Direct Data Access
One popular method, often used for batch update solutions, is to directly access the data from the host application. Although a large number of tools use this straightforward method, it has some significant drawbacks (e.g., data updates that originate outside the legacy application). Because you cannot rely upon direct data access for timely and accurate updates to the host system, it is best used for read-only applications. Otherwise, it will require extensive coding efforts within the middleware system.
Direct transaction access
You can also directly access the logic of the host application. When using this process, you call the "methods" or "transactions" within the application itself. As with direct data access, the safe use of direct transaction access must be planned out and carefully executed because you will be manipulating the application in ways it was not written to expect. The benefits of this access method can be high in terms of performance and reliability, but the cost and risk are clearly higher than the other methods. Direct transaction access offers little control over the process and falls the closest to actually rebuilding the legacy application.
Another popular method, which has been used for a long time, is to programmatically run the legacy application as if the middleware were a user of it. This method, often called screen access or screen "scraping," has evolved greatly over the years. While not always considered the most robust method, its popularity stems from its low risk and rapid timeframes. It can safely be used for both read and write access. Newer tools let this method run reliably and, unlike data transaction access, give most mid-tier-savvy workers control over the legacy application. This highly scalable method has also overcome the performance issues that plagued its earliest days. Screen access is often chosen when cost and timeframes are critical.
CICS BMS Map access
The least popular method is one that specifically targets CICS applications. While exclusivity to CICS is clearly limiting, this method is noteworthy given the large majority of host applications actually running in CICS. BMS Map access, the newest method of the four, is best thought of as a hybrid between direct transaction access and screen access. Its performance benefits are similar to direct transactions, but its implementation ease and risk reduction are similar to screen access. Essentially, this method talks directly to the CICS logic, but gives the middleware a virtual screen for building and controlling application access. This method is best used with CICS applications, especially when you want to avoid the costs and risks associated with transactional access or rebuilding.