How to Autoscale (add a node) Memcached cluster?

You can not directly scale your memcached cluster from CloudWatch. If it would be Redis, then application auto scaling would be the easy solution. Fortunately there is no problem that can not be solved with bit of custom code and Lambda function. On high-level it would go like this; CloudWatch alarm gets triggered -> CloudWatch Events rule matches the alarm -> Lambda function is triggered to add a node to the cluster. Similar case is described in below blog post.

https://aws.amazon.com/blogs/mt/customize-amazon-cloudwatch-alarm-notifications-to-your-local-time-zone-part-1/ https://docs.aws.amazon.com/autoscaling/application/userguide/what-is-application-auto-scaling.html

Ps. Adding nodes to a Memcached cluster increases the number of your cluster's partitions. When you change the number of partitions in a cluster some of your key spaces need to be remapped so that they are mapped to the right node. Remapping key spaces temporarily increases the number of cache misses on the cluster.