How to access the event object with python in AWS Lambda?

Question

To follow up on this question: Filter CloudWatch Logs to extract Instance ID

I think it leaves the question incomplete because it does not say how to access the event object with python.

My goal is to:

read the instance that was triggered by a change in running state
get a tag value associated with the instance
start all other instances that have the same tag

The Cloudwatch trigger event is:

{
  "source": [
    "aws.ec2"
  ],
  "detail-type": [
    "EC2 Instance State-change Notification"
  ],
  "detail": {
    "state": [
      "running"
    ]
  }
}

I can see examples like this:

def lambda_handler(event, context):

    # here I want to get the instance tag value
    # and set the tag filter based on the instance that 
    # triggered the event

    filters = [{
            'Name': 'tag:StartGroup',
            'Values': ['startgroup1'] 
        },
        {
            'Name': 'instance-state-name', 
            'Values': ['running']
        }
    ]

    instances = ec2.instances.filter(Filters=filters)

I can see the event object but I don't see how to drill down into the tag of the instance that had it's state changed to running.

Please, what is the object attribute through which I can get a tag from the triggered instance?

I suspect it is something like:

myTag = event.details.instance-id.tags["startgroup1"]

John Rotenstein · Accepted Answer · 2017-09-14 22:37:26Z

1

The event data passed to Lambda contains the Instance ID.

You then need to call describe_tags() to retrieve a dictionary of the tags.

import boto3
client = boto3.client('ec2')

client.describe_tags(Filters=[
        {
            'Name': 'resource-id',
            'Values': [
                event['detail']['instance-id']
            ]
        }
    ]
)

answered Sep 14, 2017 at 22:37

John Rotenstein

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1 Comment

Chrisjx Over a year ago

John, I saw references to the client object in the boto3 docs but did not see the describe_tags method. I've posted my working (maybe not best) code. It works quickly although it throws an error when I test with a non-existent instance-id. I think that's OK, because I likely won't launch an Instance from the instance list that does not exist. We'll actually, I suppose it could be terminated, still in the list, and I accidentally select it. Oh, well.

John Rotenstein · Accepted Answer · 2017-09-14 22:27:29Z

0

In the Details Section of the Event, you will get the instance Id's. Using the instance id and AWS SDK you can query the tags. The following is the sample event

{
  "version": "0",
  "id": "ee376907-2647-4179-9203-343cfb3017a4",
  "detail-type": "EC2 Instance State-change Notification",
  "source": "aws.ec2",
  "account": "123456789012",
  "time": "2015-11-11T21:30:34Z",
  "region": "us-east-1",
  "resources": [
    "arn:aws:ec2:us-east-1:123456789012:instance/i-abcd1111"
  ],
  "detail": {
    "instance-id": "i-abcd1111",
    "state": "running"
  }
}

edited Sep 14, 2017 at 22:27

John Rotenstein

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answered Sep 14, 2017 at 18:28

Vaisakh PS

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1 Comment

Chrisjx Over a year ago

Thanks, Vaisakh. That actually helps a lot. I see the event object and the detail section, but please, how does one get to a specific tag, using python, for this instance-id. Is this a json parsing exercise? Or can this be done with object attributes? The tag value for the instance does not appear to be in the event object. Thanks, Chris.

Chrisjx · Accepted Answer · 2017-09-15 01:48:42Z

This is what I came up with...

Please let me know how it can be done better. Thanks for the help.

# StartMeUp_Instances_byOne
#
# This lambda script is triggered by a CloudWatch Event, startGroupByInstance.
# Every evening a separate lambda script is launched on a schedule to stop
# all non-essential instances.
# 
# This script will turn on all instances with a LaunchGroup tag that matches 
# a single instance which has been changed to the running state.
#
# To start all instances in a LaunchGroup, 
# start one of the instances in the LaunchGroup and wait about 5 minutes.
# 
# Costs to run: approx. $0.02/month
# https://s3.amazonaws.com/lambda-tools/pricing-calculator.html
# 150 executions per month * 128 MB Memory * 60000 ms Execution Time
# 
# Problems: talk to chrisj
# ======================================

# test system
# this is what the event object looks like (see below)
# it is configured in the test event object with a specific instance-id
# change that to test a different instance-id with a different LaunchGroup

# {  "version": "0",
#   "id": "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx",
#   "detail-type": "EC2 Instance State-change Notification",
#   "source": "aws.ec2",
#   "account": "999999999999999",
#   "time": "2015-11-11T21:30:34Z",
#   "region": "us-east-1",
#   "resources": [
#     "arn:aws:ec2:us-east-1:123456789012:instance/i-abcd1111"
#   ],
#   "detail": {
#     "instance-id": "i-0aad9474",  # <---------- chg this
#     "state": "running"
#   }
# }
# ======================================

import boto3
import logging
import json

ec2 = boto3.resource('ec2')

def get_instance_LaunchGroup(iid):
    # When given an instance ID as str e.g. 'i-1234567', 
    # return the instance LaunchGroup.
    ec2 = boto3.resource('ec2')
    ec2instance = ec2.Instance(iid)
    thisTag = ''
    for tags in ec2instance.tags:
        if tags["Key"] == 'LaunchGroup':
            thisTag = tags["Value"]
    return thisTag

# this is the entry point for the cloudwatch trigger
def lambda_handler(event, context):

    # get the instance id that triggered the event
    thisInstanceID = event['detail']['instance-id']
    print("instance-id: " + thisInstanceID)

    # get the LaunchGroup tag value of the thisInstanceID
    thisLaunchGroup = get_instance_LaunchGroup(thisInstanceID)
    print("LaunchGroup: " + thisLaunchGroup)
    if thisLaunchGroup == '':
        print("No LaunchGroup associated with this InstanceID - ending lambda function")
        return

    # set the filters
    filters = [{
            'Name': 'tag:LaunchGroup',
            'Values': [thisLaunchGroup] 
        },
        {
            'Name': 'instance-state-name', 
            'Values': ['stopped']
        }
    ]

    # get the instances based on the filter, thisLaunchGroup and stopped
    instances = ec2.instances.filter(Filters=filters)

    # get the stopped instance IDs
    stoppedInstances = [instance.id for instance in instances]

    # make sure there are some instances not already started
    if len(stoppedInstances) > 0:
        startingUp = ec2.instances.filter(InstanceIds=stoppedInstances).start()

    print ("Finished launching all instances for tag: " + thisLaunchGroup)

J Roysdon · Accepted Answer · 2019-09-20 20:33:21Z

0

So, here's how I got the tags in my Python code for my Lambda function.

  ec2 = boto3.resource('ec2')
  instance = ec2.Instance(instanceId)

# get image_id from instance-id
  imageId = instance.image_id
  print(imageId)

  for tags in instance.tags:
    if tags["Key"] == 'Name':
      newName = tags["Value"] + ".mydomain.com"
  print(newName)

So, using instance.tags and then checking the "Key" matching my Name tags and pulling out the "Value" for creating the FQDN (Fully Qualified Domain Name).

answered Sep 20, 2019 at 20:33

J Roysdon

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