Replacing EC2 On-Demand Instances With New Spot Instances

with an SMS text warning two minutes before interruption, using CloudWatch Events Rules And SNS

The EC2 Spot instance marketplace has had a number of enhancements in the last couple months that have made it more attractive for more use cases. Improvements include:

  • You can run an instance like you normally do for on-demand instances and add one option to make it a Spot instance! The instance starts up immediately if your bid price is sufficient given spot market conditions, and will generally cost much less than on-demand.

  • Spot price volatility has been significantly reduced. Spot prices are now based on long-term trends in supply and demand instead of hour-to-hour bidding wars. This means that instances are much less likely to be interrupted because of short-term spikes in Spot prices, leading to much longer running instances on average.

  • You no longer have to specify a bid price. The Spot Request will default to the instance type’s on-demand price in that region. This saves looking up pricing information and is a reasonable default if you are using Spot to save money over on-demand.

  • CloudWatch Events can now send a two-minute warning before a Spot instance is interrupted, through email, text, AWS Lambda, and more.

Putting these all together makes it easy to take instances you formerly ran on-demand and add an option to turn them into new Spot instances. They are much less likely to be interrupted than with the old spot market, and you can save a little to a lot in hourly costs, depending on the instance type, region, and availability zone.

Plus, you can get a warning a couple minutes before the instance is interrupted, giving you a chance to save work or launch an alternative. This warning could be handled by code (e.g., AWS Lambda) but this article is going to show how to get the warning by email and by SMS text message to your phone.

WARNING!

You should not run a Spot instance unless you can withstand having the instance stopped for a while from time to time.

Make sure you can easily start a replacement instance if the Spot instance is stopped or terminated. This probably includes regularly storing important data outside of the Spot instance (e.g., S3).

You cannot currently re-start a stopped or hibernated Spot instance manually, though the Spot market may re-start it automatically if you configured it with interruption behavior “stop” (or “hibernate”) and if the Spot price comes back down below your max bid.

If you can live with these conditions and risks, then perhaps give this approach a try.

Start An EC2 Instance With A Spot Request

An aws-cli command to launch an EC2 instance can be turned into a Spot Request by adding a single parameter: --instance-market-options ...

The option parameters we will use do not specify a max bid, so it defaults to the on-demand price for the instance type in the region. We specify “stop” and “persistent” so that the instance will be restarted automatically if it is interrupted temporarily by a rising Spot market price that then comes back down.

Adjust the following options to suite. The important part for this example is the instance market options.

ami_id=ami-c62eaabe # Ubuntu 16.04 LTS Xenial HVM EBS us-west-2 (as of post date)
region=us-west-2
instance_type=t2.small
instance_market_options="MarketType='spot',SpotOptions={InstanceInterruptionBehavior='stop',SpotInstanceType='persistent'}"
instance_name="Temporary Demo $(date +'%Y-%m-%d %H:%M')"

instance_id=$(aws ec2 run-instances \
  --region "$region" \
  --instance-type "$instance_type" \
  --image-id "$ami_id" \
  --instance-market-options "$instance_market_options" \
  --tag-specifications \
    'ResourceType=instance,Tags=[{Key="Name",Value="'"$instance_name"'"}]' \
  --output text \
  --query 'Instances[*].InstanceId')
echo instance_id=$instance_id

Other options can be added as desired. For example, specify an ssh key for the instance with an option like:

  --key $USER

and a user-data script with:

  --user-data file:///path/to/user-data-script.sh

If there is capacity, the instance will launch immediately and be available quickly. It can be used like any other instance that is launched outside of the Spot market. However, this instance has the risk of being stopped, so make sure you are prepared for this.

The next section presents a way to get the early warning before the instance is interrupted.

Email Alerts for AWS Billing Alarms

using CloudWatch and SNS to send yourself email messages when AWS costs accrue past limits you define

The Amazon documentation describes how to use the AWS console to monitor your estimated charges using Amazon CloudWatch and includes some pointers for folks using the command line. Unfortunately, that article leaves out the commands to set up the SNS (Simple Notification Service) topics and SNS subscriptions, so I present here the complete steps I use.

I like using the command line tools as they let me automate and repeat actions without having to do lots of pointing, clicking, and re-entering data. For example, I want to set up a number of billing alerts in each new account, sometimes at $10 increments, and sometimes at $100 or $1000 increments. The steps below let me do this in seconds with a simple copy and paste.