One of the goals of the WebSphere Application Server for z/OS programming model and runtime is to significantly simplify the work required for application developers to write and deploy applications. Sometimes we say that WebSphere Application Server for z/OS relieves the application programmer of many of the plumbing tasks involved in developing applications. For example, application code in WebSphere Application Server for z/OS does not concern itself directly with remote communication--it locates objects which may be local or remote and drives methods. Therefore, you won't see any direct use of socket calls or TCP/IP programming in a WebSphere Application Server for z/OS application.
This separation of what you want to do from where you do it is one aspect of removing the application programmers from plumbing tasks. Other considerations are not having to deal with data calls for some types of beans, potentially user authentication, and threading. There are generally no calls from the application code to touch sockets, RACF calls, or management of threading. Removing this from the application programmer doesn't mean this work won't get done. Rather, it means that there may be more work for the DBA, the network administrator, the security administrator, and the performance analyst.
There are four layers of tuning that need to be addressed: