High availability design patterns in AWS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Architecture

intermediate

Understanding Multi-AZ Deployment Benefits

Which of the following best describes the primary benefit of deploying an AWS RDS database instance in Multi-AZ mode?

A It reduces the cost by using spot instances in multiple zones.

B It increases the database read throughput by distributing read requests across zones.

C It automatically replicates data across multiple Availability Zones to improve fault tolerance.

D It enables automatic scaling of the database storage based on demand.

💡 Hint

Think about what Multi-AZ means for availability and data safety.

Multi-AZ deployments create a synchronous standby replica in a different Availability Zone. This setup improves fault tolerance by allowing automatic failover if the primary instance fails.

❓ service_behavior

intermediate

Load Balancer Behavior in High Availability

What is the expected behavior of an AWS Elastic Load Balancer (ELB) when one of its registered EC2 instances becomes unhealthy?

A The ELB redirects traffic to a backup ELB in another region.

B The ELB continues to send traffic to all instances regardless of health status.

C The ELB terminates the unhealthy instance automatically.

D The ELB stops sending traffic to the unhealthy instance and routes requests only to healthy instances.

💡 Hint

Consider how ELB ensures availability by managing traffic distribution.

ELB performs health checks and stops routing traffic to instances that fail health checks, ensuring only healthy instances receive traffic.

❓ security

advanced

Designing High Availability with Secure Access

You want to design a highly available web application on AWS that restricts access to backend databases only from application servers. Which combination of AWS features best achieves this?

A Place application servers in public subnets and databases in private subnets with security groups allowing traffic only from application servers.

B Place both application servers and databases in public subnets with open security groups.

C Place application servers in private subnets and databases in public subnets with no security groups.

D Place application servers and databases in the same public subnet with security groups allowing all traffic.

💡 Hint

Think about subnet types and security groups to restrict access.

Placing databases in private subnets protects them from direct internet access. Security groups restrict traffic to only allow application servers to connect, enhancing security and availability.

✅ Best Practice

advanced

Implementing Auto Scaling for High Availability

Which Auto Scaling group configuration best supports high availability for a web application across multiple Availability Zones?

A Configure the Auto Scaling group with a fixed number of instances and no scaling policies.

B Configure the Auto Scaling group to span multiple Availability Zones with a minimum number of instances in each zone.

C Configure the Auto Scaling group in a single Availability Zone with a high maximum instance count.

D Configure the Auto Scaling group to launch instances only on demand without health checks.

💡 Hint

Consider how spreading instances across zones improves availability.

Spanning multiple Availability Zones with minimum instances in each ensures the application remains available even if one zone fails.

🧠 Conceptual

expert

Analyzing Failover Behavior in Multi-Region Architectures

In a multi-region AWS architecture using Route 53 with health checks and failover routing policy, what happens when the primary region becomes unreachable?

A Route 53 automatically routes traffic to the secondary region after detecting the primary region is unhealthy.

B Route 53 continues routing traffic to the primary region regardless of health status.

C Route 53 routes traffic to both regions simultaneously to balance load.

D Route 53 disables DNS resolution until the primary region recovers.

💡 Hint

Think about how DNS failover works with health checks in Route 53.

Route 53 monitors the health of the primary region and automatically switches DNS responses to the secondary region if the primary is unhealthy, ensuring availability.

Practice

1. What is the main goal of a high availability design pattern in AWS?

easy

A. To reduce the cost of cloud resources

B. To keep the application running even if some parts fail

C. To increase the speed of the application only

D. To limit the number of users accessing the app

Solution

1. Step 1: Understand high availability concept

   High availability means designing systems to stay online and functional despite failures.

2. Step 2: Match goal with options

   Only To keep the application running even if some parts fail describes keeping the app running during failures, which is the goal.

3. Final Answer:

   To keep the application running even if some parts fail -> Option B

4. Quick Check:

   High availability = keep running despite failures [OK]

Hint: High availability means no downtime during failures [OK]

Common Mistakes:

• Confusing cost reduction with availability
• Thinking speed alone defines high availability
• Assuming limiting users improves availability

2. Which AWS service is commonly used to distribute incoming traffic across multiple instances for high availability?

easy

A. Amazon S3

B. AWS Lambda

C. Amazon RDS

D. Elastic Load Balancer (ELB)

Solution

1. Step 1: Identify service for traffic distribution

   Elastic Load Balancer (ELB) distributes traffic to multiple instances to avoid single points of failure.

2. Step 2: Eliminate other options

   Amazon S3 stores files, RDS is a database, Lambda runs code; none distribute traffic.

3. Final Answer:

   Elastic Load Balancer (ELB) -> Option D

4. Quick Check:

   Traffic distribution = ELB [OK]

Hint: Load balancers spread traffic to keep apps available [OK]

Common Mistakes:

• Choosing storage or compute services instead of load balancer
• Confusing Lambda with traffic distribution
• Selecting database services for traffic handling

3. Consider this AWS setup: An application runs in two Availability Zones (AZs) behind an Elastic Load Balancer. If one AZ fails, what happens to the app availability?

medium

A. The app slows down but stays available in the failed AZ

B. The app becomes completely unavailable

C. The app continues running using the healthy AZ

D. The app automatically scales down to zero instances

Solution

1. Step 1: Understand multi-AZ deployment

   Running in multiple AZs means if one AZ fails, others can serve traffic.

2. Step 2: Analyze ELB behavior

   ELB routes traffic only to healthy instances, so app stays available in the healthy AZ.

3. Final Answer:

   The app continues running using the healthy AZ -> Option C

4. Quick Check:

   Multi-AZ + ELB = app stays available [OK]

Hint: Multi-AZ with ELB keeps app running if one AZ fails [OK]

Common Mistakes:

• Assuming total outage if one AZ fails
• Thinking app slows but serves from failed AZ
• Believing app scales down automatically on failure

4. You configured an Auto Scaling group with minimum 2 instances across two AZs, but during a failure, only one instance is running. What is the likely issue?

medium

A. Auto Scaling group is not set to replace unhealthy instances

B. Load balancer is misconfigured to route traffic

C. Instances are launched in the same AZ only

D. Security groups block instance communication

Solution

1. Step 1: Check Auto Scaling group settings

   Auto Scaling must replace unhealthy instances to maintain minimum capacity.

2. Step 2: Understand failure impact

   If replacement is disabled, failed instance won't be replaced, causing fewer running instances.

3. Final Answer:

   Auto Scaling group is not set to replace unhealthy instances -> Option A

4. Quick Check:

   Auto Scaling replaces unhealthy = maintain capacity [OK]

Hint: Auto Scaling must replace failed instances to keep count [OK]

Common Mistakes:

• Blaming load balancer for instance count
• Assuming instances launched only in one AZ
• Thinking security groups affect instance count

5. You want to design a highly available web app on AWS that stays online even if an entire region fails. Which combination should you use?

hard

A. Deploy instances in multiple regions with Route 53 latency-based routing and health checks

B. Deploy instances in multiple AZs within one region and use an Elastic Load Balancer

C. Use a single instance in one AZ with Auto Scaling enabled

D. Deploy instances in one AZ and use Amazon S3 for static content

Solution

1. Step 1: Understand region failure scope

   Multi-AZ protects against AZ failure, but not entire region failure.

2. Step 2: Choose multi-region design

   Deploying in multiple regions with Route 53 routing and health checks ensures availability if one region fails.

3. Final Answer:

   Deploy instances in multiple regions with Route 53 latency-based routing and health checks -> Option A

4. Quick Check:

   Multi-region + Route 53 = survive region failure [OK]

Hint: Multi-region with Route 53 handles region failures [OK]

Common Mistakes:

• Thinking multi-AZ covers region failure
• Relying on single instance with Auto Scaling
• Ignoring DNS routing for multi-region failover