TL;DR: I think we've already partially solved this with the "datastore" concept. A little bit more work will be required for a reliable message queue (see arguments below), but this should be easier and safer than using a shared read/write database.
I think there are two ways to solve this: at the storage layer, and at the application layer.
The storage layer solution is to design a multi-reader multi-writer distributed storage system that all application nodes (i.e. Alice's computer, cloud provider X's client, cloud provider Y's client) can use for a shared storage medium. It would either provide some well-defined consistency model that all applications would simply need to be aware of (the CouchDB/PouchDB approach), or the storage system itself would need to be programmable so the application could get exactly the consistency/durability semantics it needs (the Syndicate approach).
The advantage of the storage layer approach is that we solve the consistency problem once, for everyone. The disadvantages are that it's going to amount to a lot of complexity in the lower layers of the stack, and it will likely lead to implicit tight coupling between applications and the implementation (in particular, the storage system's implementation's consistency model will dictate how we build applications that use it). I say "implementation" instead of "specification" here because it is notoriously difficult to implement the desired consistency model for a spec (history has shown we can't even do it for the lax POSIX consistency model for filesystems, let alone distributed storage systems).
The tight coupling concern can be mitigated with wide-area software-defined storage, where the application may opt to prescribe behaviors to application nodes that act on the storage system's data plane (and in doing so, enforce its own data consistency, durability, access controls, etc. independent of other applications). However, such systems are still in their infancy, and have complex specifications and complex implementations as it is.
The other strategy (and the one I prefer) is an application layer solution. Each application node maintains its own data store that only it can write to, but anyone can read. An application node does a little bit of extra processing to construct a consistent view over the set of other nodes' data.
In most cases, including this example of choosing your favorite color, this is very simple (which is why I like it). Suppose that an application wants to know what Alice's latest color preference is. Suppose also that the application records Alice's favorite color to a well-defined path in a datastore (e.g.
/favorites/color). Alice updates her favorite color on her personal datastore. The application nodes in cloud X and cloud Y periodically refresh
/favorites/color from each other and from Alice's datastore, and in the event of conflicting values, they take the value that was written last. Eventually, in the absence of writes, they all see the latest
/favorites/color. It is easy to see how more advanced write-conflict-resolution algorithms can be applied as well.
As an optimization, we can extend the per-node datastore to have a per-record pub/sub interface. This can be achieved simply by giving each node a publicly-routable reliable message queue, and by having the client publish each mutation operation (create, update, delete) to it. Other application nodes can subscribe to this queue to get informed as to when certain mutations happen. In our example, Alice's datastore client would send a "write" message to her message queue, which cloud X and cloud Y have already subscribed to. Cloud X and cloud Y's application nodes would each receive the "write" message on
/favorites/color, and immediately refresh
/favorites/color from Alice's storage provider. Of course, the message queue would be designed so that messages are end-to-end confidential and authenticated (so Alice could use any queue provider without having to trust it).
Here's a (badly-drawn) figure that tries to show how Alice, cloud X, and cloud Y work together with their respective storage providers and Alice's message queue:
Red paths are instigated by Alice; blue paths by cloud X; green paths by cloud Y. Arrows show the flow of bytes. The clouds that hold the data are just commodity cloud storage providers, and the clients (circles) implement a simple one-writer-many-reader filesystem client (i.e. cloud X can use his client to read Alice's data from her storage provider).
Two key advantages of the application layer approach is that we keep both consistency and security easy to reason about. Only Alice can write to her storage provider, and only Alice can send messages via her queue (similar for cloud X and Y's data storage). However, anyone who can decrypt Alice's messages and data can read them (i.e. Alice would grant cloud X and cloud Y the ability to do so via a bootstrapping step). This is nice because it means the application developer doesn't have to think about storage edge-cases that come with dealing with shared read/write media. At the same time, this approach is probably more secure in practice, because from the get-go we require that only the data store's owner can write (whereas if we took a shared read/write approach, we'd have to be extra careful that the necessarily complex implementation still ensured that only the data owner could write to their data).
The other less-obvious advantage of the application-layer approach is that it works behind NATs. The clients for Alice, cloud X, and cloud Y do not need to be publicly routable; only the storage providers and message queue providers do. Also, the clients do not have to be online 24/7; they can simply consume their pending messages once they come back online (but before doing anything else). Alice could have a personal computer at home that operates on her data, for example.
Insofar as how to bootstrap this, Alice's, cloud X's, and cloud Y's clients would each discover each other's data store providers (e.g. Amazon S3, Dropbox, etc.) and message queue providers (e.g. a ZMQ instance running an a VM) via each other's zonefiles. Alice would grant cloud X and cloud Y the ability to read her messages by sharing a secret with them using cloud X's and cloud Y's public keys obtained by her Blockstack node. Similarly, cloud X and cloud Y would get Alice's public key, so they could verify messages and data written by her. If anyone changed their storage providers or queue providers, the others could easily find out by checking each other's zonefiles with their local Blockstack nodes.
We have already implemented the data store client functionality in the rc-0.14.1 branch. I still have to create the reliable message queue functionality.