SESAM/SQL is available in 3 product versions:
with independent DBH there is
the standard edition of SESAM/SQL with single-task operation
the enterprise edition with multitask operation
with linked-in DBH, there is the SESAM/SQL-LINK version.
The figure 24 compares linked-in DBH with independent DBH.
Figure 24: Comparison of linked-in DBH and independent DBH
Independent DBH
The figure 25 shows independent DBH and its interfaces. Independent DBH can access multiple databases and work with multiple requesters, as shown in this illustration.
Figure 25: The SESAM/SQL-DBH and its interfaces
The independent DBH consists of one or more tasks (up to 16) (multitasking only possible with the enterprise edition). The tasks of this task family must be regarded as a unit, i.e. they all have the same functionality and are started and terminated together. For the system administrator, the task family presents itself with a “single-system image”. This means:
The administrator starts SESAM/SQL in a task which, in turn, starts the other DBH tasks belonging to the task family according to the specifications made in the DBH-TASKS load option. The DBH is operational only when all DBH tasks have been started.
The administrator only communicates with the task explicitly started, but administration statements always effect all tasks. It is not possible to address an individual task within the task family.
In the course of an active session, all outputs from SESAM/SQL are made via the explicitly started DBH task.
SESAM/SQL automatically distributes the load to the individual DBH tasks. The system administrator cannot influence load distribution.
Linked-in DBH
The figure 26 illustrates linked-in DBH and its interfaces. Linked-in DBH can access multiple databases, but in contrast to independent DBH, it can only work with one requester, as shown in this illustration. It only consists of one task.
Figure 26: The SESAM/SQL-LINK-DBH and its interfaces
The linked-in DBH works exclusively with requests from a single application program and is linked directly to the application program. When the first statement is issued, the SESLINK connection module automatically loads the modules of the linked-in DBH from the SESAM/SQL module library and functions as a mediator between the application program and the DBH for the duration of the program run (see section “Connection modules”).
The program and the DBH share a common task. This means that no time is spent on handling communication between tasks when a database operation is requested. In addition, because the linked-in DBH only needs to handle requests from a single application program, it requires only a relatively short time to perform database operations. The linkedin DBH is only slower than the independent DBH if the update rate is high; this is because only one thread is generated for the linked-in DBH (see section “Multi-thread operation”).
A prerequisite for operation with the linked-in DBH is that the add-on product SESAM/SQL-LINK is installed. It constitutes the option of choice when one or more databases are to be accessed by a single application.