Instance metadata and user data in AWS - Step-by-Step Execution

Process Flow - Instance metadata and user data

Instance Launch

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Access Metadata URL

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Retrieve Metadata Info

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Access User Data URL

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Retrieve User Data Script

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Execute User Data Script

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Instance Configured

When an instance starts, it can get info about itself from metadata and run setup commands from user data.

Execution Sample

AWS

curl http://169.254.169.254/latest/meta-data/instance-id
curl http://169.254.169.254/latest/user-data

These commands get the instance ID and the user data script from the instance metadata service.

Process Table

Step	Action	URL Accessed	Response	Effect
1	Instance starts	-	-	Instance boots up
2	Request instance ID	http://169.254.169.254/latest/meta-data/instance-id	i-1234567890abcdef0	Instance learns its ID
3	Request user data	http://169.254.169.254/latest/user-data	#!/bin/bash echo Hello > /var/tmp/greeting.txt	Instance gets setup script
4	Execute user data script	-	Script runs	File /var/tmp/greeting.txt created with 'Hello'
5	Instance ready	-	-	Configured and running
6	Exit	-	-	No more metadata or user data requests

💡 User data script executed and instance fully configured

Status Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
instance_id	undefined	i-1234567890abcdef0	i-1234567890abcdef0	i-1234567890abcdef0	i-1234567890abcdef0
user_data_script	undefined	undefined	#!/bin/bash echo Hello > /var/tmp/greeting.txt	#!/bin/bash echo Hello > /var/tmp/greeting.txt	#!/bin/bash echo Hello > /var/tmp/greeting.txt
greeting_file	does not exist	does not exist	does not exist	exists with content 'Hello'	exists with content 'Hello'

Key Moments - 3 Insights

Why do we use the special IP 169.254.169.254 to get metadata?

This IP is a fixed link-local address that all instances can use to get their own metadata without needing internet access, as shown in step 2 of the execution table.

What happens if the user data script is empty or missing?

The instance will not run any setup commands from user data, so no changes happen after step 3; the instance just boots normally.

Can the instance metadata change after launch?

No, metadata like instance ID is fixed at launch and does not change during the instance lifetime, as tracked in the variable 'instance_id'.

Visual Quiz - 3 Questions

Look at the execution table, what is the instance ID returned at step 2?

A instance-001

B i-abcdef1234567890

C i-1234567890abcdef0

D No ID returned

💡 Hint

Check the 'Response' column in row for step 2 in the execution table.

At which step does the instance execute the user data script?

A Step 4

B Step 2

C Step 3

D Step 5

💡 Hint

Look for the 'Execute user data script' action in the execution table.

If the user data script was missing, what would happen to the 'greeting_file' variable?

A It would be created with 'Hello'

B It would not exist

C It would contain an error message

D It would be empty but exist

💡 Hint

Refer to the variable_tracker row for 'greeting_file' and the key moment about missing user data.

Concept Snapshot

Instance metadata is info the instance can get about itself via a special IP.
User data is a script the instance runs at launch to configure itself.
Access metadata at http://169.254.169.254/latest/meta-data/
Access user data at http://169.254.169.254/latest/user-data
User data runs only once at boot.
Metadata is read-only and fixed per instance.

Full Transcript

When an AWS instance starts, it can get information about itself called metadata by accessing a special IP address 169.254.169.254. This metadata includes things like the instance ID. The instance can also get user data, which is a script provided at launch time. The instance runs this script once to set itself up. For example, the instance can get its ID by requesting the metadata URL and get a setup script by requesting the user data URL. Then it runs the script to create files or install software. Metadata is fixed and does not change after launch. User data is optional and runs only once. This process helps automate instance configuration without manual steps.