expose isEmbedded property

[dominictarr]

expose an isEmbedded property, realized that this can be a property, not a function.

Level/levelup#279

[mcollina]

👍 for the property.

[nolanlawson]

Are we sure this can be a property? In the case of sqlDOWN, I believe it can work with Postgres/MySQL/SQLite/WebSQL, i.e. a mix of embedded and non-embedded. @calvinmetcalf can you confirm that this would be okay?

Also, let's please open a PR or an issue on *down libraries to remind them to add this. I'm guessing the majority of *down libraries are embedded, so it's kind of unfortunate that we can't reverse the semantics here, but I cannot think of a good antonym for "embedded," so here we are.

Off the top of my head:

• sqlDOWN
• localstorage-down
• memdown
• level variants (rocksdb, basho's fork)
• jsondown
• level.js and the other IndexedDB one
• medeadown

It'd be nice to all be on the same page about this.

[calvinmetcalf]

if it is to be a property of a db instance that that isn't a problem for sqldown to do

[calvinmetcalf]

ah but an issue for sqldown, assuming by isEmbedded you mean single process (e.g. sole connection to db) then that could be tricky, for instance sqlite3 is embedded but multiple processes can attach to it (e.g. wouldn't throw an error if you used it with cluster) but on the other hand @maxogden is using sqldown with postgres due to heroku's ephemeral fire structure, in other words like a remote embeded db.

[rvagg]

I think this should be done in abstract-leveldown first as a conformance test, as per Level/levelup#279

[dominictarr]

in the other thread we decided that "exclusive access" was a better term for what we are getting at here.