Detecting which cloud you are in

[Jeffrey O'Neill]

Let's say you have a GAE Python 2.7 standard environment app running in GAE, AWS, and Azure, and part of your code depends on which cloud you are running in.  

For example, you need something like this:

    if cloud == "GAE":

        do_gae_stuff()

    elif cloud == "AWS":

        do_aws_stuff()

    elif cloud == "Azure":

        do_azure_stuff()

    else:

        assert False

How do you detect which cloud you are in so you can do the above?  

[Chris Donati]

We don't have an API that will do this at the platform level, but your application can guess the infrastructure with something like the following:

from google.appengine.api import urlfetch

METADATA_URLS = {'GCE': 'http://metadata.google.internal',
                 'EC2': 'http://169.254.169.254/latest/meta-data/',
                 'Azure': 'http://169.254.169.254/metadata/v1/InstanceInfo'}

def guess_infrastructure():
    for infrastructure, url in METADATA_URLS.items():
        try:
            response = urlfetch.fetch(url)
        except urlfetch.DownloadError:
            continue

        if response.status_code != 200:
            continue

        return infrastructure

    return None

The performance of the above can be improved with async parallel fetches, short timeouts, etc. However, you probably only need to run it once on startup (maybe in your appengine_config.py) and cache the result.

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