Powered by Memcached

Their announcement doesn’t provide too many details about the inner workings of the ElastiCache, but it appears the initial beta is some form of memcached 1.4.5. It’s entirely possible that they have made modifications.

There may be future additions of different engines, such as Redis, but AWS has not provided any indications of this happening in the immediate future.

Unlike some of the recent AWS feature launches, ElastiCache sports a tab on the AWS Management Console, which is great. Administering your clusters looks to be dead simple.

Performance and benchmarks

It’s still pretty early, and I haven’t been able to find any benchmarks comparing an EC2 instance with Memcached to ElastiCache. However, keep a few things in mind:

• You can deploy your ElastiCache to single or multiple availailibty zones. Instances within the same zone can communicate with ElastiCache just as fast as they can to neighboring EC2 instances.
• It is likely that like RDS, Amazon has tweaked the instances that run ElastiCache specifically for this workload.

That said, my uneducated guess would be that performance between a properly configured EC2 instance and ElastiCache will be negliglble. But that’s not what they’re selling you on. You’re paying the premium for not having to manage that part of your stack.

Usage case

For sites with low to moderate traffic, running a dedicated memcached instance may not be worth the money, or an increase in your service’s footprint. It is often fine to run a memcached server on the same instance as your app server. Memcached is great about not using much CPU, but it will happily consume as much RAM as you allow it to.

As your site grows, you may find that memcached actively uses RAM that your app server needs, causing resource contention. Worse case, you may find yourself hitting swap space. This can be absolute death for even moderately trafficked sites.

For those that are finding their memcached process needing more and more RAM to satiate your application’s needs, it’s probably time to explore breaking your caching out into its own EC2 instance, or ElastiCache.

Price

ElastiCache clusters come in sizes similar to EC2 instances: Small, Large, XL, etc. On the lower end, the monthly price for a single-node, Small On-Demand ElastiCache cluster is about $70. To serve as a basis for comparison, a Small On-Demand EC2 instance costs about $63. For those that actually use On-Demand instances over prolonged periods of time, the difference is negligible. You’re paying a $7/month premium to not have to hassle with administering your memcached instance.

However, the price difference for Reserved instances vs. On-Demand ElastiCache is large. Let’s say you use a Small Reserved EC2 instance. By the time you take into account the up-front reservation fee and the reduced monthly fee, you’re looking at around $41/month for a 1-year term. Since ElastiCache currently has no Reservation capability like EC2 yet, this leaves us with a marked difference in price points. A single-node, Small On-Demand ElastiCache would be $756 for the whole year, vs. $491 for the Small Reserved EC2 instance with a 1-year term. That’s a $265 savings for doing it yourself and shelling out the up-front reservation fee.

Cheaper alternatives

For those that find the price for ElastiCache a little too high, there are a few different options that might be more affordable:

• Run memcached on one of your current instances, or on the same instance as the app server. This assumes lower load/traffic. Make sure that you aren’t running out of RAM.
• Run memcached on an On-Demand Micro instance. This is one of the few useful tasks I’ve found for Micro instances. Recall that memcached is extremely easy on the CPU, and Micro instances have around 613 MB to play with. This would cost around $15/month On-Demand (less with a Reserved instance). If you are free-tier eligible, this route would be free for a year, if you haven’t already fired up another Micro instance.
• If you can afford the up-front fee, a Reserved Small EC2 instance is powerful enough to handle even well-trafficked sites. You could pay a little more and get two Small Reserved EC2 instances for a moderate bit more than a single-node Small ElastiCache cluster.

The verdict

I love that Amazon is adding this, and think it’ll be a competitive service eventually. ElastiCache is currently in beta, and the prices are higher while they’re figuring out their costs and margins. The lack of a Reserved ElastiCache cluster option is unfortunately the killer. At the higher ends in particular, On-Demand ElastiCache is a good deal more expensive than the equivalent  Reserved EC2 instance.

I strongly suggest waiting for the prices to come down, and for the ability to purchase Reserved ElastiCache clusters. As it stands, this service isn’t cost-effective yet for the benefit.