Cloudster

3 minute read

Practicing JQ, JMESPath and then discovering I didn’t need them

Intro

I was revisiting some of my old scripts, and found this messy piece of code that attempts to grab the latest “minimal” AWS AMI - HVM that’s EBS backed:

aws --query 'Images[*].[Name,ImageId]' \
  --output text \
  ec2 describe-images \
    --owners amazon \
    --filters \
      "Name=root-device-type,Values=ebs" \
      "Name=architecture,Values=x86_64" \
      "Name=virtualization-type,Values=hvm" \
      "Name=image-type,Values=machine" \
      "Name=is-public,Values=true" | grep minimal
        | sort | tail -n1 | awk '{print $2}'

I knew that there is a better way of doing this, but first I wanted to flex my JQ muscles before googling.

First attempt

One thing I forgot is to sanitize the “query” portion of the previously used command.

Bad command, notice the --query:

aws --query 'Images[*].[Name,ImageId]' \
  --output json \
  ec2 describe-images \
    --owners amazon \
    --filters \
      "Name=root-device-type,Values=ebs" \
      "Name=architecture,Values=x86_64" \
      "Name=virtualization-type,Values=hvm" \
      "Name=image-type,Values=machine" \
      "Name=is-public,Values=true"

Output I had to deal with:

[
  [
    "Windows_Server-2008-R2_SP1-English-64Bit-SQL_2012_RTM_SP2_Enterprise-2018.07.11",
    "ami-ffe1e514"
  ],
  [
    "amzn-ami-hvm-2016.03.2.x86_64-ebs",
    "ami-fff61890"
  ]
]

Increase of difficulty, for sure, but not impossible to filter the way I want. To re-iterate: I want a single, latest “minimal” Amazon Linux AMI.

This is what I came up with:

aws --query 'Images[*].[Name,ImageId]' \
  --output text \
  ec2 describe-images \
    --owners amazon \
    --filters \
      "Name=root-device-type,Values=ebs" \
      "Name=architecture,Values=x86_64" \
      "Name=virtualization-type,Values=hvm" \
      "Name=image-type,Values=machine" \
      "Name=is-public,Values=true" |
        jq -r '
          [
            .[] | select(.[0] | test("^amzn-ami-minimal-hvm")) |
            {
              ami: .[1],
              name: .[0],
              day: (.[0] | match("\\d{8}") | .string)
            }
          ] | sort_by(.day)[-1].ami
        '

Translation:

  1. Pick elements that pass the regex test over the name leaving us only with Amazon Linux images: select(.[0] | test("^amzn-ami-minimal-hvm"))
  2. Instead of list of lists, create a list of objects with k/v pairs.
  3. Every object of that list of objects will have a new property .day, which I will craft based on the image Name: day: (.[0] | match("\\d{8}") | .string). Value of the “day” property will have the YYYYMMDD format.
  4. Sort the remaining list by values of the newly created “day” property: sort_by(.day)
  5. Select only the value of the .ami property of the last element of the list: [-1].ami

This solves my task, but before I got to optimizing JQ, I noticed the malicious --query. I was wondering why the pre-JQ AWS CLI output was so sparse!

Second attempt

Removing the “malicious” --query option shows us that the output we have to deal with is substantial:

{
  "Images": [
    {
      "Architecture": "x86_64",
      "CreationDate": "2016-06-03T23:22:31.000Z",
      "ImageId": "ami-fff61890",
      "ImageLocation": "amazon/amzn-ami-hvm-2016.03.2.x86_64-ebs",
      "ImageType": "machine",
      "Public": true,
      "OwnerId": "137112412989",
      "State": "available",
      "BlockDeviceMappings": [
        {
          "DeviceName": "/dev/xvda",
          "Ebs": {
            "DeleteOnTermination": true,
            "SnapshotId": "snap-c70259f1",
            "VolumeSize": 8,
            "VolumeType": "standard",
            "Encrypted": false
          }
        }
      ],
      "Description": "Amazon Linux AMI 2016.03.2 x86_64 HVM EBS",
      "Hypervisor": "xen",
      "ImageOwnerAlias": "amazon",
      "Name": "amzn-ami-hvm-2016.03.2.x86_64-ebs",
      "RootDeviceName": "/dev/xvda",
      "RootDeviceType": "ebs",
      "SriovNetSupport": "simple",
      "VirtualizationType": "hvm"
    },
    ...
  ]
}

This allowed me to eliminate JQ. The only reason I previously used JQ was because of the ability to extract matches from the match() function. I am not aware whether JMESPath can do this. Let’s use a proper --query this time!

aws --output text \
  ec2 describe-images \
    --owners amazon \
    --filters \
      "Name=root-device-type,Values=ebs" \
      "Name=architecture,Values=x86_64" \
      "Name=virtualization-type,Values=hvm" \
      "Name=image-type,Values=machine" \
      "Name=is-public,Values=true" \
      --query '
        Images[?starts_with(ImageLocation,`amazon/amzn-ami-minimal-hvm-`) == `true`] |
        sort_by(@, &CreationDate)[-1:].ImageId
      '

Much better, but the execution time is still 5-ish seconds long. I do most of the processing after AWS returns a lot of results.

Google is your friend

The last thing to do was to check whether someone has done it better. The first result that came up was from an AWS blog post on how to do exactly what I was trying to do.

First thing that I was not aware of is that you can use wildcards in the --filters, so I improved my last iteration:

aws --output text \
  ec2 describe-images \
    --owners amazon \
    --filters \
      "Name=root-device-type,Values=ebs" \
      "Name=architecture,Values=x86_64" \
      "Name=virtualization-type,Values=hvm" \
      "Name=image-type,Values=machine" \
      "Name=is-public,Values=true" \
      "Name=name,Values=amzn-ami-minimal-hvm-*" \
      --query '
        sort_by(Images, &CreationDate)[-1:].ImageId
      '

Execution time is now sub-second, and the code is much cleaner. This is thanks to this line: "Name=name,Values=amzn-ami-minimal-hvm-*"

However, and this is the second thing I was not aware of, the blog post shows the most deterministic way to get the latest image as per my spec using SSM. That’s how I found out about the AWS SSM Parameter Store. Quite handy!

So, the command I ended up going with is this:

aws ssm get-parameter \
  --name /aws/service/ami-amazon-linux-latest/amzn-ami-minimal-hvm-x86_64-ebs \
  --query 'Parameter.Value' \
  --output text

Google is your friend, but you end up learning a lot by trying things out for yourself :)

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