Why per-wallet db.log and database?

[jnano]

In a GitHub discussion related to multi-wallet there was a mention of (optionally?) each wallet having "its own bdb environment", and storing each wallet.dat in a directory along with db.log and a /database subdirectory.

What's the idea here and what's the database directory supposed to hold?

[achow101]

Both the file db.log and the directory database are files used by the BDB database engine. The directory database is a temporary directory that is used to store data about the wallet while the wallet database is open. If Bitcoin Core is uncleanly closed, the BDB database will also be uncleanly closed. If it is uncleanly closed, then in order to open it again without having any problems, you must have the database directory as it contains some BDB state data. If they are separated (wallet.dat moved out or database is deleted), then the wallet database can be corrupted and private keys corrupted. This can result in a loss of funds.

Having separate wallet directories containing their own database environments allows for better dynamic wallet handling and a failure to close the BDB environment cleanly may only just effect one wallet instead of all wallets.

[jnano]

So wallet.dat is BDB, and ./database/ is a transactional log?

[achow101]

So wallet.dat is BDB, and ./database/ is a transactional log?

database/ is used for whatever other temporary data BDB needs. This includes (but is not necessarily limited to) database transaction logs.