Enable abstract-leveldown stores to emit events for levelup

In some cases, an underlying store may change via some other external event, it would be nice if it were possible to capture this and utilize it in the levelup event emitter.

Example: browser localStorage can be updated on a different browser window; if we catch it we could potentially trigger a levelup.emit('put', key, val).

[vweevers]

But I hope this discussion can continue to consider this feature being an added part to abstract leveldown. As mentioned, there are certainly at least a few stores which could be extended to take advantage of this already.

Yes!

[u8sand]

Google Drive documents can be modified at any time, suppose we’re writing key/values to a database which is built out of a google drive document–say just a JSON text file, location refers to the name of the file. We can do batch writes and reads to that file and eventually flush it to the remote store (google drive).

But because it’s google drive, that file may have changed since the last time we looked at it, effectively causing some batch put/delete to our store (not by us directly, so we would not have called “put” or “del” explicitly)–if we can emit those events, our application will be aware of how our underlying store has mutated. It can then update whatever information used to be in it if keys were overwritten or deleted, etc…

The alternative to this is to constantly sync to an in-memory store of some kind, which may not be desirable if, for instance it’s too big to fit in memory. We can use things like cachedown to speed up these potentially slow remote dbs.