# Elastic Compute Cloud

1. [TL;DR](#tldr)
1. [Burstable instances](#burstable-instances)
1. [Disks](#disks)
1. [Further readings](#further-readings)
   1. [Sources](#sources)

## TL;DR

Use an instance profile to pass an IAM role to an EC2 instance.

`T` instances launch as `unlimited` by default. Launch them in `standard` mode to avoid paying for surplus credits.

The instance type [_can_ be changed][change the instance type]. The procedure depends on the root volume, but does
require downtime.

Clone EC2 instances by:

1. Creating an AMI from the original instance.
   Mind the default behaviour of the AMI creator is to **shutdown** the instance, take a snapshot, and boot it again
   [to guarantee the image's filesystem integrity][create an ami from an amazon ec2 instance].
1. Using that AMI to launch clones identical to the original.

<details>
  <summary>Real world use cases</summary>

```sh
# Get the IDs of running nginx instances in 'dev'.
aws ec2 describe-instances --output 'text' \
  --query 'Reservations[].Instances[].InstanceId[]'
  --filters \
    'Name=instance-state-name,Values=running' \
    'Name=tag:env,Values=dev' \
    'Name=tag:app,Values=nginx' \

# Start SSM sessions to specific machines.
aws ec2 describe-instances --output text \
  --query 'Reservations[].Instances[].InstanceId' \
  --filters \
    'Name=app,Values=mysql' \
    'Name=instance-state-name,Values=running' \
| xargs -ot aws ssm start-session --target

# Show images details.
aws ec2 describe-images --image-ids 'ami-8b8c57f8'
aws ec2 describe-images --filters \
  'Name=name,Values=["al2023-ami-minimal-*"]' \
  'Name=owner-alias,Values=["amazon"]' \
  'Name=architecture,Values=["arm64","x86_64"]' \
  'Name=block-device-mapping.volume-type,Values=["gp3"]'

# Describe security groups.
aws ec2 describe-security-groups --group-names 'pulumi-workshop'

# Delete security groups.
aws ec2 delete-security-group --group-name 'pulumi-workshop'
aws ec2 delete-security-group --group-id 'sg-0773aa724d0c2dd51'

# Query the onboard IMDSv1 metadata server.
curl 'http://instance-data/latest/meta-data/instance-id'
curl 'http://169.254.169.254/latest/meta-data/instance-type'
curl 'http://[fd00:ec2::254]/latest/meta-data/local-ipv4'
```

</details>

## Burstable instances

`T` instances are burstable.

Refer [Burstable performance instances] and [Key concepts and definitions for burstable performance instances].

Traditional EC2 instance types provide fixed CPU resources.<br/>
Burstable performance instances provide a baseline level of CPU utilization, with the ability to burst CPU utilization
above the baseline level.

One only pays for the baseline CPU, plus any additional burst CPU usage over a 24-hour period.

The baseline utilization and ability to burst are governed by **CPU credits**.<br/>
Burstable performance instances continuously earn credits when they stays **below** the CPU baseline, and continuously
spend credits when they bursts above the baseline.<br/>
**Accrued credits** can be used later to burst above baseline CPU utilization.<br/>
Credits can be accrued only up to a point. How high this limit is depends on the instance type.<br/>
When the credits spent are more than credits earned, the instance behavior depends on the credit configuration mode
(_Standard_ or _Unlimited_).

In Standard mode, burstable instances:

- Use the accrued credits to burst above baseline CPU utilization when they are available.
- **Gradually** come down to baseline CPU utilization if there are no accrued credits remaining.
- **Cannot** burst above baseline until they accrue more credits.

In Unlimited mode, burstable instances:

- Use the accrued credits to burst above baseline CPU utilization when they are available.
- Spend surplus credits to continue bursting above baseline if there are no accrued credits remaining.
- Use CPU credits they earn to pay down the surplus credits they spent earlier when CPU utilization falls below the
  baseline again.

The ability to earn CPU credits to pay down surplus credits is what enables EC2 to average the CPU utilization of
instances over a 24-hour period.<br/>
If the average CPU usage over a 24-hour period **exceeds** the baseline, instances are billed for the additional usage
at a [flat additional rate per vCPU-hour](https://aws.amazon.com/ec2/pricing/on-demand/#T2.2FT3.2FT4g_Unlimited_Mode_Pricing).

## Disks

See [EBS].

## Further readings

- [Amazon Web Services]
- [AWS EC2 Instance pricing comparison]
- [EC2Instances.info on vantage.sh]
- AWS' [CLI]
- [SSM]
- [Connect to your instances without requiring a public IPv4 address using EC2 Instance Connect Endpoint]
- [Unlimited mode for burstable performance instances]
- [Standard mode for burstable performance instances]
- [Configuring EC2 Disk alert using Amazon CloudWatch]

### Sources

- [Using instance profiles]
- [DescribeImages] API
- [`describe-images`][describe-images] CLI subcommand
- [Best practices for handling EC2 Spot Instance interruptions]
- [IAM roles for Amazon EC2]
- [Retrieve instance metadata]
- [Burstable performance instances]
- [Change the instance type]
- [How to Clone instance EC2]
- [Create an AMI from an Amazon EC2 Instance]

<!--
  Reference
  ═╬═Time══
  -->

<!-- Knowledge base -->
[amazon web services]: README.md
[cli]: cli.md
[ebs]: ebs.md
[ssm]: ssm.md

<!-- Upstream -->
[best practices for handling ec2 spot instance interruptions]: https://aws.amazon.com/blogs/compute/best-practices-for-handling-ec2-spot-instance-interruptions/
[burstable performance instances]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-performance-instances.html
[change the instance type]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-resize.html
[connect to your instances without requiring a public ipv4 address using ec2 instance connect endpoint]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/connect-with-ec2-instance-connect-endpoint.html
[create an ami from an amazon ec2 instance]: https://docs.aws.amazon.com/toolkit-for-visual-studio/latest/user-guide//tkv-create-ami-from-instance.html
[describe-images]: https://docs.aws.amazon.com/cli/latest/reference/ec2/describe-images.html
[describeimages]: https://docs.aws.amazon.com/AWSEC2/latest/APIReference/API_DescribeImages.html
[how to clone instance ec2]: https://repost.aws/questions/QUOrWudF3vRL2Vqtrv0M9lfQ/how-to-clone-instance-ec2
[iam roles for amazon ec2]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html
[key concepts and definitions for burstable performance instances]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-credits-baseline-concepts.html
[retrieve instance metadata]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/instancedata-data-retrieval.html
[standard mode for burstable performance instances]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-performance-instances-standard-mode.html
[unlimited mode for burstable performance instances]: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/burstable-performance-instances-unlimited-mode.html
[using instance profiles]: https://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_use_switch-role-ec2_instance-profiles.html

<!-- Others -->
[aws ec2 instance pricing comparison]: https://ec2instances.github.io/
[ec2instances.info on vantage.sh]: https://instances.vantage.sh/
[configuring ec2 disk alert using amazon cloudwatch]: https://medium.com/@chandinims001/configuring-ec2-disk-alert-using-amazon-cloudwatch-793807e40d72
