Managed instance groups in GCP - Step-by-Step Execution

Process Flow - Managed instance groups

Create Instance Template

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Create Managed Instance Group

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Group uses Template to launch VMs

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Autoscaling & Autohealing Monitor

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Add or Remove Instances Automatically

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Update Group with New Template

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Rolling Update of Instances

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Group Stable

This flow shows how a managed instance group is created from a template, then automatically manages VM instances by scaling, healing, and updating them.

Execution Sample

GCP

gcloud compute instance-templates create web-template \
  --machine-type=e2-medium --image-family=debian-11 \
  --image-project=debian-cloud

gcloud compute instance-groups managed create web-group \
  --base-instance-name=web --size=2 \
  --template=web-template --zone=us-central1-a

gcloud compute instance-groups managed set-autoscaling web-group \
  --max-num-replicas=5 --min-num-replicas=1 --target-cpu-utilization=0.6 \
  --zone=us-central1-a

This code creates an instance template, then a managed instance group with 2 VMs, and sets autoscaling based on CPU usage.

Process Table

Step	Action	State Change	Result
1	Create instance template 'web-template'	Template created with Debian 11 image and e2-medium machine type	Template ready for use
2	Create managed instance group 'web-group' with size 2	2 VM instances launched using 'web-template'	Group has 2 running instances
3	Set autoscaling: min 1, max 5, target CPU 60%	Autoscaler monitors CPU usage	Group can scale between 1 and 5 instances
4	CPU usage rises above 60%	Autoscaler adds 1 instance	Group scales up to 3 instances
5	CPU usage drops below 60%	Autoscaler removes 1 instance	Group scales down to 2 instances
6	Update instance template with new software	New template version created	Ready for rolling update
7	Perform rolling update on 'web-group'	Instances replaced one by one with new template	All instances updated and stable
8	Instance fails health check	Autohealing deletes and recreates instance	Group maintains desired size and health
9	Manual resize to 4 instances	Group adds 2 instances	Group now has 4 running instances
10	Autoscaler disables	No automatic scaling	Group size fixed until autoscaler re-enabled
11	Exit	No further actions	Group stable with current instances

💡 Execution stops as group is stable and no further scaling or updates occur

Status Tracker

Variable	Start	After Step 2	After Step 4	After Step 5	After Step 7	After Step 9	Final
Instance Template	None	web-template created	web-template created	web-template created	Updated template created	Updated template created	Updated template created
Managed Instance Group Size	0	2	3	2	2 (rolling update ongoing)	4	4
Autoscaler Status	Disabled	Enabled	Enabled	Enabled	Enabled	Enabled	Disabled
Instance Health	N/A	Healthy	Healthy	Healthy	Healthy (rolling update)	Healthy	Healthy

Key Moments - 3 Insights

Why does the managed instance group add or remove instances automatically?

Because the autoscaler monitors CPU usage and adjusts the number of instances to keep CPU near the target, as shown in steps 3, 4, and 5 in the execution table.

What happens during a rolling update of the managed instance group?

Instances are replaced one by one with new versions from the updated template to avoid downtime, as shown in step 7.

How does autohealing keep the group healthy?

If an instance fails health checks, the group deletes and recreates it automatically, maintaining the desired size and health, as shown in step 8.

Visual Quiz - 3 Questions

Look at the execution table, at which step does the group scale up due to high CPU usage?

A Step 4

B Step 5

C Step 7

D Step 9

💡 Hint

Check the 'Action' and 'Result' columns for scaling events related to CPU usage.

According to the variable tracker, what is the group size after step 9?

A 2

B 4

C 3

D 5

💡 Hint

Look at the 'Managed Instance Group Size' row and the 'After Step 9' column.

If autoscaling is disabled at step 10, what happens to the group size afterward?

A It continues to scale automatically

B It immediately scales down to minimum size

C It remains fixed until autoscaler is re-enabled

D It deletes all instances

💡 Hint

Refer to step 10 in the execution table under 'Result' for autoscaler status effects.

Concept Snapshot

Managed instance groups use an instance template to launch identical VMs.
They automatically scale instances based on load and heal unhealthy ones.
Rolling updates replace instances gradually to avoid downtime.
Autoscaling can be configured with min/max sizes and CPU targets.
Autohealing ensures group health by recreating failed instances.

Full Transcript

Managed instance groups start by creating an instance template that defines VM settings. Then, a managed instance group is created using this template, launching a set number of VM instances. Autoscaling monitors CPU usage and adjusts the number of instances automatically to keep performance steady. When CPU usage rises, the group adds instances; when it falls, it removes them. Updates to the group are done by creating a new template version and performing a rolling update, replacing instances one at a time to avoid downtime. Autohealing monitors instance health and replaces any failing instances to keep the group stable. Manual resizing and disabling autoscaling are also possible, giving control over the group size. This process ensures a reliable, scalable, and maintainable VM environment.