Local / remote id mapping implementation

From TODO.txt:

I want to leave out the remote saving out of the Deploy module. We should
only care about the UUID and skip the whole cross communication to figure out
the remote IDs that happened in Deploy 6.x. So it's up to the remote to save
its entities based on the UUID. So we need to create a new module called
'services_uuid_entity'. That module will expose a REST service that saves
entities based on their UUID. The mapping is done locally on the site. And
for this to work the UUID module needs a function called 'uuid_entity_save()'
that can save entities based on their UUID. That function might need to be
abstracted to it's own module to depend on the Entity API module that
provides the proper 'entity_save()'.

So, the plan is to let the remote handle the content id mapping based on UUIDs?

While it sounds very nice, there are some problems I see in this approach:

1. You need this functionality implemented on the remote.
We'd need to write a plugin / module for every system we wanted to use and not only a REST / XMLRPC / ... Interface.
(Drupal, Wordpress, Yoomla, ...)
2. You need the full control over the remote to install the functionality.
We'd need to install the plugin / module.
3. It is not restful, as you can't decide whether it's an create or an update
- Create should issue something like: POST http://example.org/rest/node & data
- Updateshould issue something like: PUT http://example.org/rest/node/{nid} & data

That means we can't use other systems that implement a REST Interface without implementing a mapping mechanisme first and we can't use public APIs like Flickr or Youtube which require us to distinguish between a POST request for creating content and a PUT request for updating content.

My suggestion is to keep the local <-> remote id mapping in deploy itself as this reduces the dependencies on the remote and enables us to deploy to public APIs aswell.