这两周，我被分配的任务是实现 AWS 的 Auto Scaling功能。多亏有了 Boto， 很快就实现了创建 Auto Scaling Group 和添加 Scaling Policy。但是有一个问题却花费了一些时间才顺利解决。
我们的团队每周四发布新的代码。为了确保服务不中断，更新代码的步骤如下：
这样可以保证在 AWS 上运行的 Instance 依次得到更新，并且服务没有中断。
现在的问题是：使用了 Auto Scaling 之后，每次尝试改变 Auto Scaling Group 当中的 Instance 数目，都会激发 Scaling Policy。比如我想关闭一个旧的 Instance，这样 Auto Scaling Group 当中 Instance 的数量就会小于 Desired Capacity，Auto Scaling Group 就会新创建一个 Instance， 而不是等我激活一个 Instance 去替代。
我想到的办法步骤如下：
因为 AWS Auto Scaling 的特性（比如 Auto Scaling Group 被挂起的时候不能够添加 Instance，desired_capacity 不能小于 min_size），实现第三步并不容易。具体来说第三步又要分成下面几个步骤：
desired_capacity 和 min_size 分别置为 0（防止恢复 Auto Scaling Group 的时候自动创建 Instance）；min_size。下面提供一份示例代码以供参考：
#!python
import sys
import time
from boto.ec2.autoscale import AutoScaleConnection
from boto.ec2.connection import EC2Connection
AUTO_SCALING_GROUP = "test_auto_scaling_group"
NEW_INSTANCES = ["i-4acb0666", "i-4acbqa9"]
CHECK_INTERVAL = 5
def suspend(as_conn):
"""
Suspend auto scaling group
"""
as_conn.suspend_processes(AUTO_SCALING_GROUP)
print "Suspend Done\n"
def resume(as_conn):
"""
Resume auto scaling group
"""
as_conn.resume_processes(AUTO_SCALING_GROUP)
print "Resume Done\n"
def get_instance_by_id(ec2conn, inst_id):
"""
Get instance by its id
"""
...
pass
def deactivate_instance(inst):
"""
Deactivate instance
"""
...
pass
def delete_instances(as_conn, ec2conn):
"""
Delete instances in auto scaling group
"""
as_group = as_conn.get_all_groups(names=[AUTO_SCALING_GROUP])[0]
instances = [get_instance_by_id(ec2conn, i.instance_id) for i in as_group.instances]
for instance in instances:
if instance.state != "terminated":
print "*** Terminate Instance %s" % instance.id
deactivate_instance(instance)
# All instances should be terminated
while any([inst.update() != "terminated" for inst in instances]):
sys.stdout.write(".")
sys.stdout.flush()
time.sleep(CHECK_INTERVAL)
print ".\nDelete Done\n"
def attach_instances(as_conn):
"""
Attach instances into auto scaling group
"""
while True and NEW_INSTANCES:
as_group = as_conn.get_all_groups(names=[AUTO_SCALING_GROUP])[0]
if len(as_group.instances) == 0:
as_conn.attach_instances(AUTO_SCALING_GROUP, NEW_INSTANCES)
print "Attach Done\n"
return
else:
print "There are instances still in the group"
time.sleep(CHECK_INTERVAL)
def set_capacity(as_conn):
"""
Set desired capacity and minimum size as 0
"""
as_group = as_conn.get_all_groups(names=[AUTO_SCALING_GROUP])[0]
as_group.min_size = 0
as_group.desired_capacity = 0
as_group.update()
print "Set Capacity Done\n"
def resume_capacity(as_conn):
"""
Resume minimum size as the original value
"""
as_group = as_conn.get_all_groups(names=[AUTO_SCALING_GROUP])[0]
setattr(as_group,"min_size", 1)
as_group.update()
print "Resume Capacity Done\n"
def main():
as_conn = AutoScaleConnection()
ec2conn = EC2Connection()
print "Suspend Auto Scaling Group"
suspend(as_conn)
print "Delete Instances"
delete_instances(as_conn, ec2conn)
print "Set Capacity"
set_capacity(as_conn)
print "Resume Auto Scaling Group"
resume(as_conn)
print "Attach Instances"
attach_instances(as_conn)
print "Resume Capacity"
resume_capacity(as_conn)
if __name__ == "__main__":
main()