The 3270 bridge
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 z/OS® 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 IBM® 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.
- 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 IBM 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.
The 3270 bridge and FEPI
To help you decide between the 3270 bridge technology and FEPI, the following table summarizes the major characteristics.
| START Bridge | Link3270 Bridge | FEPI |
|---|---|---|
| Enabling technology | Enabling technology | An application programming interface |
| Based on application data structure | Based on application data structure | Based on the 3270 data stream |
| Enables optimization due to integral knowledge of the target | Enables optimization due to integral knowledge of target | Easier to create generic driver (data structure is designed) |
| Efficient; no terminal control involved | Efficient, no terminal control involved | z/OS Communications Server managed connection between source and target |
| Single COMMAREA API and user replaceable program | COMMAREA API | Requires system programming and z/OS Communications Server skills |
| CICS specific: source and target must be in the same region | LINK, DPL, EXCI or ECI interface supported | Ideal for driving remote applications, not just CICS |
| Driven exit decides method of communication with the client | Client interface is LINK, DPL, EXCI or ECI | Can be freed from the workings of the target; terminal emulation |
| Knowledge of UOW | Standard CICS LINK coordination | No coordination |
| Ideal when the routing is done elsewhere | Supports workload balancing | Sysplex support requires three regions |