The ledger system in Hyperledger Fabric uses what database by default?

The ledger system in Hyperledger fabric uses levelDB. By definition, LevelDB allows concurrent

writers to safely insert data into the database by providing internal synchronization. LevelDB uses

very coarse-grained synchronization which forces all writes to proceed in an ordered, first-come-first-

served basis, effectively reduces throughput to a single thread. State database options include

LevelDB and CouchDB. LevelDB is the default key-value state database embedded in the peer

process. CouchDB is an optional alternative external state database. Like the LevelDB key-value store,

CouchDB can store any binary data that is modeled in chaincode (CouchDB attachment functionality

is used internally for non-JSON binary data). But as a JSON document store, CouchDB additionally

enables rich query against the chaincode data, when chaincode values (e.g. assets) are modeled as

JSON data