Availability Guide for Application Design

What Is Application Availability?
Availability Guide for Application Design—525637-004
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Design Program Modules for Availability
Section 7, Availability Through Process-Pairs and Monitors, for a discussion of process
pairs and checkpointing.
Application designers can design applications that use process monitors and NonStop
process pairs. However, for most needs, that part of the application design is
transparently done for you by the transaction services of the NonStop Tuxedo or
Pathway transaction-processing environments.
Transaction management is done by TMF on behalf of the application. This product
maintains an audit trail of before-image and after-image records that ensure that the
database can be restored in the unlikely event of a failure. In a correctly configured
system, the database can always be restored to its state before the failure, except for
changes made by transactions that were active at the time of the failure.
Applications That Run in the NonStop Distributed
Computing Environment
The NonStop Distributed Computing Environment (DCE) product provides an open
interface for developing or porting distributed applications for execution on NonStop
systems. The combination of this product and the NonStop architecture bring the
NonStop fundamentals, including availability, to applications that use this standard
interface.
The NonStop DCE product provides for highly available multithreaded applications
using the client/server model. Server processes typically run on an NonStop server as
OSS processes, while client processes typically run on client PCs or workstations and
communicate using TCP/IP or UDP/IP protocols. Server processes are highly available
because they can be run as multiple copies called replicas, executing on more than
one system. In addition, the cell directory service (CDS) server is implemented using
SQL/MP, which improves availability by:
•
Treating a set of logically related updates to the CDS namespace database as a
single transaction, to increase database consistency
•
Allowing concurrent writes
•
Caching in the database manager
•
Eliminating the need for blocking checkpoints
The DCE application interfaces provide access to:
•
The NonStop system
•
Distributed application services such as the cell directory service, security service,
and the distributed time service
•
NonStop transaction processing environments
The use of multiple threads in applications increases availability by reducing the
amount of time that the application spends waiting for the completion of blocking
operations.