Open Stack

On Fri, Sep 04, 2015 at 04:20:23PM EDT, Kevin Benton wrote:
> Right, it depends on your perspective of who 'owns' the API. Is it
> cloud-init or EC2?
> 
> At this point I would argue that cloud-init is in control because it would
> be a large undertaking to switch all of the AMI's on Amazon to something
> else. However, I know Sean disagrees with me on this point so I'll let him
> reply here.


Here's my take:

Cloud-Init is a *client* of the Metadata API. The OpenStack Metadata API
in both the Neutron and Nova projects should all the details of the
Metadata API that is documented at:

http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-metadata.html

This means that this is a compatibility layer that OpenStack has
implemented so that users can use appliances, applications, and
operating system images in both Amazon EC2 and an OpenStack environment.

Yes, we can make changes to cloud-init. However, there is no guarantee
that all users of the Metadata API are exclusively using cloud-init as
their client. It is highly unlikely that people are rolling their own
Metadata API clients, but it's a contract we've made with users. This
includes transport level details like the IP address that the service
listens on. 

The Metadata API is an established API that Amazon introduced years ago,
and we shouldn't be "improving" APIs that we don't control. If Amazon
were to introduce IPv6 support the Metadata API tomorrow, we would
naturally implement it exactly the way they implemented it in EC2. We'd
honor the contract that Amazon made with its users, in our Metadata API,
since it is a compatibility layer.

However, since they haven't defined transport level details of the
Metadata API, regarding IPv6 - we can't take it upon ourselves to pick a
solution. It is not our API.

The nice thing about config-drive is that we've created a new mechanism
for bootstrapping instances - by replacing the transport level details
of the API. Rather than being a link-local address that instances access
over HTTP, it's a device that guests can mount and read. The actual
contents of the drive may have a similar schema as the Metadata API, but
I think at this point we've made enough of a differentiation between the
EC2 Metadata API and config-drive that I believe the contents of the
actual drive that the instance mounts can be changed without breaking
user expectations - since config-drive was developed by the OpenStack
community. The point being that we call it "config-drive" in
conversation and our docs. Users understand that config-drive is a
different feature.

I've had this same conversation about the Security Group API that we
have. We've named it the same thing as the Amazon API, but then went and
made all the fields different, inexplicably. Thankfully, it's just the
names of the fields, rather than being huge conceptual changes.

http://lists.openstack.org/pipermail/openstack-dev/2015-June/068319.html

Basically, I believe that OpenStack should create APIs that are
community driven and owned, and that we should only emulate
non-community APIs where appropriate, and explicitly state that we only
are emulating them. Putting improvements in APIs that came from
somewhere else, instead of creating new OpenStack branded APIs is a lost
opportunity to differentiate OpenStack from other projects, as well as
Amazon AWS.

Thanks for reading, and have a great holiday.

-- 
Sean M. Collins