DDM overview
This topic describes the purpose of distributed data management (DDM), the functions that DDM supplies, and the concepts of IBM® i DDM.
DDM is part of the IBM i licensed program. IBM i DDM as a source supports Level 2.0 and below of the DDM architecture. IBM i DDM as a target supports Level 2.0 and below for record file (a file on disk in which the data is read and written in records) types and Level 3.0 and below of the DDM architecture for stream files (documents) and directories (folders).
IBM i DDM support allows application programs or users to access data files that reside on remote systems, and also allows remote systems to access data files on the local IBM i operating system, as shown in Figure 1. Any system that supports the DDM architecture as a client system can access data (if authorized to do so) on any other system to which it is attached. The attached system must support DDM as a server system (the system that receives a request from another system to use one or more files located on the system). However, the client and server systems must support compatible subsets and levels of the DDM architecture.
The folder management services (FMS) support allows personal computer users to access folders and documents that reside on an IBM i server system. Remote systems that support Level 3.0 or Level 2.0 of the DDM architecture for the stream access method can access folders and documents on the local system.
DDM extends the file accessing capabilities of the IBM i database management support. In this topic collection, database management refers to the system function that controls local file processing; that is, it controls access to data in files stored on the local system, and it controls the transfer of that data to requesting programs on the same system.
Distributed data management controls remote file processing. DDM enables IBM i application programs to access data files stored on another system supporting DDM. Similarly, other systems that have DDM can access files in the database of the local system. DDM makes it easier to distribute file processing between two or more systems.
Systems that use DDM communicate with each other using the Advanced Program-to-Program Communication (APPC) support, Advanced Peer-to-Peer Networking (APPN) support, or TCP/IP. See the Communications Management manual on the i5/OS PDF files and manuals page and the APPC, APPN, and HPR topic for information needed to use APPC and APPN.
Folder management services (FMS) allows local access to documents or folders that are on the IBM i operating system. Personal computers can access folder management functions on the system by using DDM.
As shown in Figure 2, the system on which a user application issues a request involving a remote file is called a client system. The system that receives the request for one of its files is called the server system. A system can be both a client and server system for separate requests received at the same time.
Using DDM, an application program can get, add, change, and delete data records in a file that exists on a server system. It can also perform file-related operations, such as creating, deleting, renaming, or copying a file from the server system to the client system.
When DDM is in use, neither the application program nor the program user needs to know if the file that is needed exists locally or on a remote system. DDM handles remote file processing in essentially the same way as local file processing is handled on the local system, and the application program normally does not receive any indication of where the requested file is located. (However, in error conditions, messages are returned to the user that indicate, when necessary, that a remote system was accessed.) Informational messages about the use of server system files are included in the client system's job log.
When DDM is to be used, only application programmers need to know where the file is located and, using control language (CL) commands outside of the high-level language (HLL) programs, they can control which file is used. However, the programmers can also choose to use specific recovery functions to handle certain communications failures; the HLL programs might need to be changed to include handling any such failure.
Therefore, BASIC, ILE COBOL, ILE RPG, ILE C, and IBM i programs that are compiled to process database files on the local system might not need to be changed or recompiled for DDM to process those same files when they are moved to or exist on a remote system.
