The metadata are in the [[ https://github.com/chrislusf/seaweedfs/wiki/Components#master-service | master service ]] which is a custom database. It must be updated when an object is written and read must lookup this database to figure out on which [[ https://github.com/chrislusf/seaweedfs/wiki/Components#volume-service | volume ]] to get the data.
The metadata database is a [[ https://github.com/chrislusf/seaweedfs/blob/master/weed/server/master_server.go | implemented in seaweedfs ]]. It is strongly consistent (Raft), meaning that the leader handles all writes and acknowledges an update once all running servers have a copy. Reads can be from any servers.
The test coverage is unclear and appears to be low. There does not seem to be integration tests.