Scaling & Reliability

Your traffic is spiky and unpredictable during business hours. Design an EC2 Auto Scaling policy that reacts fast without overspending overnight.

Target tracking should be the default scaling policy, since it only needs one metric and handles the math itself. Layer scheduled actions on top for the predictable part of the day, and treat step scaling as the exception for spikes that need a bigger, immediate jump than target tracking gives you.

When would you use EC2 Spot Instances in production, and how do you design a workload to survive interruptions?

Spot is unused EC2 capacity sold at a steep discount, usually 60 to 90 percent off On-Demand, with the trade that AWS can reclaim it with two minutes of warning. It's a good fit for anything stateless, retryable, or checkpointed, and a bad fit for anything that can't tolerate an interruption.

Multi-AZ deployments, read replicas, and Aurora all get mentioned as RDS availability features. What's actually different between them, and when do you use each?

Multi-AZ is a synchronous standby that exists purely for failover, not for serving traffic. A read replica is asynchronous and exists to serve read traffic, with failover as a secondary, slower use. Aurora replaces both mechanisms with a shared, distributed storage layer, which is why an Aurora Replica can do double duty that a standard RDS replica can't.

You have 50M embeddings, need p99 under 50ms, and 95% recall is acceptable. Which vector index do you choose?

Vector index choice is a three-way trade between recall, latency and memory. At tens of millions of vectors with a tight latency budget and tolerance for approximate results, HNSW with quantization is the standard answer: exact search cannot meet the latency, and unquantized HNSW cannot meet the memory budget affordably.

An ecommerce application sends order events to a processing service. During sales events the processing service is overwhelmed and drops orders. Orders for a given customer must be processed in the sequence they were submitted, and no order may be processed twice. Which solution meets these requirements?

Two requirements in the stem, ordering and no duplicates, point at exactly one service. An SQS FIFO queue guarantees first-in-first-out delivery within a message group and provides exactly-once processing through deduplication. A standard queue gives neither.

An Amazon RDS for MySQL database serves a reporting application. Read queries have grown until they slow down writes, and the business separately requires that the database survive the loss of a single Availability Zone with automatic failover. Which combination meets both requirements?

Multi-AZ and read replicas solve different problems and the exam tests whether you conflate them. A Multi-AZ standby is synchronous and cannot serve reads. A read replica is asynchronous and does serve reads. Needing both availability and read scaling means deploying both.

A web application runs on Amazon EC2 instances in an Auto Scaling group behind an Application Load Balancer. Users report being logged out at random. Investigation shows that session data is held in memory on each instance, so a request routed to a different instance loses the session. Which solution fixes this while keeping the web tier horizontally scalable?

Session state held on an instance makes that instance special, which is what breaks scaling and termination. Moving sessions to a shared store such as ElastiCache or DynamoDB makes every instance interchangeable, which is the property the rest of the architecture assumes.

You are asked to back up the cluster state of a cluster where etcd runs as a static Pod on the control plane node. Which approach produces a restorable backup?

Take an etcdctl snapshot save against the etcd endpoint, passing the CA certificate, client certificate and key that etcd is configured with. That one file is the cluster state.

A media company stores original video files in an Amazon S3 bucket in us-east-1. Regulators require that a copy of every new file exists in a second AWS Region within 15 minutes of upload, and that no file can be permanently deleted by mistake. Which TWO actions should a solutions architect take?

Cross-Region Replication copies new objects to a bucket in another Region automatically, and versioning is its prerequisite as well as the control that makes deletion recoverable. The pair answers both halves of the requirement with one dependency between them.

A nightly batch job renders video segments. The work is split into thousands of independent tasks, any task can be retried safely if it fails, and the job must finish before 06:00 but has no other timing constraint. The company wants to minimise compute cost. Which approach should a solutions architect recommend?

Independent, retryable tasks with a loose deadline is the definition of an interruption-tolerant workload, and that is what Spot Instances are for. Spreading the request across several instance types and Availability Zones is what keeps a Spot fleet from being reclaimed all at once.

A HorizontalPodAutoscaler targeting 70% average CPU never scales, and kubectl describe hpa shows unknown for the current metric. Which TWO conditions would cause this?

CPU-based autoscaling needs a metrics source serving the Metrics API, normally metrics-server, and it needs CPU requests on the Pods' containers, since utilisation is a percentage of the request.