[quassnoi]

InnoDB was initially designed as a KVS. MySQL only added the SQL frontend to it.

Initially, MySQL also supported BerkeleyDB, another KVS, but as for now they dropped the support.

From InnoDB's point of view, MySQL is just a client, as is HandlerSocket the author is using.

PostgreSQL's storage engine is much more closely integrated into the system and has no low-level API easily available from the outside.

[morgo]

Yes, InnoDB was standalone. But it had it's own SQL parser and things like Stored Procedures as well ;) It was rudimentary, but I don't know if KVS is the best description.

How MySQL and InnoDB talk is very much a K<->V / row based relationship.