Create/Open Persistent Databases in Python

To create or open a persistent database, the application ...

Parameter Settings

Please refer to the general guidelines discussed in the Creating Databases page for determining the proper values for MemPageSize, DiskPageSize, MaxConnections, MaxDiskDatabaseSize and LogType.

Log Type and Transaction Commit Policy

Applications set the DatabaseParameters.LogType to RedoLog, UndoLog or NoLog for the Database method open(). Depending on the log type selected, the additional Database.Parameters property RedoLogLimit or DelayedCommitThreshold can be set. (Please see the Persistent Database I/O page for detailed explanations.)

Note that if the NoLog log type is selected, the application can explicitly force the cache to be flushed to the media with the Connection method diskFlush().

Memory Device Specification

As explained in the Python memory devices page, applications using persistent databases must specify at least four memory devices:

• dictionary and metadata, and possible transient class data: a private or shared memory device
• persistent data storage: a file, multifile or raid device
• persistent data cache: a private or shared memory device
• transaction log: a file, multifile or raid device

To specify these devices, the application instantiates a tuple of devices and passes this to the open_database() call. For example, the following code snippet defines memory for an in-memory or persistent database, then creates the database:

 
    if self.disk:
 
        devices = (self.exdb.fileDevice(self.exdb.Device.Data, "open_db.dbs"),
            self.exdb.fileDevice(self.exdb.Device.TransactionLog, "open_db.log"),
            self.exdb.privateMemoryDevice(self.exdb.Device.Data, 4*1024*1024),
            self.exdb.privateMemoryDevice(self.exdb.Device.DiskCache, 16*1024*1024)
        )
 
    else:
        devices = ( self.exdb.privateMemoryDevice(self.exdb.Device.Data, 4 * 1024*1024),  )
 
    self.db = self.exdb.open_database(dbname='opendb', dictionary=dict, is_disk=self.disk, devices=devices);
     

Note that in order to open an existing persistent database, the application must specify the same open_database() parameters that were used to create the database.