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Redis – How to get public_dns_name according to ec2 resource instance?

Now I want to get the public_dns_name, and then log on to an instance of aws to get the CPU utilization. I know that I can check it with cloudwatch, but now I don’t want to rely on the function provided by amazon to check it directly based on Linux. How can I get it?

ec2 = boto3.resource('ec2',
                     aws_access_key_id=settings.AWS.get("aws_access_key_id"),
                     aws_secret_access_key=settings.AWS.get("aws_secret_access_key"),
                     region_name=settings.AWS.get("region_name"))

user_data_script = """#cloud-config
   runcmd:
    - sudo systemctl start docker
    - cd /srv/app/kfquantum
    - sudo docker run --env broker=%s 
                      --env result=redis://10.0.0.245:6379/0 
                      --env host=kf-db.cluster-crietao0wrdu.rds.cn-northwest-1.amazonaws.com.cn 
                      --env user=db 
                      --env pw=y6y83aGagY8hgXZ 
                      -w /kfquantum -d --restart unless-stopped --name kf-celery kf-celery 
                      celery -A kfquantum worker --pool=solo -l info
   """ % settings.BROKER_URL
instance_type = "c5.large"
instance = ec2.create_instances(
    BlockDeviceMappings=[
        {
            "DeviceName": "/dev/xvda",
            'Ebs': {
                'DeleteOnTermination': True,
                'VolumeSize': 8
            },
        },
    ],
    ImageId="ami-09feea664bfc54194",
    MinCount=1,
    MaxCount=1,
    Placement={'AvailabilityZone': 'cn-northwest-1a'},
    InstanceType=instance_type,
    UserData=user_data_script,
    NetworkInterfaces=[
        {
            'DeviceIndex': 0,
            'SubnetId': "subnet-093161e8b00deb367",
            'AssociatePublicIpAddress': True
        },
    ]
)
instance = instance[0]
print("1",instance.private_ip_address)
print("2",instance.public_dns_name)

Console

1,10.0.0.73
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2

Answers


  1. 0

    The value for public_dns_name is not assigned yet.

    This could be due to

    1. The instance is not running yet. You are trying to fetch the DNS name as soon as the create_instances method is invoked. It takes a few seconds for the instance to reach running state. The public_dns_name is not available until the instance reaches running state. Check for the instance state and wait for it to be running before trying to get the Public DNS Name.

    An easy fix without disturbing the rest of your code would be,

    Update:
    The instance object must be reloaded after the waiter for the updated instance attributes to be detected.

    instance = instance[0]
instance.wait_until_running()
instance.reload()
print("1",instance.private_ip_address)
print("2",instance.public_dns_name)
    1. DNS hostnames are not enabled in your VPC.
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  2. 0

    Try this…

    instance= instance[0]
print('***Success!! instance:', instance.id, 'created, and instance-state-name:', instance.state['Name'])           

instance.wait_until_running()           
instance.reload()

print('***instance:', instance.id, 'is now up, and instance-state-name:', instance.state['Name'])
print('***private ipa:', instance.private_ip_address)
print('***public ipa:', instance.public_dns_name)

    Hope it helps…
    r0ck

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