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Logical Databases

Overview​

Garnet supports multiple logical databases in a single server instance. This feature is made available only when cluster mode is turned off (by default cluster mode is turned off).
The number of allowed logical databases in a server instance can be altered by changing the MaxDatabases configuration option (either in your garnet.conf file, or via the command line with --max-databases). By default, MaxDatabases is set to 16.

New clients will always connect to the default database (of index 0). To switch database context, you can use the SELECT command.

Design​

Each logical database in Garnet is represented by a GarnetDatabase instance. Each such instance holds a reference to the database stores, AOF device as well as other database-specific data.
When the Garnet server instance is created, StoreWrapper instantiates a server-wide IDatabaseManager, which by default is a SingleDatabaseManager that that holds the default database.
The IDatabaseManager can be later upgraded to a MultipleDatabaseManager, if a non-zero database index is selected.
StoreWrapper in turn calls the IDatabaseManager to perform actions as checkpointing, AOF commits etc., which at each different implementation of IDatabaseManager will handle those in either a single-database or multiple-database context.

Each RespServerSession manages a map of GarnetDatabaseSession instances, which represent per-database session data. Each GarnetDatabaseSession holds an instance of StorageSession as well as GarnetApi instances and a TransactionManager instance.
Whenever a client chooses to interact with a database it hasn't interacted with before (using SELECT, for instance), a new GarnetDatabaseSession will be created.
Each time the client calls SELECT, the RespServerSession would switch its context based on the appropriate GarnetDatabaseSession instance.

Checkpointing, AOF & Recovery​

Each database's store data will be separately stored on its own directory. Checkpoints are stored in CheckpointDir defined in the confguration (and if not specified defaults to LogDir). Each store checkpoint data will then be stored in Store/checkpoints and ObjectStore/checkpoints respectively for the default database or in Store/checkpoints_i and ObjectStore/checkpoints_i for database of index i.
Similarly, AOF data will be stored in the same CheckpointDir (and if not specified defaults to LogDir). Each database AOF data will be stored in AOF for the default database or in AOF_i for database of index i.

Upon recovery, Garnet will extract the indexes of the saved databases from the aforementioned directory name pattern and recover any saved data matching the database index.