Operating and maintaining high-availability architectures (for example, application failovers, database failovers)

How can you ensure high availability for a stateful application like an RDBMS in AWS?

By using Amazon RDS with Multi-AZ deployments for failovers and Amazon Aurora for automatic failover in the case of a primary database instance failure. This involves setting up a primary RDS instance and multiple read replicas across different Availability Zones. In the event of a failure, RDS will automatically failover to a standby instance with minimal disruption.

Describe the role of a load balancer in achieving high availability in AWS.

Load balancers like AWS Elastic Load Balancing (ELB) distribute incoming application traffic across multiple targets, such as EC2 instances, in multiple Availability Zones, which increases the fault tolerance of your application. ELB detects unhealthy instances and reroutes traffic to healthy ones until the unhealthy instances have been restored.

What AWS services can be used to monitor application health and perform automatic recoveries of EC2 instances?

AWS CloudWatch can be used to monitor the health and performance of AWS resources and applications, while AWS Auto Scaling and EC2 Auto Recovery can be used for automatic recovery. If an instance becomes impaired or unreachable, the Auto Recovery feature will attempt to automatically recover the instance.

How can you handle failover for a multi-tier web application using AWS services?

For the web tier, use an Elastic Load Balancer to distribute traffic. For the application tier, implement Auto Scaling groups across different Availability Zones for redundancy. For the database tier, use RDS Multi-AZ deployment or Aurora DB for automated failovers.

Can you explain the concept of a stateless architecture and how it contributes to high availability?

Stateless architecture means that each request from clients doesn’t depend on any prior state that might have been established. It contributes to high availability because any server in a pool can handle any request. This model enables horizontal scaling, more resilience to failures, and more efficient failover procedures.

Describe an approach for maintaining session continuity in a stateless architecture.

Session state can be maintained using distributed caching systems like Amazon ElastiCache, storing sessions state outside of the web server layer, or services like Amazon DynamoDB. Another way is using token-based authentication methods like JWT (JSON Web Tokens) to maintain state while keeping the servers stateless.

What steps would you take to implement a disaster recovery plan in AWS?

I would define Recovery Time Objectives (RTO) and Recovery Point Objectives (RPO), choose an appropriate backup strategy (snapshotting, cross-region replication), and set up infrastructure in multiple regions (if necessary). Automate the replication of data and configuration using services like AWS CloudFormation, and test the DR plan regularly.

What is the difference between scalability and high availability, and how do both concepts interplay in AWS architectures?

Scalability is the ability of a system to handle growing amounts of work by adding resources, while high availability is the ability of a system to remain operational even after one or more of its components fail. In AWS, both are achieved through the use of elastic services that can scale out (for scalability) and are distributed across Availability Zones or regions (for high availability).

How would you use Amazon S3 for high availability?

Amazon S3 inherently provides high availability by storing data redundantly across multiple devices in multiple facilities within an AWS Region. By using S3, you’re leveraging its 999999999% (11 9’s) durability and 99% availability. It also supports cross-region replication to further ensure high availability and data protection.

Explain the concept of “pilot light” in the context of disaster recovery on AWS.

The “pilot light” scenario involves having a minimal replica of the production environment running in AWS. The core elements necessary to run the application (like RDS databases) are always on but typically at a smaller scale. In case of disaster, the environment quickly scales up to handle the production load, akin to a gas heater’s pilot light that’s always ready to ignite the full system.

How can AWS’s Route 53 contribute to high availability and fault tolerance?

AWS Route 53 provides DNS services with features such as health checks and failover routing policies. It can route traffic to healthy endpoints and avoid those that are experiencing issues, which ensures high availability by decreasing the probability of traffic reaching unresponsive servers.

What is the purpose of using AWS CloudFormation in the context of high-availability architecture?

AWS CloudFormation allows for the creation, management, and provisioning of a collection of related AWS resources in a predictable and controlled manner. This facilitates the setup of auto-scaling, load balancing, and deployment across multiple Availability Zones, which is pivotal for maintaining a high-availability architecture. It also helps in enabling infrastructure as code which makes the whole setup reproducible and easily recoverable in case of disasters.