AutoscalingLearn how AWS Autoscaling dynamically adjusts your application resources to meet traffic demands, optimize costs, and address DNS challenges.
ByAnis Mer_

Autoscaling

Adapting Resources to Traffic Demands

As your application grows, fluctuating traffic can strain resources during peak times or leave them underutilized during lulls. This is where Autoscaling comes in. AWS Autoscaling ensures your infrastructure scales dynamically to meet demand while optimizing costs.

In this article, we’ll explore how Autoscaling works, its components, and practical tips for configuration. We’ll also address common DNS-related challenges such as errors like DNS_PROBE_FINISHED_NXDOMAIN and strategies to ensure high availability.

What is Autoscaling?

Autoscaling is an AWS feature that automatically adjusts the number of resources in your application to match demand. It helps maintain application performance during traffic spikes and reduces costs during quieter periods by scaling down unnecessary resources.

Mental Model: Autoscaling as a Responsive Theater

Imagine your application as a theater:

  • During peak showtimes, more ticket counters (resources) open to handle the crowd.
  • During off-hours, the counters close to save costs.
    Autoscaling acts like a manager, ensuring the right number of counters are open at any time.

Key Components of Autoscaling

1. Auto Scaling Groups (ASGs)

An Auto Scaling Group is a collection of EC2 instances managed together. ASGs define:

  • The minimum number of instances to keep running.
  • The maximum number of instances to scale up to.
  • The desired capacity based on your application’s requirements.

Example:
For an e-commerce site, you may configure an ASG with:

  • Minimum: 2 instances (always available).
  • Maximum: 10 instances (for peak shopping times).
  • Desired: 5 instances (normal operation).

2. Scaling Policies

Scaling policies define the conditions that trigger scaling actions.

Types:

  1. Target Tracking: Scales resources based on metrics like CPU utilization or request count.
    • Example: Add instances when CPU usage exceeds 70%.
  2. Step Scaling: Adjusts capacity incrementally based on predefined thresholds.
    • Example: Add 1 instance if CPU > 50%, add 2 if CPU > 70%.
  3. Scheduled Scaling: Scales resources based on known patterns.
    • Example: Add instances at 6 PM daily when user traffic peaks.

Setting Up Autoscaling

1. Create an Auto Scaling Group

  1. Go to the EC2 Console > Auto Scaling Groups.
  2. Click Create Auto Scaling Group.
  3. Specify:
    • Launch Template: Define the AMI, instance type, and other configurations.
    • Group Size: Set minimum, maximum, and desired capacity.
    • Subnets: Select subnets across multiple Availability Zones for high availability.

2. Configure Scaling Policies

Set up target tracking or step scaling policies to define how the group adjusts based on demand.

  • Example: Use Target Tracking to maintain CPU usage at 50%.

3. Test Your Autoscaling Setup

Simulate traffic spikes with a load-testing tool to ensure the scaling policies behave as expected.

DNS Challenges in Autoscaling

Dynamic scaling can lead to DNS-related issues if records are not configured properly or if caching interferes with DNS updates.

Error: DNS_PROBE_FINISHED_NXDOMAIN

This error occurs when a DNS query fails to resolve the domain name, meaning the requested domain doesn’t point to any valid resource. In the context of AWS, this can happen if the DNS records in Route53 are misconfigured or if they haven’t updated after scaling events.

Fix:

  1. Verify DNS Records:
    Check that your Route53 configuration points to the correct ELB endpoint, such as a CNAME or Alias record linking your domain (e.g., www.example.com) to the ELB’s DNS name (myapp-alb-123456.us-east-1.elb.amazonaws.com).

    Example:
    Suppose you’ve set up an Application Load Balancer (ALB) with the DNS name: myapp-alb-123456.us-east-1.elb.amazonaws.com

    You want your custom domain www.example.com to point to this ALB.

    • Go to the AWS Management Console and open Route53.
    • Navigate to the Hosted Zones section and select the zone for example.com.
    • Look for a record that matches www.example.com.
    • Ensure it is configured as follows:
      • Record Type: CNAME (or Alias if the domain is a root/apex domain).
      • Value: myapp-alb-123456.us-east-1.elb.amazonaws.com.

    If the value points to an outdated ELB or another resource, update it to the correct ALB DNS name. For instance:

    • Change old-alb-987654.us-west-2.elb.amazonaws.com to myapp-alb-123456.us-east-1.elb.amazonaws.com.

    Save the updated record and use tools like nslookup www.example.com or dig www.example.com to verify the record resolves to the correct ALB DNS name.

  2. Set Low TTL:
    Ensure the Time-to-Live (TTL) value for DNS records is low (e.g., 60 seconds) to propagate changes quickly during scaling or updates.

  3. Test Resolution:
    Use tools like nslookup or dig to test domain resolution and confirm that the ELB’s DNS name resolves correctly.

This ensures traffic to www.example.com is routed to the right Application Load Balancer and prevents errors like DNS_PROBE_FINISHED_NXDOMAIN.

Flushing DNS Cache – When and Why

For most AWS setups, flushing DNS cache isn’t necessary. However, in rare cases involving custom DNS resolvers or applications, it may help resolve persistent cache issues.

  1. Custom Applications or Resolvers: If applications cache DNS internally, flushing ensures they reflect newly scaled instances.
  2. Persistent Cache Issues: If DNS records don’t update despite scaling, clearing caches on instances or application libraries can help.

Best Practices:

  • Use Amazon-provided DNS for dynamic updates.
  • Set low TTL values in Route53 to minimize stale records.
  • Avoid manual flushing unless troubleshooting specific DNS resolution issues.

Scaling DNS Traffic

Autoscaling doesn’t just apply to compute resources. DNS traffic itself can fluctuate, especially during promotional events or viral moments. For example, imagine a viral product launch where DNS queries spike overnight. Route53’s latency-based routing ensures traffic is directed to the nearest regional server, minimizing delays and maintaining a smooth user experience.

  • Use Route53’s Traffic Flow: Implement latency-based routing to direct users to the nearest resources.
  • Set Low TTL Values: Ensure DNS changes propagate quickly to handle dynamic scaling.
  • Monitor DNS Queries: Use AWS CloudWatch to track DNS query volume and detect potential bottlenecks.

Big Words Defined

  • Autoscaling: Automatically adjusting resources to match demand.
  • Auto Scaling Group (ASG): A group of EC2 instances managed together for scaling.
  • Scaling Policy: Rules that define when and how scaling actions occur.
  • TTL (Time-to-Live): Determines how long DNS records are cached.
  • DNS Cache: Locally stored DNS query results to speed up future lookups.

What’s Next?

Want to ensure your application stays healthy while scaling? Next, we’ll dive into CloudWatch and Alarms — tools that provide real-time monitoring and automated responses to keep your infrastructure running smoothly.

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