<strike>`name` is guaranteed to be constant.  So is `site_url`*.

Use one of them.

`/users/{id}/associated` is not vectorized (as a part of stackauth and not api.* it doesn't follow the "all {id}'s are vectors" rule) and thus you should be able to keep track of that information yourself easily.

<sub>*There *might* be some goofiness with area51 created sites in this regard, we'll keep you posted when the details shake out.</sub></strike>

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Update.

The proper way** to lookup site data is to key off of the current domain (http://stackoverflow.com, for instance, `site_url` basically) and resolve old/cached entries using `aliases`.

Its probably going to be months from now before any site returns anything in `aliases` - whenever WebApps becomes... not WebApps, assuming it ever does - buts its spec'd.

<sub>**As opposed to synthesizing some "key" and trying to hold it constant in the face of changing site data.  Pro-tip, this is how the SO engine works!</sub>