I occasionally run into an issue with the ec2_elb module that I think needs
to be addressed. This module is used to add and remove Amazon EC2
instances to/from an Amazon load balancer. The API into Amazon is
asynchronous, so when you use it to add an instance to a load balancer it
issues a "insert" command then it goes into a loop where it sleeps for a
second, checks the status, and continues to loop until the instance reaches
an "In Service" state in the load balancer. There's also a check to see if
the instance enters an error state, and if it does then the module
immediately returns an error. Here's a bit of pseudocode to demonstrate the
behavior:
while True:
get_instance_state
if InService:
return success;
else if instance_error:
return error;
sleep 1
So when an instance is added to the load balancer, the module waits until
it reaches the InService state. If at any point along that way it enters
an error state then it immediately fails.
The problem I have is that it's not terribly uncommon for an instance to
enter a transient unhealthy state for a couple of seconds prior to being
successfully put into service. I have on a number of occasions had my
Ansible playbook fail because the ec2_elb module throws an error and yet
the EC2 instance is successfully put into service in the load balancer. If
the module had simply waited a few more seconds to check on the health of
the instance then my playbook would have run successfully.
I would like to propose making a change to the ec2_elb module to address
these sorts of transient errors. There really should be a timeout
associated with this while loop in the module. It should only fail if the
instance is not put into service during that period of time, and any errors
that occur within that time period should be ignored. To maintain the
current state it shouldn't be too difficult to add an optional timeout
parameter that changes the behavior only if it is set. So if a timeout
parameter is added then the above loop woudl look something like this:
while not timeout_exceeded:
get_instance_state
if InService:
return success;
else if instance_error AND timeout_exceeded:
return error;
sleep 1
return timeout
Any comments/suggestions about this, especially from other folks using the
ec2_elb module?
-Bruce
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