Availability Guide for Application Design

Instrumenting an Application for Availability
Availability Guide for Application Design—525637-004
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The DSM Subsystem Environment
You might issue this type of command in response to a file-full threshold event
message indicating that the file-full threshold has been crossed.
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Switch to stand-in processing.
You would typically issue this kind of command when it is no longer possible to run
the application normally. In response to the command, some other processing
takes over, perhaps with reduced function, until the normal application is able to
resume.
Response Messages
After receiving and processing a command message, the application SPI interface
typically returns a response message to the management application that issued the
command. It creates the message using the same set of SPI procedures as the
management application used to send the command message.
These messages contain at least one item indicating the outcome of the command.
The message might contain additional items and error lists if the application is unable
to carry out the command. In some cases, the amount of information returned can be
more than will fit into one message buffer; in such cases, the application sends
additional messages, using a special protocol that is provided by SPI.
The DSM Subsystem Environment
Your application must contain a management interface if it is to be a part of the DSM
subsystem environment and benefit from the increased availability supported by DSM.
You must add extra logic to your application in order to provide an interface with the
rest of the DSM architecture. That is, you must provide logic to generate event
messages for capture by EMS and to receive command messages from management
applications and generate responses to those commands.
In addition to generating event messages, you might also consider gathering statistics
about your application resources. Although not formally a part of DSM, the Measure
product does allow you to define counters for measuring resource usage and to write
the values of those counters to a disk file.
Several tools and techniques are available for providing these capabilities. Of the
following subset, the first three are helpful in generating EMS messages and delivering
those messages to an EMS collection process. The third provides a command
management interface. The last provides a mechanism for generating statistical
information about resource usage.
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EMS FastStart provides an easy way to generate and test event messages.
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SPI programming uses a procedural interface for:
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Building your own event messages
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Receiving command and query messages
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Responding to command and query messages