AWS Compute and Elasticity: Match Runtime, Scaling Unit, and Failure Model

Compute questions become easier when you separate runtime choice, scaling unit, traffic entry, and purchasing model.

Choose the runtime boundary

Lambda has execution, package, concurrency, and networking constraints. Containers fit longer-running or portable processes. EC2 is appropriate when control matters enough to justify patching and capacity management.

Scale stateless compute across AZs

An Auto Scaling Group replaces unhealthy instances and adjusts desired capacity. Target tracking is the default for a stable metric; step scaling reacts by alarm severity; scheduled scaling handles known demand. Predictive scaling helps cyclical workloads. Keep session state outside instances when possible.

ALB routes HTTP/HTTPS by host, path, headers, or other Layer 7 rules. NLB handles very high-performance TCP/UDP/TLS and static IP requirements. GWLB inserts virtual network appliances. Health checks determine whether traffic should reach a target; they do not repair application data.

Price the workload shape

• On-Demand: unknown or short-lived demand.
• Savings Plans/Reserved commitments: steady baseline.
• Spot: interruption-tolerant, checkpointable work.
• Dedicated Host: host-level licensing or compliance.
• Capacity Reservation: reserve capacity, not a discount by itself.

Exam lens: high availability means at least two failure domains plus health-based routing and replacement. A larger single instance is only vertical scale.

Review card

• Runtime choice is a control-versus-operations decision.
• ASG scales instances; ECS service scaling scales tasks.
• ALB is Layer 7; NLB is Layer 4; GWLB inserts appliances.
• Externalize state before horizontal scaling.
• Match purchase model to predictability and interruption tolerance.