3270 bridge concepts

The 3270 bridge allows you to introduce new GUI front ends to access existing 3270-based CICS® applications without modifying them.

This means that you can concentrate your efforts on the new user interfaces and avoid, or at least postpone, rewriting stable mainframe applications. You do not need to restructure your applications to separate the business logic from the presentation logic; the bridge effectively does this for you.

The same applications can be used both by 3270 terminals, and by the new client applications. This allows a phased migration of users from the 3270 applications to the new client applications. Applications written for 3270 terminals can be run on CICS systems without the z/OS® Communications Server.

The bridge can process commands faster than existing front-end methods, such as FEPI and EPI, because the terminal emulation is part of the same CICS transaction. With the START BREXIT bridge mechanism, there is only a single unit of work. This means that the bridge can use a recoverable WebSphere® MQ queue. This greatly simplifies recovery.

For BMS user transactions, there is no need to convert BMS data to 3270 format, because the client application receives the BMS Application Data Structure, rather than a 3270 datastream. This provides an easier method for the application programmer to interface with the user transaction compared to FEPI. A utility program (DFHBMSUP) is provided to re-create map source code from existing load modules, so that installations that do not have access to the original source code can still exploit the new ADS descriptor provided by the BMS macros.

The target transaction is unchanged, but because of the way it now executes in the bridge environment, there are some restrictions on what it can do. These restrictions are described in Link3270 programming considerations.

CICS provides two types of 3270 bridge mechanism:

The Link3270 mechanism: This mechanism provides a simplified interface using LINK, ECI or EXCI. All messages have a fixed format and you are not required to provide any user-written supporting programs.
The START BREXIT mechanism: This 3270 bridge mechanism requires a bridge monitor transaction to initiate the bridge environment by issuing a START BREXIT command, which specifies the target user transaction and also the name of a user-written bridge exit. The bridge exit is called to intercept 3270 requests and pass them in the form of messages to the client application. You can write your own bridge exit and also define your own message formats. Bridge exits are provided to support client applications using Temporary Storage, the Web and Websphere MQ as transport mechanisms for requests, using sample message formats.

The START BREXIT mechanism is still supported, and the sample bridge exits are still provided, but it is better to use the simpler Link3270 mechanism, and migrate to it where possible. START BREXIT is not documented in this release of CICS.