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An Overview of the Host Data Replicator Architecture

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The Host Data Replicator (HDR) resides with Microsoft SQL Server on a Microsoft Windows NT Server system. When a replication begins from the SQL Server side, HDR accesses the ODBC Driver for DB2 driver using a standard ODBC interface. The ODBC Driver for DB2 then converts ODBC commands to DRDA (Distributed Relational Database Architecture) messages.

Note   The ODBC Driver for DB2 is bundled with SNA Server.

The ODBC Driver for DB2 then uses the Advanced Program-to-Program Communications (APPC) interface of SNA Server to send data over an SNA Logical Unit 6.2 session to the mainframe.

ODBC SQL commands are transferred to the mainframe in DRDA messages, which are an efficient format for specifying one or more SQL commands and their result sets. For efficiency, one DRDA message may contain multiple SQL commands.

Communication to the mainframe takes place over any link protocol and media supported by Microsoft SNA Server, the hardware on which SNA Server runs, and the hardware of the mainframe. For example, links may be Token Ring, Ethernet, Synchronous Data Link Control (SDLC), or channel attachment. Connection to the mainframe normally is through the standard SNA facilities of IBM processors. In MVS systems, connection is typically through front-end processors running Network Control Program (NCP) and through Virtual Telecommunications Access Method (VTAM) facilities. Using channel attachment hardware, users can bypass front-end processors.

On the mainframe, the DRDA messages are processed by DRDA facilities that are standard for all IBM implementations of DB2. These mainframe-resident facilities are called DRDA Application Servers, and they handle the ODBC Driver for DB2 and other clients, which are called DRDA Application Requesters. In MVS, the DRDA Application Server is called Distributed Data Facility (DDF). DRDA is the most efficient way of processing DB2 facilities from a network. Other methods, such as running an application in TSO, CMS, CICS, or IMS, usually require three times as much mainframe processing.

Upon processing the SQL commands sent by HDR, DB2 returns result sets in DRDA messages. These go to HDR in the Windows NT system and are inserted into SQL Server or DB2 as required for the replication. After the transfer and insertion, the replication is complete.