Question: How can one implement auto scaling with Memcached?

Answer

Auto-scaling with Memcached involves dynamically scaling your Memcached clusters based on the load. It helps to accommodate fluctuating workloads by automatically adjusting capacity in response to changing demands.

AWS ElastiCache for Memcached provides a simple way to implement this functionality. However, it's important to note that AWS ElastiCache does not natively support auto-scaling for Memcached. Instead, one could use AWS Application Auto Scaling service to monitor and adjust ElastiCache resources as needed.

Here are the steps to set up auto-scaling for Memcached in an AWS environment:

1. Create an AWS Identity and Access Management (IAM) Role - This role will grant the Application Auto Scaling service permissions to access the required resources.

First, create a trust policy file trust_policy.json with the following content:

{
  "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Service": "application-autoscaling.amazonaws.com"
      },
      "Action": "sts:AssumeRole"
    }
  ]
}

Then, create the IAM role using the policy file:

aws iam create-role --role-name ecsAutoscaleRole --assume-role-policy-document file://trust_policy.json

2. Attach the necessary policies to your IAM Role – The AmazonElastiCacheFullAccess policy allows the role to perform actions on ElastiCache resources.

aws iam attach-role-policy --role-name ecsAutoscaleRole --policy-arn arn:aws:iam::aws:policy/AmazonElastiCacheFullAccess

3. Register Scalable Targets - This step informs the Application Auto Scaling service about the resource you want to scale.

aws application-autoscaling register-scalable-target --service-namespace elasticache --resource-id cache-cluster-id --scalable-dimension elasticache:nodegroup:Count --min-capacity 1 --max-capacity 10 --role-ARN ecsAutoscaleRole

4. Set up Scaling Policies - Define when the service should scale out (add more nodes) or scale in (remove nodes).

aws application-autoscaling put-scaling-policy --policy-name scale-out-policy --service-namespace elasticache --resource-id cache-cluster-id --scalable-dimension elasticache:nodegroup:Count --policy-type StepScaling --step-scaling-policy-configuration AdjustmentType=ChangeInCapacity,StepAdjustments=[{MetricIntervalLowerBound=0,ScalingAdjustment=1}],Cooldown=300

In this command, we're creating a policy named 'scale-out-policy' which adds 1 node whenever the specified metric exceeds its defined threshold.

Remember, these scripts require AWS CLI and appropriate credentials configured on your machine. Also, replace 'cache-cluster-id' with your actual cluster ID where necessary.

Naturally, there are many factors to consider when setting up auto-scaling, including how quickly the loads change, how long it takes new nodes to start handling requests efficiently, and costs associated with running additional nodes.

Please note that auto-scaling isn't always the best solution for managing variable loads and can bring complexities. For non-AWS environments or tailor-made solutions, consider using consistent hashing algorithms that make adding or removing servers easier without causing significant cache invalidation.