TS/MP Pathsend and Server Programming Manual (G06.24+, H06.03+)

Introduction to Pathway Application Programming
NonStop TS/MP Pathsend and Server Programming Manual–132500
1-5
Fault Tolerance
commonly known as a transaction log. You can use the audit trail to rebuild the database
in the event of a hardware or software failure.
The design of Pathway servers supports the integrity of individual transactions and
therefore transaction processing protection as a whole. Because the requester/server
model allows a clear division of processing functions, application programmers can code
each server program to handle a specific set of transaction types: for example, checking
an account balance, entering a new customer, or updating the parts inventory. The server
processes service their transactions by performing the same set of tasks over and over
again. In this way, a valid transaction is defined as a specific set of tasks both by the
requester program and within the server logic.
If for any reason a server is unable to complete all tasks involved in processing a
transaction, it can abort the transaction and thereby maintain the transaction’s integrity.
The server does not have to wait for the requester to abort the transaction.
Fault Tolerance
Because OLTP systems automate core business operations and deliver key business
services, companies depend on OLTP applications to stay up and running—even if a
hardware or software component fails.
Tandem NonStop systems, which are specifically intended for online transaction
processing, are designed to remain continuously available during the hours when
transactions are being entered and business is being conducted. Typically, a Tandem
NonStop system can continue processing despite the failure of any single software or
hardware component within that system. This ability is referred to as fault tolerance.
In the Pathway environment, automatic fault tolerance (that is, fault tolerance that does
not require any additional programming effort on your part) is provided by the use of
process pairs and the actions of the PATHMON process, the TMF subsystem, and the
terminal control process (TCP) provided with the Pathway/TS product.
In the Guardian operating environment, the functions and tasks of an application are
performed by processes, which are running programs. A process pair consists of a
primary process, which does some specific function in the overall work of the
application, and a secondary (backup) process, which remains ready to take over if the
primary process fails. During processing, the primary process keeps the backup process
informed of what it is doing (for example, sending a request) by means of special
interprocess messages, in an activity called checkpointing. Through checkpointing, the
backup process has enough information to take over and continue if the primary process
fails.
Both the PATHMON process and the TCP can be configured as process pairs to support
Pathway applications. When the PATHMON process is configured as a process pair, you
are ensured the ability to control and monitor OLTP system operation even if the
primary PATHMON process fails. When a TCP is configured as a process pair and the
primary TCP fails, terminals controlled by the TCP can still be used.
Pathway server classes provide additional fault tolerance by allowing requests to be
rerouted to surviving server processes in a server class if one server process fails.