The user defines a DB configuration by telling the DBH which databases are to be attached. This can be done in the following ways, depending on the type of DBH used:
linked-in DBH
using DBH load parameters
independent DBH
using DBH load parameters and
using DAL commands
In mono-DB operation, the configuration name may be identical to the name of the database (cf. load parameter DBNAME, "DBH load parameters").
The independent DBH can also be started with an "empty DB configuration" if no databases are specified in the load parameters on starting the DBH. It is possible to use DAL commands to attach databases at a later stage.
A DB configuration may be altered during a session or after a session abort.
DAL commands can be used to attach databases (ADD) and detach databases (DROP); see section “The Database Administrator Language DAL”.
Altering the DB configuration during the session
To attach or detach databases to or from a DB configuration, a request to attach or detach one or more databases is sent to the DBH in the form of the DAL command ADD or DROP. This request is not implemented until the DAL command PERFORM is issued.
If the database which is to be attached is inconsistent, the DBH executes a warm start before making it accessible to transactions.
This warm start comprises:
rollback of all uncompleted transactions in the database
rerunning transactions which have made changes not yet implemented in the database
The SLF maintains a fail-safe log of the current DB configuration as a basis for an automatic session restart after a session abort. Consequently, the DBH obtains the information required to perform a session restart from the SLF. This information includes the currently applicable DBH load parameters and the currently attached databases.
Note on UDS-D
If the DB configuration is altered during the session, the database administrator must adapt the distribution table to match via the administration interface (DAL commands). If there are still transactions in the PTC (prepared to commit) condition even after a database warm start, the associated database cannot be detached from the DB configuration (see section “Altering the DB configuration: notes for UDS-D”).
Altering the DB configuration after a session abort (reconfiguration)
After a session abort, the database administrator may dispense with a session restart and start a new session instead with a new DB configuration setup, new DBH load parameters, the same (or a different) configuration name, and any DBH variant.
Reconfiguration is typically useful when a session abort occurs shortly before the scheduled DBH end, i.e. when there is no point in resuming the session by means of a session restart.
To carry out a reconfiguration, the database administrator deletes the SLF file of the aborted session (see section “Session log file (SLF)”), thereby declaring the aborted session terminated.
Reconfigurations are subject to a number of restrictions (see “Notes on reconfiguration” in chapter "DBH load parameters").