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Understanding the Keystone Service Catalog

Keystone is the authentication and identity service for the OpenStack cloud services. To handle the authentication chores, it provides a specialized API; to authenticate, the user sends a POST request to an authentication URL, sending the username and password (or API key), and Keystone returns an access token along with a service catalog that lists all of the services that are available to the user. Because authentication can be expensive, the access token can be used (for a limited period of time) to access any of the supported services without the need to hit the authentication service for each request. For the purposes of this post, I’m going to use the actual service catalog returned for my account on the Rackspace cloud (with the security-critical components redacted, of course).

This is a JSON object; at its highest level, it has three elements:

The “access” element (line 2)

This has one sub-element, token, which has three sub-elements: id, expires, and tenant.

The “serviceCatalog” element (line 11)

This is the service catalog proper; it consists of an array of objects identified as “endpoints”:

Here are the key elements of the “endpoints” object:

Now, this is the important point: to know which service to use, you must be able to identify it by the region (“DFW”), the type (“compute”), and the name (“cloudServersOpenStack”). This is because there may be multiple instances of the same service type; in the larger example, you’ll see that there are two “compute” services, one named “cloudServers” and the other “cloudServersOpenStack.” There can also be multiple regions that support the same service. You’ll need to have documentation from your provider as to what values to use. At Rackspace, “cloudServers” refers to our first-generation Cloud Servers product, while “cloudServersOpenStack” is the next-generation Cloud Servers built using the OpenStack Nova product. How you retrieve the exact URL you need, of course, depends on the programming language you’re using. In PHP, for example, you can use the json_decode() function to convert the JSON into a PHP object. However, PHP doesn’t really have an easy way to represent anonymous array elements, so it turns them into a regular (numerically-indexed) array that you must iterate over to find the right endpoint.

Glen Campbell
June 4, 2012