Tutorial / Cram Notes
To configure a load balancer to recover from backend failure effectively, you can use AWS Elastic Load Balancing (ELB), which offers different types of load balancers—Application Load Balancer (ALB), Network Load Balancer (NLB), and Classic Load Balancer (CLB). Each serves a specific use case, with ALB being well-suited for HTTP/HTTPS traffic and NLB excelling in high-performance, low-latency TCP traffic scenarios.
Health Checks
Firstly, a load balancer must monitor the health of backend resources to detect failures. Health checks are configured to ping services at regular intervals using a specified protocol and port to ensure they respond correctly. If a resource fails a certain number of checks consecutively, it is deemed unhealthy and the load balancer stops sending traffic to it until it recovers.
For example, in an ALB, you can configure health checks with the following parameters:
- Timeout: The amount of time to wait for a response from a target before considering it failed.
- Interval: The time between health checks.
- Unhealthy threshold: The number of consecutive failed health checks required before considering the target unhealthy.
- Healthy threshold: The number of consecutive successful health checks required before considering an unhealthy target healthy again.
- Path (for HTTP/HTTPS checks): The destination on the target to which the load balancer sends the health check.
Auto Scaling Integration
AWS Auto Scaling is pivotal for recovering from backend failures. By integrating load balancers with Auto Scaling groups, new instances can be launched automatically to replace failed ones, and the load balancer can start routing traffic to these instances once they pass health checks.
Cross-Zone Load Balancing
It’s essential to enable cross-zone load balancing. This feature ensures that each load balancer node distributes traffic evenly across all backend instances, regardless of the Availability Zone. This is particularly useful in scenarios where a failure impacts all instances inside a single Availability Zone.
Deregistration Delay
For connection-based protocols, you can configure a deregistration delay (also known as connection draining) to ensure that existing connections are not dropped abruptly when an instance is de-registered or becomes unhealthy. This gives active requests additional time to complete.
Example of Load Balancer Health Check Configuration:
{
“HealthCheck”: {
“Target”: “HTTP:80/ping”,
“Interval”: 30,
“Timeout”: 5,
“UnhealthyThreshold”: 2,
“HealthyThreshold”: 2
}
}
Handling Failures in Different Load Balancer Types
| Feature | Application Load Balancer (ALB) | Network Load Balancer (NLB) | Classic Load Balancer (CLB) |
|---|---|---|---|
| Health Check Protocol | HTTP, HTTPS, TCP | TCP, TLS | HTTP, HTTPS, TCP, SSL |
| Deregistration Delay | Supported | Supported | Supported (as connection draining) |
| Path-Based Routing | Supported | Not Supported | Not Supported |
| Cross-Zone Load Balancing | Always On (no extra charge) | Optional (extra charge) | Optional (extra charge) |
| Failure Recovery | Targets added/removed based on health checks and Auto Scaling | Targets added/removed based on health checks and Auto Scaling | Instances added/removed based on health checks and Auto Scaling |
Proactive Steps to Enhance Recovery
- Deployment Strategies: Use rolling updates or blue/green deployments to ensure that new versions of your application can be deployed without affecting the entire service.
- Multi-AZ Setup: Deploy backend instances across multiple Availability Zones for higher fault tolerance.
- Pre-Warming: If expecting high traffic, inform AWS support to ‘pre-warm’ the load balancer, which adjusts its initial capacity to handle the traffic surge.
- Monitoring and Alerting: Set up CloudWatch alarms to monitor the health and performance of your load balancers and trigger alerts or automated actions in response to predefined metrics.
- Logging: Enable access logs on your load balancer to capture detailed information about requests. These logs can be used for troubleshooting and understanding traffic patterns, which can be crucial after a backend failure event.
In conclusion, effectively configuring a load balancer for backend failure recovery necessitates proper health check setup, Auto Scaling integration, use of features like cross-zone load balancing and connection draining, and proactive management of deployment strategies and geographic distribution. By mastering these elements, a candidate aiming for the AWS Certified DevOps Engineer – Professional certification will be well-prepared for designing resilient and scalable architectures on AWS.
Practice Test with Explanation
True or False: AWS Elastic Load Balancer automatically performs health checks on the registered instances.
- True
- False
Answer: True
Explanation: AWS Elastic Load Balancer regularly performs health checks on registered instances to ensure traffic is only directed to healthy backends.
Which of the following can be used as a load balancer on AWS?
- Amazon Elastic Load Balancer
- Amazon EC2 Auto Scaling groups
- Amazon Simple Notification Service
- Amazon CloudFront
Answer: Amazon Elastic Load Balancer
Explanation: Amazon Elastic Load Balancer is specifically designed to distribute application traffic across multiple servers or availability zones.
True or False: If a backend instance fails a health check, the load balancer will stop sending traffic to that instance immediately.
- True
- False
Answer: True
Explanation: When an instance fails a health check, the load balancer stops sending traffic to it to maintain the overall health of the application.
Which type of health checks can AWS Elastic Load Balancers perform? (Select TWO)
- ICMP health checks
- HTTP health checks
- TCP health checks
- DNS health checks
Answer: HTTP health checks, TCP health checks
Explanation: AWS Elastic Load Balancers can perform HTTP and TCP health checks but not ICMP or DNS health checks.
True or False: It is possible to configure AWS Load Balancer health checks to expect a specific status code from the backend instances.
- True
- False
Answer: True
Explanation: Health checks can be configured to expect specific response codes to determine instance health.
When configuring a load balancer, an acceptable health check interval range is _____.
- 5 to 300 seconds
- 1 to 60 seconds
- 30 to 600 seconds
- 10 to 120 seconds
Answer: 5 to 300 seconds
Explanation: AWS ELB health checks can be configured with intervals ranging from 5 to 300 seconds.
In which scenarios will a load balancer stop sending requests to an EC2 instance? (Select TWO)
- When the instance is stopped.
- When the instance CPU utilization reaches 100%.
- When the instance fails health checks.
- When the instance type is changed.
Answer: When the instance is stopped, When the instance fails health checks.
Explanation: A load balancer stops routing traffic to instances that are stopped or have failed health checks, not necessarily based on CPU utilization or instance type changes.
True or False: AWS Network Load Balancers do not support path-based routing.
- True
- False
Answer: True
Explanation: AWS Network Load Balancers operate at the transport layer (layer 4) and do not support path-based routing; this is a feature of Application Load Balancers.
An application load balancer health check is configured with a threshold of What does this mean?
- The instance is marked unhealthy after 5 consecutive failed health checks.
- The instance is marked healthy after 5 minutes of continuous operation.
- The load balancer checks the health of the instance every 5 seconds.
- The load balancer waits for 5 seconds before performing the first health check.
Answer: The instance is marked unhealthy after 5 consecutive failed health checks.
Explanation: The threshold setting determines the number of consecutive failed health checks before an instance is deemed unhealthy.
True or False: You must use Amazon CloudWatch to configure alarms for unhealthy hosts in a load balanced environment.
- True
- False
Answer: False
Explanation: While Amazon CloudWatch can be used to monitor and alert on the health of instances, it is not a requirement for configuring a load balancer to handle backend failure. The load balancer itself handles the rerouting of traffic based on health checks.
What happens to the in-flight requests to an instance that has just been declared unhealthy by the load balancer?
- They are retried on a healthy instance.
- They are dropped immediately.
- They are completed, and future requests are routed away.
- They are queued until the instance is healthy again.
Answer: They are completed, and future requests are routed away.
Explanation: The load balancer allows in-flight requests to complete but will route new requests to healthy instances.
After releasing a new version of your application, you notice an increase in backend connection errors. What can you do to avoid this scenario in the future?
- Perform a blue/green deployment.
- Increase the health check interval.
- Disable health checks during deployment.
- Manually deregister the backend instances during deployment.
Answer: Perform a blue/green deployment.
Explanation: Blue/green deployments minimize downtime and reduce the risk of errors by ensuring a new version can handle traffic before routing to it, with an easy rollback strategy.
Interview Questions
Explain how the Elastic Load Balancer (ELB) detects a failed backend instance?
ELB detects a failed backend instance using health checks. These are configured to perform regular checks of the EC2 instances’ health by attempting to make a connection to a specified port and path on the instance. If a health check fails, ELB considers the instance unhealthy and stops routing traffic to it until it passes health checks again.
What are the different types of load balancers provided by AWS, and which support backend failure recovery?
AWS provides three types of load balancers: Application Load Balancer (ALB), Network Load Balancer (NLB), and Classic Load Balancer (CLB). All three types support backend failure recovery through health checks and automatic re-routing of traffic to healthy instances.
How can you configure Auto Scaling to work with an Elastic Load Balancer to recover from a backend failure?
Auto Scaling can be configured with an ELB by setting up scaling policies based on the desired performance metrics. If an instance fails health checks, Auto Scaling can replace it by launching a new instance based on the pre-configured launch configuration or template, thus recovering from a backend failure.
Describe briefly what cross-zone load balancing is and how it can help in backend failure scenarios?
Cross-zone load balancing evenly distributes the traffic across instances in multiple Availability Zones. If an instance in one zone fails, the load balancer can redirect traffic to healthy instances in other zones, helping to ensure high availability and fault tolerance.
In AWS, how can you automate the response to a backend EC2 instance failure?
AWS enables automation through CloudWatch alarms and EC2 Auto Scaling policies. When an instance fails, CloudWatch alarms can trigger an Auto Scaling policy to replace it or an AWS Lambda function to perform recovery steps.
What steps would you take to configure an Application Load Balancer for a high-traffic application to recover from a failure of an application server?
To configure ALB for high-traffic application recovery:
– Enable health checks with appropriate thresholds.
– Use Auto Scaling to adjust the capacity based on demand and replace failed instances.
– Implement cross-zone load balancing to distribute traffic evenly.
– Configure sticky sessions if needed to maintain user sessions.
– Optionally, use AWS WAF or Shield with your ALB for additional security.
Can you adjust the health check interval and timeout settings of an AWS ELB? What implications do these settings have on backend failure recovery?
Yes, health check intervals and timeout settings can be adjusted in ELB. A shorter health check interval can detect and recover from backend failures faster, but might increase the load on the backend servers, whereas longer intervals might reduce load but lead to slower recovery times.
How do you ensure zero downtime deployments with ELB during backend updates or patches?
Zero downtime deployments can be ensured by using a blue-green deployment strategy or rolling updates. This involves routing traffic to a new “green” fleet while the “blue” fleet is updated, or gradually replacing old instances with new ones, thereby minimizing service disruption.
How does AWS’s Elastic Load Balancer integrate with services like Amazon CloudWatch and Amazon S3 for logging and monitoring backend health?
ELB automatically integrates with Amazon CloudWatch to provide metrics and logs, reporting detailed information about request counts, error rates, and backend connection errors. Logs can be stored and analyzed in Amazon S3 for further analysis or to archive data for compliance purposes.
In a multi-tier application architecture, how would you configure a load balancer to ensure that a failure in the web server tier doesn’t cause a service disruption?
In a multi-tier architecture, the load balancer should be set up with health checks that are specific to the web server tier. Additionally, you’d configure the load balancer in front of the web server tier and potentially another in front of the application server tier, allowing each tier to scale and recover independently from failures.
Great blog post! Configuring a load balancer to recover from backend failure has always been a tricky part of my deployments.
I completely agree. Automation tools like AWS CloudFormation make it easier, though.
Thanks for the article. It really clarified the steps needed.
Can anyone share more about using Route 53 for load balancer failover?
Appreciate the detailed guide. It’s a lifesaver!
What about using ALB vs NLB in terms of failover?
Is it necessary to set up Cross-Zone Load Balancing?
Amazing post. Very insightful.