EC2 Architecture & Resilience

Ref: https://learn.cantrill.io/courses/1820301/lectures/41301315

EC2 architecture

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🔧 An instance is a VM (OS + logical resources)
Characterized by its type → Properties:
- Family
- Generation
- Size
- Extra capabilities

🔧 Physical HW where instances run
- CPU, memory, storage, networking
Managed by AWS
Types:
- Shared Host: shared across different AWS customers
  - No ownership of HW
  - Billed for how long instances & their allocated resources run
  - Instances still isolated from each other → no interaction with instances from different customers, no awareness of what other stuff is hosted
  - Default
- Dedicated Host: dedicated to a single AWS account
  - No sharing with other customers
  - Billed for the entire host (NOT for the instances you're running in it)
A host is of a certain year, processor type, memory type, storage type, storage configuration
- Generally contains instances of same type but different sizes → two instances of different types will likely end up in a different host
- Instances of different types & generations can still share the same host
An instance stays on its host until:
- Host fails
- Host is taken into maintenance by AWS
- Instance is firstly stopped, and then started again
  - ‼️ Instances stay on their host on simple restarts → NOT THE SAME!

‼️ EC2 has AZ resiliency - very reliant on the AZ
- Everything runs in one AZ and is locked in that AZ: HW, NW, storage…
- If AZ fails → host fails → its instances fail
- ❗ Actually a great feature/advantage when designing HA systems:
  - 👍 AZ isolation → Failure blast radius contained within one AZ
  - 👍 Load balancing between AZs handy for HA
❗ Instances can NOT natively move between AZs
- Even if reallocated to a new host, that host will be in the same AZ
- Migration to another AZ = create a copy of the instance in a different AZ
EBS volumes or NWs from one AZ can NOT be used in an instance in another AZ!