ODBC Server Reference Manual

Architecture Overview

HP NonStop ODBC Server Reference Manual—429151-002

2-32

SQL Communication Subsystem

A client request is one logical message, but it can be sent in multiple 512-byte

transport messages. As SCS receives messages from the client, it forwards them

without change to the NonStop ODBC server.

When a request is being received, the NonStop ODBC server reads all the incoming

messages before it starts processing the request. As it is acquiring result data, it writes

messages to SCS as soon as the messages are ready.

While the NonStop ODBC server is processing a request, it usually has a WRITEREAD

posted to SCS. If the client issues a cancellation request, the WRITEREAD can

terminate; the NonStop ODBC server then cancels its activities.

If a stored procedure is being executed, the NonStop ODBC server uses a NOWAIT

ServerClass_Send so the NonStop ODBC server process can process the cancel

message. If NonStop SQL/MP is being executed, which is done in the WAIT mode, the

NonStop ODBC server does not process the cancel message until it has completed the

SQL request. However, if a cursor SELECT is being executed, the NonStop ODBC

server checks for cancellation between fetches of the cursor.

Server Classes

Server classes are named and have a number of attributes pertaining to the NonStop

ODBC server processes in the class (for example, allowable processors and initial

priority). There are also attributes limiting the number of servers and specifying the

Figure 2-26. Interprocess Communication Between SCS and NonStop ODBC

Server

NEWPROCESS

WRITEREAD

Open

Initiate

Open

[ Data ]

Read

Write

Send

Receive

SCS NonStop ODBC Server

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