I asked this question on the Amazon Forums and apparently there is a recent bug in the creation of Alarms that automatically sets the "ActionsEnabled" property to False.
From AWS:
We have identified an issue in the AutoScaling console regarding the binding of
AutoScaling policies to CloudWatch alarms and are working on a fix. We will post
an update to this thread once the fix is rolled out. Thanks for bringing this to
our attention.
The workaround for now:
In the meantime, please try calling the DescribeAlarms CloudWatch API. If the
alarms associated with your policies have ActionsEnabled=false, then this could
cause your policies to not be invoked when the alarm is triggered. Please try
calling the PutMetricAlarm CloudWatch API to update ActionsEnabled=true for the
affected alarms, and that should fix the issues you are experiencing.
I have confirmed the bug and the workaround with my own Alarms through the API.
Let's go over your questions.
So basically I want my original instance to be running at all times. Then when it starts going over capacity I want the Auto Scaling Group to start launching instances and the Load Balancer to distribute the load across them. Is my thinking here sound?
I'd say yes, but I do have a couple reservations. If I understand correctly, you've placed your "main" instance outside of the auto scaling group. Theoretically, that would ensure that auto scaling doesn't kill off your original instance. There are a couple of things I'd like to mention:
- You're not making full use of the possibilities of Auto Scaling. Auto Scaling not only enables your setup to scale, but it can also ensure limits. If, for whatever reason, your "main" instance dies, your auto scaling policy won't come into action. If you keep your instance in an auto scaling group with a
min-size of 1, Auto Scaling automatically replaces the failed instance.
- When auto scaling, it's often best practise to treat your instances as being "disposable", because that's how you build resilient systems. Don't depend on one instance to always be available.
- You can set the termination policy for your auto scaling group so that it always terminates the newest instances first. That would ensure your "main" instance will be kept (as long as it's healthy). My previous comment still applies though.
When I make code and data changes to my original instance, do I have to remake the image my Launch Configuration uses?
I'd say no, but that's more of a design issue. Your image should describe the state of your server, but it shouldn't be responsible for code distribution. Consider a situation where you'd have to update your application because of an urgent bug, but your servers are under high load. Does updating your main server, creating an AMI, updating your launch config and killing off your auto scaled servers so they can be respawned with the latest AMI sound like an attractive solution? My answer to that would be no (again). Look into source code version control and deployment strategies (I'm a Rails developer 60% of the time and use git and capistrano, for instance).
There are situations where your approach would work as well and there is a lot of middle ground here (I would recommend also looking into Chef and userdata scripts). I myself actually rarely use custom AMIs, thanks to Chef.
What needs to be down with DNS names and IPs? I'm currently using Route 53, do I make that point to my Load Balancer and that's it?
Basically, yes. You can select the loadbalancer(s) that should be attached to new instances when creating your auto scaling group. I haven't used the GUI for Auto Scaling yet, but I'm quite sure it's in there somewhere. If not, the CLI still supports it. Point your Route53 record to your ELB alias and that's basically it.
Response to additional questions (2014/02/23):
If you're developing using Rails, I can highly recommend Capistrano for deployments, which can take a specific version of your app in your preferred version control system (like git) and deploy it to a number of servers in a specific environment. There are a bunch of tutorials out there, but Ryan Bates' revised (and pro) Railscasts on the subject are very concise, although you need a subscription to his website to watch both of them. Most of the other tutorials will get you going as well though. Fire up a new server with your AMI and a launch script that pulls a temporary clone of your git repo and runs a local Capistrano command to get your app going. This ensures that, later on, you can also deploy new versions of your application using Capistrano with just one command to all running servers.
Capistrano also provides a couple of other benefits, including enabling you to execute specific tasks on all or just one of your servers and roll back a deployment, which is very hard to accomplish using just git.
Best Answer
Cloudformation's AWS::CloudWatch::Alarm is useful for setting up machine-specific alarms. It's especially great combined with autoscaling, ELB, and EC2 instances. Here's a snippet used with an autoscaling group. Perhaps someone can suggest an edit to give an EC2-specific example.
Otherwise, you could roll something with Cloudinit and mon-enable-alarm-actions. Or OpsWorks using, uh, I don't know.