Compute services and EC2 purchasing options

Foundational notes for AWS Certified Cloud Practitioner (CLF-C02). Read them in order the first time through, then use them as a revision sweep before you book.

12 study points. Everything here is exam-oriented: each point is a fact or a distinction that CLF-C02 items are built on. Test yourself against the practice exam once you can explain a section without re-reading it.

Compute

Elasticity is a core concept in cloud computing that refers to the ability to automatically scale computing resources up or down based on demand. In AWS, the primary product that implements elasticity is Auto Scaling. For example, imagine your e-commerce site normally handles 1,000 users but spikes to 50,000 during a flash sale. Auto Scaling detects the increased load and launches additional EC2 instances, then terminates them when traffic returns to normal, so you only pay for what you use.

To clone or back up an EBS (Elastic Block Store) volume, you create a snapshot. A snapshot is a point-in-time copy of the volume stored in Amazon S3. You can use that snapshot to create a new EBS volume in the same or a different Availability Zone, which is extremely useful for disaster recovery or migrating data. Snapshots are incremental, meaning only the blocks that have changed since the last snapshot are saved, which keeps storage costs low.

Auto Scaling ensures you always have the optimal number of EC2 instances running to handle your application’s load. You define a minimum, maximum, and desired number of instances, and Auto Scaling maintains that balance. If an instance becomes unhealthy, Auto Scaling terminates it and launches a replacement automatically. This removes the need for manual monitoring and intervention, letting your operations team focus on higher-value work.

Amazon EC2 (Elastic Compute Cloud) is the foundational compute service in AWS. It lets you rent virtual servers in the cloud with your choice of operating system, CPU, memory, and storage. You can launch an instance in minutes and terminate it when you’re done, paying only for the seconds it runs. EC2 is ideal for applications where you need full control over the server environment, such as running a custom web application, a database, or a machine learning training job.

EC2 instance types are designed for different use cases and are grouped into families. General Purpose (T and M families) offer a balance of compute, memory, and networking. Compute Optimized (C family) is best for CPU-intensive tasks like batch processing. Memory Optimized (R and X families) is suited for in-memory databases. Storage Optimized (I and D families) is designed for high sequential read and write access to large datasets. Choosing the right instance type can dramatically affect both performance and cost.

EC2 offers several purchasing options to help you reduce costs. On-Demand instances are the most flexible but most expensive, billed by the second with no commitment. Reserved Instances offer up to 72% savings for a one-year or three-year commitment. Spot Instances let you bid on unused capacity at discounts of up to 90%, but AWS can reclaim them with two minutes’ notice. Savings Plans offer flexible pricing similar to Reserved Instances but apply across instance families and regions.

An Amazon Machine Image (AMI) is a template that contains the software configuration (operating system, application server, and applications) needed to launch an EC2 instance. You can use AWS-provided AMIs, community AMIs from the marketplace, or create your own custom AMIs. Custom AMIs are powerful because they let you pre-bake your application and dependencies into an image, so every new instance launches fully configured in seconds rather than requiring a lengthy setup process.

Amazon Elastic Container Service (ECS) and Amazon Elastic Kubernetes Service (EKS) are container orchestration services that let you run Docker containers on AWS without managing the underlying server infrastructure. ECS is AWS’s proprietary solution and is simpler to set up, while EKS runs standard Kubernetes and is preferred if you want portability across cloud providers. Both integrate deeply with other AWS services like IAM, CloudWatch, and Application Load Balancer.

AWS Fargate is a serverless compute engine for containers that works with both ECS and EKS. With Fargate, you don’t need to provision or manage any EC2 instances at all. You simply define your container’s CPU and memory requirements, and Fargate handles the rest. This is ideal for teams that want the benefits of containers without the operational overhead of managing a cluster. You pay only for the resources your containers actually consume.

Placement Groups let you influence how EC2 instances are physically placed on underlying hardware. A Cluster placement group packs instances close together within a single AZ for low-latency communication, ideal for high-performance computing. A Spread placement group distributes instances across distinct hardware to reduce the risk of simultaneous failures. A Partition placement group spreads instances across logical partitions so that groups of instances don’t share the same rack. Choosing the right strategy depends on whether you prioritize performance or resilience.

Elastic Load Balancing automatically distributes incoming application traffic across multiple targets, such as EC2 instances, containers, or IP addresses. It continuously monitors the health of registered targets and only routes traffic to healthy ones. This means if one of your servers becomes unresponsive, the load balancer stops sending traffic to it and redirects users to a healthy server. Combined with Auto Scaling, Elastic Load Balancing forms the backbone of any scalable, fault-tolerant architecture on AWS.

User Data is a feature that lets you pass a startup script to an EC2 instance at launch time. The script runs automatically as root when the instance first boots, allowing you to install packages, configure software, or pull application code from a repository. For example, you could write a User Data script that installs a web server, downloads your latest application build, and starts serving traffic, all without any manual SSH login required.

Where to go next

Back to the AWS Cloud Practitioner overview.
Look up any service you could not name in the AWS services glossary.
Sit the 80-item practice exam once two or three note pages are solid.