How to setup cloudwatch alarm for beanstalk environment memory

What I understand from your question are the following requirements:

1. You have multiple metrics and want to use a logical OR condition when configuring an alarm, e.g. (avg metric1 > x || avg metric2 > y) ==> set alarm state to ALARM
2. You want the alarm to consider new metrics as they become available when new instances are launched by elastic beanstalk during scale out.
3. You want old metrics to not be considered as soon as elastic beanstalk scales in.

I think this is currently not possible.
There is an ongoing discussion on aws discussion forums [1] which reveals that at least (1) is possible using Metric Math. The Metric Math feature supports max. 10 metrics.

Solution

What you need to do is, to create a single metric which transports the information whether the alarm should be triggered or not ('computed metric'). There are multiple ways to achieve this:

• For complex metrics you could write a bash script and run it on an EC2 instance using cron. The script would first query existing metrics using a dimension filter ('list-metrics'), then gather each metric ('get-metric-data'), aggregate it and then push the computed metric data point ('put-metric-data').
• If the metric is rather simple, you could try the aggregate option of the AWS put-metric-data script [2]:

option_settings:
  "aws:elasticbeanstalk:customoption" :
    CloudWatchMetrics : "--mem-util --mem-used --mem-avail --disk-space-util --disk-space-used --disk-space-avail --disk-path=/ --auto-scaling --aggregated"

The documentation for the aggregated option says:

Adds aggregated metrics for instance type, AMI ID, and overall for the region.

References

[1] https://forums.aws.amazon.com/thread.jspa?threadID=94984
[2] https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/mon-scripts.html#put-metric-data

In the Elastic Beanstalk console for your environment:

1. Click the Monitoring link in the left-hand side navigation links.
2. Underneath the Overview, in the Monitoring section, click the Edit button.
3. Choose AWSEBAutoScalingGroup for the Resource.
4. Choose MemoryUtilization under CloudWatch Metric.
5. Modify Statistic and Description as desired.
6. Click the Add button, and then click the Save button in the Monitoring section.
7. Scroll down to find the new panel that was added. Click the bell icon in the upper right hand corner of the panel. This will take you to the settings to set up a new alarm.

If you do not see the MemoryUtilization metric available, verify that you have correctly set up the collection of the memory metrics.

Cloudwatch cannot create alarms in a generic way. There are only 2 ways to accomplish the task.

1) Create a startup script in your AMI. When a new instance is launched, it is responsible for its own Cloudwatch alarms. I used this a long time ago, and the approach is solid. However, running scripts on termination isn't reliable, so you'll have to periodically clean out the old alarms.

2) Use a tool that has decent capabilities (ahem.... not Cloudwatch). I recommend Blue Matador. With them, you don't even have to setup the alarms or thresholds, the machine learning automatically baselines your resources and creates alerts for you.

If you got here and don't know Beanstalk or Cloudwatch well enough to contribute, start here: How to Monitor AWS Elastic Beanstalk with CloudWatch