An Open Server application that is acting as a gateway between an Open Client application and an Adaptive Server can pass TDS packets between client and server without examining their contents. An Open Server that handles TDS packets in this way operates in passthrough mode.
Because the Open Server gateway application does not have to unpack the TDS information as it arrives from the client, and repacks information before sending it to the Adaptive Server, passthrough mode is very efficient.
For Open Client Server 12.5.1 and earlier, passthrough mode ensures that the negotiated packet size is correct by limiting the packet size requested by the client to the maximum size supported by the Open Server.
When a remote server supporting server-specified packetsize sets a packetsize larger than that configured in Open Server, the larger packetsize is used, regardless of the configured SRV_S_NETBUFSIZE.
There are two types of passthrough modes:
Regular passthrough mode
Event handler passthrough mode
Both types of passthrough modes use the passthrough routines srv_recvpassthru, ct_sendpassthru, ct_recvpassthru, and srv_sendpassthru. The differences are as follows:
In regular passthrough mode, the Open Server application recognizes events and triggers event handlers. These event handlers are coded to call the passthrough routines.
For more information on regular passthrough mode, see “Regular passthrough mode”.
In event handler passthrough mode, the Open Server application does not recognize most types of events on the connection. Instead, the full passthrough event handler is triggered whenever a network read for the connection completes. The full passthrough event handler is coded to call the passthrough routines.
For more information on event handler passthrough mode, see “Event handler passthrough mode”.
DB-Library also provides routines to support passthrough mode. See the Open Client DB-Library/C Reference Manual for details.
