Auto scaling based on SQS size
- CloudWatch Metrics + CloudWatch Alarm is required to keep track of the size of SQS

Section5: EC2 Fundamentals

User Data
EC2 On-demand for development and Reserved Instance for Production (running 24*7)
Reserved Instances
- for cost-saving purpose
- Standard RI
- Convertible RI (can change OS, ..)
- e.g. for high availability and cost saving, reserve two instances in two AZs
EC2 Spot Instances
- cost-effective
  - spot instances are cheaper than reserved instances
- not reliable to use Spot Instance for Dev environment
- used for the workflow can withstand disruptions and can be started and stopped multiple times. (not highly available)
  - e.g. it is possible to run this kind of workflow using lambdas, but more costly
- spot instance concepts:
  - spot fleet
    - a fleet of spot instances + on-demand instances(optional)
    - maintain a target capacity automatically. If a spot instance is terminated, starts a new one
  - spot instance interruption
  - spot instance request
  - spot instance pool
- spot blocks
  - not interrupted for the duration you select
  - in rare situations, it can be interrupted
Dedicated Host vs Dedicated Instance
- they both run an application on single-tenant hardware(single physical server, and you have control over how your application is placed on the physical server. Hardware isolation)
- Bring your own license(BYOL) is supported on Dedicated Host
- BYOL is not supported on Dedicated Instance
- Dedicated Instance is cheaper
- you can change between dedicated instance ↔ dedicated host, after launch
  - you cannot go back to default, or vice versa
how to copy an ec2 instance in region A to region B
- copy AMI of ec2 instance in region A to region B, and create an instance in region B using the copied AMI
  - at this point, there are a snapshot, AMI, and an instance in region B(snapshot is copied as well because AMI is based on a snapshot)
Security Groups
- cannot restrict requests based on geo location
- stateful
- for an instance
- allows all outbound traffic by default
- you can set outbound rule to only allow traffic to another security group port 1433(Microsoft SQL server, only listening to port 1433)
- you can set inbound rule to only allow traffic from another security group port 1433
Network Access Control List(NACL)
- stateless
- for a subnet
- by default, allows all inbound and outbound traffic
EC2 Hibernate
- saves the content of RAM to EBS
- can resume where you left off
- can reboot the instance quickly (help speed up the instance start time)
IAM policy
```
"Condition": {
        "StringEquals": {
          "aws:RequestedRegion": "eu-west-1"
        }
```
Only allowed to launch instances in eu-west-1, but API calls can be anywhere
- aws:RequestedRegion represents the target of the API call

Section5: EC2 Fundamentals

User Data