execution

Process Flow - ECS service auto scaling

Start: ECS Service Running

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Monitor Metrics (CPU, Memory)

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Check if Metric > Scale Out Threshold?

No→Check if Metric < Scale In Threshold?

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Increase Desired Task Count

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Update ECS Service Desired Count

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Wait for Stabilization

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Repeat Monitoring

The ECS service auto scaling monitors resource metrics and adjusts the number of running tasks by increasing or decreasing desired count based on thresholds.

Execution Sample

AWS

1. Monitor CPU usage
2. If CPU > 70%, increase tasks by 1
3. If CPU < 30%, decrease tasks by 1
4. Update ECS service desired count
5. Wait and repeat

This process automatically adjusts ECS tasks based on CPU usage thresholds.

Process Table

Step	CPU Usage (%)	Condition	Action	Desired Task Count	Notes
1	50	50 > 70? No; 50 < 30? No	No scaling	2	Initial state, no change
2	75	75 > 70? Yes	Scale out +1	3	Increased tasks due to high CPU
3	80	80 > 70? Yes	Scale out +1	4	Further increase tasks
4	65	65 > 70? No; 65 < 30? No	No scaling	4	CPU normal, no change
5	25	25 < 30? Yes	Scale in -1	3	Decreased tasks due to low CPU
6	20	20 < 30? Yes	Scale in -1	2	Further decrease tasks
7	35	35 > 70? No; 35 < 30? No	No scaling	2	CPU normal, no change
8	50	50 > 70? No; 50 < 30? No	No scaling	2	Stable state
9	Exit	Reached stable desired count	Stop scaling	2	Scaling stabilized

💡 Scaling stops when CPU usage is stable and desired task count matches load.

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6	After Step 7	After Step 8	Final
CPU Usage (%)	50	50	75	80	65	25	20	35	50	50
Desired Task Count	2	2	3	4	4	3	2	2	2	2
Scaling Action	None	None	Scale Out +1	Scale Out +1	None	Scale In -1	Scale In -1	None	None	None

Key Moments - 3 Insights

Why does the desired task count not change when CPU usage is 50%?

Because 50% CPU is between the scale out threshold (70%) and scale in threshold (30%), so no scaling action is taken as shown in step 1 of the execution table.

What happens when CPU usage is above 70% multiple times in a row?

The desired task count increases by 1 each time CPU is above 70%, scaling out the service, as seen in steps 2 and 3.

Why does scaling stop at the end?

Scaling stops when CPU usage stabilizes within thresholds and desired task count matches the load, indicated by 'Exit' in step 9.

Visual Quiz - 3 Questions

Look at the execution table, what is the desired task count after step 3?

A 2

B 4

C 3

D 5

💡 Hint

Check the 'Desired Task Count' column at step 3 in the execution table.

At which step does the CPU usage first trigger a scale in action?

A Step 6

B Step 4

C Step 5

D Step 7

💡 Hint

Look for the first 'Scale in -1' action in the 'Scaling Action' column.

If the scale out threshold was lowered to 60%, how would the action at step 4 change?

A Scale out +1

B Scale in -1

C No change, still no scaling

D Stop scaling

💡 Hint

At step 4 CPU is 65%, which is above 60%, so scaling out would occur.

Concept Snapshot

ECS Service Auto Scaling:
- Monitors resource metrics (CPU, Memory)
- If metric > scale out threshold, increase desired tasks
- If metric < scale in threshold, decrease desired tasks
- Update ECS service desired count accordingly
- Wait for stabilization before next check

Full Transcript

ECS service auto scaling works by monitoring resource usage like CPU. When CPU goes above a set high threshold, it increases the number of running tasks to handle more load. When CPU drops below a low threshold, it decreases tasks to save resources. This process repeats continuously to keep the service responsive and efficient. The execution table shows CPU values and scaling actions step by step, with the desired task count adjusting up or down. Scaling stops when CPU stabilizes within thresholds and the task count matches the load.