[AWS

Posted 2021-06-14 Answer1215

The gola of an Auto Scaling Group (ASG) is to:

• Scale out (add EC2 instances) to match an increased load
• Scale in (remove EC2 instances) to match a decreased load
• Ensure we have a minimum and maximum number of machines running
• Automatically Register new instances to a load balancer

 

Atttributes

• A launch configuration
  • AMI + Instance Type
  • EC2 User Data
  • EBS Volumes
  • Security Groups
  • SSH Key Pair
• Min Size / Max Size / Initial Capacity
• Network + Subnets information
• Load Balancer Information
• Scaling Policies

Auto Scaling Alarms

• It is possible to scale an ASG based on CloudWatch alarms
• An Alarm monitors a metric (Such as Average CPU)
• Metrics are computed for the overall ASG instances
• Based on the alarm:
  • We can create scale-out policies (increase the number of instances)
  • We can create scale-in policies (decrease the number of instances)

 

Auto Scaling New Rules

• It is possible to define "better" auto scaling rules that are directly managed by EC2
  • Target Average CPU usage
  • Number of requests on the ELB per instance
  • Average Network In
  • Average Network Out
• There rules are easier to set up and can make more sense

 

Auto Scaling Custom Metric

• We can auto scale based on custom metric (ex: number of connected users)

1. Send Custom metric from application to EC2 to CloudWatch (PutMetric API)
2. Create CloudWatch alaram to react to low / high values
3. Use CloudWatch alaram as the scaling policy for ASG

 

ASG Brain Dump

• Scaling policies can be on CPU, Network... and can even be on custom metrics based on schedule (if you know your visitor patterns)
• ASGs use Launch configurations or Launch Templates (newer)
• To update an ASG, you must provide a new launch configuration / launch template
• IAM role attached to an ASG will get assigned to EC2 instances
• Having instances under an ASG means that if they get terminated for whatever reason, the ASG will automatically create new ones as a replacement.
• ASG can terminate instances marked as unhealthy by an LB (and hence replace them)

Scaling Policies

Target Tracking Scaling

• Most simple and easy to set-up
• Example: Iwant the average ASG cup to stay at around 40%

Simple / Step Scaling

• When a CloudWatch alarm is triggered (CPU > 70%), then add 2 units
• When a CloudWatch alarm is triggered (CPU < 30%), then remove 1 unit

Scheduled Actions

• Anticipate a scaling based on known usage patterns
• Example: increase the min capacity to 10 at 5 pm on Fridays

 

 

Server Name Indication

• SNI solves the problem of loading multiple SSL cerificates onto one web server (to serve multiple websites)
• It\'s a newer protocol and requires the client to indicate the hostname of the target server in the initial SSL hadshake
• The server will then find the correct certifcate, or return the default one
• Only works for ALB & NLB, CloudFront
• Doesn\'t work for CLB

 

ELB - Connection Draining

• Deregistration Delay for ALB & NLB
• TIme to complete "in-fight requests" while the instnace is de-registering or unhealth 
• Stops sending new request to the instance which is de-regsitering
• Between 1 to 3600 seconds, default is 300 seconds
• Can be disabled (set value to 0)
• Set to a low value if your requests are short