High availability design patterns in AWS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of high availability design patterns?

To ensure that a system or service remains operational and accessible with minimal downtime, even during failures or maintenance.

intermediate

Explain the concept of 'Active-Active' high availability pattern.

In Active-Active, multiple instances of a service run simultaneously in different locations. Traffic is distributed among them, so if one fails, others continue serving without interruption.

intermediate

What is the difference between 'Active-Passive' and 'Active-Active' patterns?

Active-Passive has one active instance and one or more standby instances that take over if the active fails. Active-Active runs all instances actively and shares the load.

beginner

How does AWS Elastic Load Balancer (ELB) support high availability?

ELB distributes incoming traffic across multiple healthy instances in different Availability Zones, helping maintain service availability if some instances fail.

beginner

Why is multi-AZ deployment important for high availability in AWS?

Multi-AZ deployment places resources in different physical locations (Availability Zones) to protect against data center failures, ensuring the system stays available.

Which pattern involves multiple active instances serving traffic simultaneously?

A Cold Standby

B Active-Passive

C Active-Active

D Single Instance

Active-Active means all instances are active and share the load.

What AWS service helps distribute traffic to multiple instances for high availability?

A Amazon S3

B AWS Lambda

C Amazon RDS

D Elastic Load Balancer (ELB)

ELB balances traffic across healthy instances to maintain availability.

Why deploy resources in multiple Availability Zones (AZs)?

A To protect against data center failures

B To improve security

C To reduce costs

D To increase latency

Multi-AZ deployment protects against failures in one data center.

In an Active-Passive pattern, what happens when the active instance fails?

A The passive instance takes over

B The system shuts down

C Traffic is lost

D Both instances fail

The passive instance becomes active to keep the system running.

Which of these is NOT a benefit of high availability design patterns?

A Reduced downtime

B Increased single point of failure

C Improved fault tolerance

D Better user experience

High availability patterns reduce single points of failure, not increase them.

Describe the Active-Active and Active-Passive high availability patterns and when you might use each.

Think about how many instances run at the same time and how traffic is handled.

Explain why deploying resources across multiple Availability Zones improves system availability in AWS.

Consider what happens if one data center goes down.

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