AWS Application Load Balancer-friendly Fabric Deploys using boto3

fabfile.py

from awsfabrictasks.ec2.api import *
from fabric.api import *
from functools import wraps

import boto3
import time


def aws_hosts(load_balancers):
    
    instance_ids = []
    
    session = boto3.Session(profile_name=env.aws_profile_name)
    alb_client = session.client('elbv2')

    load_balancers = alb_client.describe_load_balancers(Names=env.load_balancer_name)
    load_balancer_arn = load_balancers['LoadBalancers'][0]['LoadBalancerArn']

    target_groups = alb_client.describe_target_groups(LoadBalancerArn=load_balancer_arn)
    target_group_arn = target_groups['TargetGroups'][0]['TargetGroupArn']

    target_health = alb_client.describe_target_health(TargetGroupArn=target_group_arn)
    instance_ids = map(lambda t: t['Target']['Id'], target_health['TargetHealthDescriptions'])

    if not instance_ids:
        raise ValueError('No instances found! I\'m getting out of here.')

    ec2_client = session.client('ec2')
    reservations = ec2_client.describe_instances(InstanceIds=instance_ids)

    ec2_hosts = {}
    for reservation in reservations['Reservations']:
        for i in reservation['Instances']:
            ec2_hosts[i['PublicDnsName']] = i['InstanceId']

    return ec2_hosts


def elb_operation(operation, instance_id, load_balancer_names):
    session = boto3.Session(profile_name=env.aws_profile_name)
    alb_client = session.client('elbv2')


    load_balancers = alb_client.describe_load_balancers(Names=load_balancer_names)
    load_balancer_arn = load_balancers['LoadBalancers'][0]['LoadBalancerArn']

    target_groups = alb_client.describe_target_groups(LoadBalancerArn=load_balancer_arn)
    target_group_arn = target_groups['TargetGroups'][0]['TargetGroupArn']

    return getattr(alb_client, operation)(TargetGroupArn=target_group_arn, Targets=[{'Id': instance_id}])


def remove_from_elbs():
    print "Removing %s from the load balancer..." % env.aws_hosts[env.host]
    elb_operation('deregister_targets', env.aws_hosts[env.host], env.load_balancer_name)
    time.sleep(5)    # allow connections to fully drain


def add_to_elbs():
    print "Adding %s back to the load balancer..." % env.aws_hosts[env.host]
    elb_operation('register_targets', env.aws_hosts[env.host], env.load_balancer_name)
    time.sleep(15)   # generous to prevent cascading effect of removing machines from ELB


def elb_managed(func):
    @wraps(func)
    def decorated(*args, **kwargs):
        
        if env.load_balancer_name:
            remove_from_elbs()

        func(*args, **kwargs)
        
        if env.load_balancer_name:
            add_to_elbs()
            
    return decorated
  
  
def production():
    env.load_balancer_name = ['your-load-balancer-name']
    env.aws_profile_name = 'your-aws-profile-name'
    env.aws_hosts = aws_hosts(env.load_balancer_name)
    env.hosts = env.aws_hosts.keys()

    
@elb_managed
def deploy():
    # ???
    # profit