Availability Guide for Application Design

Designing Applications for Change

Availability Guide for Application Design—525637-004

10-16

Isolating Data From the Application

Updating the Interface Module

As a step towards continuous operation of the application, you must allow the old

version of the interface module to run concurrently with the new version.

You cannot simply overwrite the old version with the new. Currently executing or

suspended processes might be in the module code at the time you overwrite it. These

processes would be forced to switch to the new code part way through the module,

causing unpredictable results.

It is necessary for both versions of the module to coexist to allow processes that are

already executing the old module to continue to do so, while processes just entering

the module can call the new version. Changing the address of the module to that of the

new module is one way of achieving the desired effect. Assuming that the addressing

is indirect, this change can be performed without any change to the modules that call

the interface module as shown in Figure 10-2 on page 10-17. New calls then route to

the new module without interrupting processes that are currently executing in the old

module.

The old version of the interface must remain until all processes have exited the

module. How long this takes depends on the design of the application and on how it is

being used.

Figure 10-1. Isolating Data From the Application Through Data Encapsulation

01 Data Item1

01 Data Item2

Subitem1

Module 1

Module 2

Module 3

Module n

Data

Interface

Module

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