AWS High Availability & Fault Tolerance Architecture – Certification

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AWS High Availability & Fault Tolerance Architecture

  • Amazon Web Services provides services and infrastructure to build reliable, fault-tolerant, and highly available systems in the cloud.
  • Fault-tolerance defines the ability for a system to remain in operation even if some of the components used to build the system fail.
  • Most of the higher-level services, such as S3, SimpleDB, SQS, and ELB, have been built with fault tolerance and high availability in mind.
  • Services that provide basic infrastructure, such as EC2 and EBS, provide specific features, such as availability zones, elastic IP addresses, and snapshots, that a fault-tolerant and highly available system must take advantage of and use correctly.

AWS High Availability and Fault Tolerance

NOTE: Topic mainly for Professional Exam Only

Regions & Availability Zones

  • Amazon Web Services are available in geographic Regions and with multiple Availability zones (AZs) within a region, which provide easy access to redundant deployment locations.
  • AZs are distinct geographical locations that are engineered to be insulated from failures in other AZs.
  • Regions and AZs help achieve greater fault tolerance by distributing the application geographically and help build multi-site solution.
  • AZs provide inexpensive, low latency network connectivity to other Availability Zones in the same Region
  • By placing EC2 instances in multiple AZs, an application can be protected from failure at a single data center
  • It is important to run independent application stacks in more than one AZ, either in the same region or in another region, so that if one zone fails, the application in the other zone can continue to run.

Amazon Machine Image – AMIs

  • EC2 is a web service within Amazon Web Services that provides computing resources.
  • Amazon Machine Image (AMI) provides a Template that can be used to define the service instances.
  • Template basically contains a software configuration (i.e., OS, application server, and applications) and is applied to an instance type
  • AMI can either contain all the softwares, applications and the code bundled or can be configured to have a bootstrap script to install the same on startup.
  • A single AMI can be used to create server resources of different instance types and start creating new instances or replacing failed instances

Auto Scaling

  • Auto Scaling helps to automatically scale EC2 capacity up or down based on defined rules.
  • Auto Scaling also enables addition of more instances in response to an increasing load; and when those instances are no longer needed, they will be automatically terminated.
  • Auto Scaling enables terminating server instances at will, knowing that replacement instances will be automatically launched.
  • Auto Scaling can work across multiple AZs within an AWS Region

Elastic Load Balancing – ELB

  • Elastic Load balancing is an effective way to increase the availability of a system and distributes incoming traffic to application across several EC2 instances
  • With ELB, a DNS host name is created and any requests sent to this host name are delegated to a pool of EC2 instances
  • ELB supports health checks on hosts, distribution of traffic to EC2 instances across multiple availability zones, and dynamic addition and removal of EC2 hosts from the load-balancing rotation
  • Elastic Load Balancing detects unhealthy instances within its pool of EC2 instances and automatically reroutes traffic to healthy instances, until the unhealthy instances have been restored seamlessly using Auto Scaling.
  • Auto Scaling and Elastic Load Balancing are an ideal combination – while ELB gives a single DNS name for addressing, Auto Scaling ensures there is always the right number of healthy EC2 instances to accept requests.
  • ELB can be used to balance across instances in multiple AZs of a region.

Elastic IPs – EIPs

  • Elastic IP addresses are public static IP addresses that can be mapped programmatically between instances within a region.
  • EIPs associated with the AWS account and not with a specific instance or lifetime of an instance.
  • Elastic IP addresses can be used for instances and services that require consistent endpoints, such as, master databases, central file servers, and EC2-hosted load balancers
  • Elastic IP addresses can be used to work around host or availability zone failures by quickly remapping the address to another running instance or a replacement instance that was just started.

Reserved Instance

  • Reserved instances help reserve and guarantee computing capacity is available at a lower cost always.

Elastic Block Store – EBS

  • Elastic Block Store (EBS) offers persistent off-instance storage volumes that persists independently from the life of an instance and are about an order of magnitude more durable than on-instance storage.
  • EBS volumes store data redundantly and are automatically replicated within a single availability zone.
  • EBS helps in failover scenarios where if an EC2 instance fails and needs to be replaced, the EBS volume can be attached to the new EC2 instance
  • Valuable data should never be stored only on instance (ephemeral) storage without proper backups, replication, or the ability to re-create the data.

EBS Snapshots

  • EBS volumes are highly reliable, but to further mitigate the possibility of a failure and increase durability, point-in-time Snapshots can be created to store data on volumes in S3, which is then replicated to multiple AZs.
  • Snapshots can be used to create new EBS volumes, which are an exact replica of the original volume at the time the snapshot was taken
  • Snapshots provide an effective way to deal with disk failures or other host-level issues, as well as with problems affecting an AZ.
  • Snapshots are incremental and back up only changes since the previous snapshot, so it is advisable to hold on to recent snapshots
  • Snapshots are tied to the region, while EBS volumes are tied to a single AZ

Relational Database Service – RDS

  • RDS makes it easy to run relational databases in the cloud
  • RDS Multi-AZ deployments, where a synchronous standby replica of the database is provisioned in a different AZ, which helps increase the database availability and protect the database against unplanned outages
  • In case of a failover scenario, the standby is promoted to be the primary seamlessly and will handle the database operations.
  • Automated backups, enabled by default, of the database provides point-in-time recovery for the database instance.
  • RDS will back up your database and transaction logs and store both for a user-specified retention period.
  • In addition to the automated backups, manual RDS backups can also be performed which are retained until explicitly deleted.
  • Backups help recover from higher-level faults such as unintentional data modification, either by operator error or by bugs in the application.
  • RDS Read Replicas provide read-only replicas of the database an provides the ability to scale out beyond the capacity of a single database deployment for read-heavy database workloads
  • RDS Read Replicas is a scalability and not a High Availability solution

Simple Storage Service – S3

  • S3 provides highly durable, fault-tolerant and redundant object store
  • S3 stores objects redundantly on multiple devices across multiple facilities in an S3 Region
  • S3 is a great storage solution for somewhat static or slow-changing objects, such as images, videos, and other static media.
  • S3 also supports edge caching and streaming of these assets by interacting with the Amazon CloudFront service.

Simple Queue Service – SQS

  • Simple Queue Service (SQS) is a highly reliable distributed messaging system that can serve as the backbone of fault-tolerant application
  • SQS is engineered to provide “at least once” delivery of all messages
  • Messages are guaranteed for sent to a queue are retained for up to four days( by default, and can be extended upto 14 days)  or until they are read and deleted by the application
  • Messages can be polled by multiple workers and processed, while SQS takes care that a request is processed by only one worker at a time using configurable time interval called visibility timeout
  • If the number of messages in a queue starts to grow or if the average time to process a message becomes too high, workers can be scaled upwards by simply adding additional EC2 instances.

Route 53

  • Amazon Route 53 is a highly available and scalable DNS web service.
  • Queries for the domain are automatically routed to the nearest DNS server and thus are answered with the best possible performance.
  • Route 53 resolves requests for your domain name (for example, www.example.com) to your Elastic Load Balancer, as well as your zone apex record (example.com).

CloudFront

  • CloudFront can be used to deliver website, including dynamic, static and streaming content using a global network of edge locations.
  • Requests for your content are automatically routed to the nearest edge location, so content is delivered with the best possible performance.
  • CloudFront is optimized to work with other Amazon Web Services, like S3 and EC2
  • CloudFront also works seamlessly with any non-AWS origin server, which stores the original, definitive versions of your files.

AWS Certification Exam Practice Questions

  • Questions are collected from Internet and the answers are marked as per my knowledge and understanding (which might differ with yours).
  • AWS services are updated everyday and both the answers and questions might be outdated soon, so research accordingly.
  • AWS exam questions are not updated to keep up the pace with AWS updates, so even if the underlying feature has changed the question might not be updated
  • Open to further feedback, discussion and correction.
  1. You are moving an existing traditional system to AWS, and during the migration discover that there is a master server which is a single point of failure. Having examined the implementation of the master server you realize there is not enough time during migration to re-engineer it to be highly available, though you do discover that it stores its state in a local MySQL database. In order to minimize down-time you select RDS to replace the local database and configure master to use it, what steps would best allow you to create a self-healing architecture[PROFESSIONAL]
    1. Migrate the local database into multi-AWS RDS database. Place master node into a multi-AZ auto-scaling group with a minimum of one and maximum of one with health checks.
    2. Replicate the local database into a RDS read replica. Place master node into a Cross-Zone ELB with a minimum of one and maximum of one with health checks. (Read Replica does not provide HA and write capability and ELB does not have feature for Min and Max 1 and Cross Zone allows just the equal distribution of load across instances)
    3. Migrate the local database into multi-AWS RDS database. Place master node into a Cross-Zone ELB with a minimum of one and maximum of one with health checks. (ELB does not have feature for Min and Max 1 and Cross Zone allows just the equal distribution of load across instances)
    4. Replicate the local database into a RDS read replica. Place master node into a multi-AZ auto-scaling group with a minimum of one and maximum of one with health checks. (Read Replica does not provide HA and write capability)
  2. You are designing Internet connectivity for your VPC. The Web servers must be available on the Internet. The application must have a highly available architecture. Which alternatives should you consider? (Choose 2 answers)
    1. Configure a NAT instance in your VPC. Create a default route via the NAT instance and associate it with all subnets. Configure a DNS A record that points to the NAT instance public IP address (NAT is for internet connectivity for instances in private subnet)
    2. Configure a CloudFront distribution and configure the origin to point to the private IP addresses of your Web servers. Configure a Route53 CNAME record to your CloudFront distribution.
    3. Place all your web servers behind ELB. Configure a Route53 CNAME to point to the ELB DNS name.
    4. Assign EIPs to all web servers. Configure a Route53 record set with all EIPs. With health checks and DNS failover.
  3. When deploying a highly available 2-tier web application on AWS, which combination of AWS services meets the requirements? 1. AWS Direct Connect 2. Amazon Route 53 3. AWS Storage Gateway 4. Elastic Load Balancing 4. Amazon EC2 5. Auto scaling 6. Amazon VPC 7. AWS Cloud Trail [PROFESSIONAL]
    1. 2,4,5 and 6
    2. 3,4,5 and 8
    3. 1 through 8
    4. 1,3,5 and 7
    5. 1,2,5 and 6
  4. Company A has hired you to assist with the migration of an interactive website that allows registered users to rate local restaurants. Updates to the ratings are displayed on the home page, and ratings are updated in real time. Although the website is not very popular today, the company anticipates that It will grow rapidly over the next few weeks. They want the site to be highly available. The current architecture consists of a single Windows Server 2008 R2 web server and a MySQL database running on Linux. Both reside inside an on -premises hypervisor. What would be the most efficient way to transfer the application to AWS, ensuring performance and high-availability? [PROFESSIONAL]
    1. Export web files to an Amazon S3 bucket in us-west-1. Run the website directly out of Amazon S3. Launch a multi-AZ MySQL Amazon RDS instance in us-west-1a. Import the data into Amazon RDS from the latest MySQL backup. Use Route 53 and create an alias record pointing to the elastic load balancer. (Its an Interactive website, although it can be implemented using Javascript SDK, its a migration and the application would need changes. Also no use of ELB if hosted on S3)
    2. Launch two Windows Server 2008 R2 instances in us-west-1b and two in us-west-1a. Copy the web files from on premises web server to each Amazon EC2 web server, using Amazon S3 as the repository. Launch a multi-AZ MySQL Amazon RDS instance in us-west-2a. Import the data into Amazon RDS from the latest MySQL backup. Create an elastic load balancer to front your web servers. Use Route 53 and create an alias record pointing to the elastic load balancer. (Although RDS instance is in a different region which will impact performance, this is the only option that works.)
    3. Use AWS VM Import/Export to create an Amazon Elastic Compute Cloud (EC2) Amazon Machine Image (AMI) of the web server. Configure Auto Scaling to launch two web servers in us-west-1a and two in us-west-1b. Launch a Multi-AZ MySQL Amazon Relational Database Service (RDS) instance in us-west-1b. Import the data into Amazon RDS from the latest MySQL backup. Use Amazon Route 53 to create a hosted zone and point an A record to the elastic load balancer. (does not create a load balancer)
    4. Use AWS VM Import/Export to create an Amazon EC2 AMI of the web server. Configure auto-scaling to launch two web servers in us-west-1a and two in us-west-1b. Launch a multi-AZ MySQL Amazon RDS instance in us-west-1a. Import the data into Amazon RDS from the latest MySQL backup. Create an elastic load balancer to front your web servers. Use Amazon Route 53 and create an A record pointing to the elastic load balancer. (Need to create a aliased record without which the Route 53 pointing to ELB would not work)
  5. Your company runs a customer facing event registration site. This site is built with a 3-tier architecture with web and application tier servers and a MySQL database. The application requires 6 web tier servers and 6 application tier servers for normal operation, but can run on a minimum of 65% server capacity and a single MySQL database. When deploying this application in a region with three availability zones (AZs) which architecture provides high availability? [PROFESSIONAL]
    1. A web tier deployed across 2 AZs with 3 EC2 (Elastic Compute Cloud) instances in each AZ inside an Auto Scaling Group behind an ELB (elastic load balancer), and an application tier deployed across 2 AZs with 3 EC2 instances in each AZ inside an Auto Scaling Group behind an ELB. and one RDS (Relational Database Service) instance deployed with read replicas in the other AZ.
    2. A web tier deployed across 3 AZs with 2 EC2 (Elastic Compute Cloud) instances in each AZ inside an Auto Scaling Group behind an ELB (elastic load balancer) and an application tier deployed across 3 AZs with 2 EC2 instances in each AZ inside an Auto Scaling Group behind an ELB and one RDS (Relational Database Service) Instance deployed with read replicas in the two other AZs.
    3. A web tier deployed across 2 AZs with 3 EC2 (Elastic Compute Cloud) instances in each AZ inside an Auto Scaling Group behind an ELB (elastic load balancer) and an application tier deployed across 2 AZs with 3 EC2 instances m each AZ inside an Auto Scaling Group behind an ELS and a Multi-AZ RDS (Relational Database Service) deployment.
    4. A web tier deployed across 3 AZs with 2 EC2 (Elastic Compute Cloud) instances in each AZ Inside an Auto Scaling Group behind an ELB (elastic load balancer). And an application tier deployed across 3 AZs with 2 EC2 instances in each AZ inside an Auto Scaling Group behind an ELB. And a Multi-AZ RDS (Relational Database services) deployment.
  6. For a 3-tier, customer facing, inclement weather site utilizing a MySQL database running in a Region which has two AZs which architecture provides fault tolerance within the region for the application that minimally requires 6 web tier servers and 6 application tier servers running in the web and application tiers and one MySQL database? [PROFESSIONAL]
    1. A web tier deployed across 2 AZs with 6 EC2 (Elastic Compute Cloud) instances in each AZ inside an Auto Scaling Group behind an ELB (elastic load balancer), and an application tier deployed across 2 AZs with 6 EC2 instances in each AZ inside an Auto Scaling Group behind an ELB. and a Multi-AZ RDS (Relational Database Service) deployment. (As it needs Fault Tolerance with minimal 6 servers always available)
    2. A web tier deployed across 2 AZs with 3 EC2 (Elastic Compute Cloud) instances in each A2 inside an Auto Scaling Group behind an ELB (elastic load balancer) and an application tier deployed across 2 AZs with 3 EC2 instances in each AZ inside an Auto Scaling Group behind an ELB and a Multi-AZ RDS (Relational Database Service) deployment.
    3. A web tier deployed across 2 AZs with 3 EC2 (Elastic Compute Cloud) instances in each AZ inside an Auto Scaling Group behind an ELB (elastic load balancer) and an application tier deployed across 2 AZs with 6 EC2 instances in each AZ inside an Auto Scaling Group behind an ELB and one RDS (Relational Database Service) Instance deployed with read replicas in the other AZs.
    4. A web tier deployed across 1 AZs with 6 EC2 (Elastic Compute Cloud) instances in each AZ Inside an Auto Scaling Group behind an ELB (elastic load balancer). And an application tier deployed in the same AZs with 6 EC2 instances inside an Auto scaling group behind an ELB and a Multi-AZ RDS (Relational Database services) deployment, with 6 stopped web tier EC2 instances and 6 stopped application tier EC2 instances all in the other AZ ready to be started if any of the running instances in the first AZ fails.
  7. You are designing a system which needs, at minimum, 8 m4.large instances operating to service traffic. When designing a system for high availability in the us-east-1 region, which has 6 Availability Zones, you company needs to be able to handle death of a full availability zone. How should you distribute the servers, to save as much cost as possible, assuming all of the EC2 nodes are properly linked to an ELB? Your VPC account can utilize us-east-1’s AZ’s a through f, inclusive.
    1. 3 servers in each of AZ’s a through d, inclusive.
    2. 8 servers in each of AZ’s a and b.
    3. 2 servers in each of AZ’s a through e, inclusive. (You need to design for N+1 redundancy on Availability Zones. ZONE_COUNT = (REQUIRED_INSTANCES / INSTANCE_COUNT_PER_ZONE) + 1. To minimize cost, spread the instances across as many possible zones as you can. By using a though e, you are allocating 5 zones. Using 2 instances, you have 10 total instances. If a single zone fails, you have 4 zones left, with 2 instances each, for a total of 8 instances. By spreading out as much as possible, you have increased cost by only 25% and significantly de-risked an availability zone failure. Refer link)
    4. 4 servers in each of AZ’s a through c, inclusive.
  8. You need your API backed by DynamoDB to stay online during a total regional AWS failure. You can tolerate a couple minutes of lag or slowness during a large failure event, but the system should recover with normal operation after those few minutes. What is a good approach? [PROFESSIONAL]
    1. Set up DynamoDB cross-region replication in a master-standby configuration, with a single standby in another region. Create an Auto Scaling Group behind an ELB in each of the two regions DynamoDB is running in. Add a Route53 Latency DNS Record with DNS Failover, using the ELBs in the two regions as the resource records. (Use DynamoDB cross-regional replication version with two ELBs and ASGs with Route53 Failover and Latency DNS. Refer link)
    2. Set up a DynamoDB Multi-Region table. Create an Auto Scaling Group behind an ELB in each of the two regions DynamoDB is running in. Add a Route53 Latency DNS Record with DNS Failover, using the ELBs in the two regions as the resource records. (No such thing as DynamoDB Multi-Region table)
    3. Set up a DynamoDB Multi-Region table. Create a cross-region ELB pointing to a cross-region Auto Scaling Group, and direct a Route53 Latency DNS Record with DNS Failover to the cross-region ELB. (No such thing as Cross Region ELB or cross-region ASG)
    4. Set up DynamoDB cross-region replication in a master-standby configuration, with a single standby in another region. Create a cross-region ELB pointing to a cross-region Auto Scaling Group, and direct a Route53 Latency DNS Record with DNS Failover to the cross-region ELB. (No such thing as DynamoDB cross-region table or cross-region ELB)
  9. You are putting together a WordPress site for a local charity and you are using a combination of Route53, Elastic Load Balancers, EC2 & RDS. You launch your EC2 instance, download WordPress and setup the configuration files connection string so that it can communicate to RDS. When you browse to your URL however, nothing happens. Which of the following could NOT be the cause of this.
    1. You have forgotten to open port 80/443 on your security group in which the EC2 instance is placed.
    2. Your elastic load balancer has a health check, which is checking a webpage that does not exist; therefore your EC2 instance is not in service.
    3. You have not configured an ALIAS for your A record to point to your elastic load balancer
    4. You have locked port 22 down to your specific IP address therefore users cannot access your site using HTTP/HTTPS
  10. A development team that is currently doing a nightly six-hour build which is lengthening over time on-premises with a large and mostly under utilized server would like to transition to a continuous integration model of development on AWS with multiple builds triggered within the same day. However, they are concerned about cost, security and how to integrate with existing on-premises applications such as their LDAP and email servers, which cannot move off-premises. The development environment needs a source code repository; a project management system with a MySQL database resources for performing the builds and a storage location for QA to pick up builds from. What AWS services combination would you recommend to meet the development team’s requirements? [PROFESSIONAL]
    1. A Bastion host Amazon EC2 instance running a VPN server for access from on-premises, Amazon EC2 for the source code repository with attached Amazon EBS volumes, Amazon EC2 and Amazon RDS MySQL for the project management system, EIP for the source code repository and project management system, Amazon SQL for a build queue, An Amazon Auto Scaling group of Amazon EC2 instances for performing builds and Amazon Simple Email Service for sending the build output. (Bastion is not for VPN connectivity also SES should not be used)
    2. An AWS Storage Gateway for connecting on-premises software applications with cloud-based storage securely, Amazon EC2 for the resource code repository with attached Amazon EBS volumes, Amazon EC2 and Amazon RDS MySQL for the project management system, EIPs for the source code repository and project management system, Amazon Simple Notification Service for a notification initiated build, An Auto Scaling group of Amazon EC2 instances for performing builds and Amazon S3 for the build output. (Storage Gateway does not provide secure connectivity, still needs VPN. SNS alone cannot handle builds)
    3. An AWS Storage Gateway for connecting on-premises software applications with cloud-based storage securely, Amazon EC2 for the resource code repository with attached Amazon EBS volumes, Amazon EC2 and Amazon RDS MySQL for the project management system, EIPs for the source code repository and project management system, Amazon SQS for a build queue, An Amazon Elastic Map Reduce (EMR) cluster of Amazon EC2 instances for performing builds and Amazon CloudFront for the build output. (Storage Gateway does not provide secure connectivity, still needs VPN. EMR is not ideal for performing builds as it needs normal EC2 instances)
    4. A VPC with a VPN Gateway back to their on-premises servers, Amazon EC2 for the source-code repository with attached Amazon EBS volumes, Amazon EC2 and Amazon RDS MySQL for the project management system, EIPs for the source code repository and project management system, SQS for a build queue, An Auto Scaling group of EC2 instances for performing builds and S3 for the build output. (VPN gateway is required for secure connectivity. SQS for build queue and EC2 for builds)

References

29 thoughts on “AWS High Availability & Fault Tolerance Architecture – Certification

  1. Hello Sir, Thank you for putting all of this together.

    I wanted to humbly submit a correction for question #4 on this page.

    One cannot add an “A record” to Route 53 that points to the ELB because the ELB does not have an IP address, only a DNS endpoint. The “A record” requires an IP address if I am not mistaken. The first option is not correct as there is no such thing as “Amazon 53 bucket”. Therefore the correct answer is the second option which correctly uses an ALIAS record to point to the ELB and does not mention anything that does not exist.

    I have been wrong in life more times than I have been right. This may be no different so I am open to correction myself. However, I believe I am correct in this instance.

    Thank you again for this outstanding resource.

    1. Thanks Luis, you might be right but i would still stick with D.
      With B the RDS is in different region which would impact performance heavily, which is one of the main question point.
      Also, agreed that a A record does not point to ELB directly, however you do create a A record and just add an alias internally pointing to ELB, which is open to question in option D.

  2. Regarding Q4, I believe the answer is A.

    An interactive website does not mean it cant be hosted in S3 – as long as it is client side. You simply cannot have an A record pointed to an ELB, so this breaks D

    Thanks

  3. Re: Q4.

    None of these answers are particularly good. C and D both require Route 53 to create an A record pointing to an ELB, which is not possible/recommended. Answer A implies that the website is JavaScript, but it also wants to use Route 53 to point to an ELB, when there isn’t one. It would need to be the s3-website URL. Therefore B is the only possible answer, despite the latency.

    It should also be noted that you cannot launch a Multi-AZ RDS Instance into a specific AZ.

    1. Yup you are correct, I liked the option D which is very close to the implementation except to the fact that it does not create a Alias record.
      Also agreed to the Multi AZ RDS instance into a specific AZ.

      1. In Route53 you create DNS Record Type as A record and then select Alias Yes for the ELB. So technically you create A record for ELB. So I think D is the right answer for Q4 as it is the most efficient.

        1. Technically its a A record. But A record does not support it and hence the need of the custom Alias record. Its just the words and interpretation 🙂

  4. I think D should be the best option.
    Read line Create Route 53 and point to ELB ……NOT S3 endpoint .

    No use of ELB in it …

    Also interactive site …

    1. Option A is wrong, Option D the Route 53 does not create an Alias record hence it would not work. Option B is the only valid option that would work even though the RDS instance is in different Region.

  5. Question 6 is incorrect and should be option A (N+1 redundancy)

    The questions states “minimally requires 6 web tier servers and 6 application tier servers..”

    With Option B, if an AZ fails there is not enough servers left to support this part of the requirement therefore it is incorrect.

  6. Q 6

    Answer should be B

    As Autoscaling provide fault tolerance ,as soon 2nd AZ down ,autoscaling initiate another 3 instances in first AZ.

    We use autoscaling for fault tolerance…

    1. That isn’t fault tolerance. Fault tolerance is the ability to lose an AZ or region without any interruption to service. Having to autoscale new instances takes time this causes interruption of service

    1. What you are talking about there is *high availability*. The application is still available even with the failure of an AZ. Fault tolerance is *no impact to the application despite the loss of an AZ*.

      In answer A, your site, which requires 6 servers in web tier and 6 servers in the backend tier minimum, as soon as you lose an AZ your application does not satisfy minimum requirements, and you need to take 3-5-10 minutes to spin up the necessary instances to bring yourself back up to capacity. In answer B, you can lose a whole AZ without any impact.

      References:

      https://acloud.guru/forums/aws-certified-solutions-architect-professional/discussion/-KW2-4by33fyMW4Xf_YI/sample-fault-tolerance-questions

      https://www.quora.com/What-is-the-difference-between-a-highly-fault-tolerant-and-a-highly-available-system

  7. You mention for RDS read replicas they are used for scalability, not high availability. This might be true fundamentally, but if your primary DB fails, you can promote the read replica accordingly. Of course you should always turn on multi-az anyway. But as far as exam details go, what are your thoughts?

    As per AWS

    “Increased Availability
    Read replicas in Amazon RDS for MySQL, MariaDB, and PostgreSQL provide a complementary availability mechanism to Amazon RDS Multi-AZ Deployments. You can promote a read replica if the source DB instance fails. You can also replicate DB instances across AWS Regions as part of your disaster recovery strategy. This functionality complements the synchronous replication, automatic failure detection, and failover provided with Multi-AZ deployments.”

    1. Read Replicas are mostly projected as scalability solution, cause for HA there is Multi-AZ deployments.
      But agreed, Read Replicas can act as HA as you can promote a Read Replica to master if the DB fails.
      However, you would need to take into account the RTO and RPO as Read replicas are not synchronized.
      Read Replicas surely work for DR across Regions, as Multi-AZ deployments are not possible across regions.

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