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| All MFSes must be located in the SYSPROG account. OpenInsight does not do any inheritance checking. Thus it is not possible for different versions of an MFS in different accounts to be recognized. |
Filing System Access
OpenInsight file operation requests originate in BASIC+ commands such as READ, WRITE, DELETE, LOCK, UNLOCK, SELECT, READNEXT, or in System Monitor routines such as ATTACH_TABLE, DELETE_ROW and LIST_TABLES. Developers write these commands into their programs, or execute the routines from the System Monitor, to control the flow of data to and from the file.
When a program or routine executes, each file-oriented statement becomes a call to the filing system. For example, if an BASIC+ program encounters a READ statement, and the file being read is a Linear Hash file, OpenInsight generates a call to the program RTP57, the Linear Hash BFS, with the appropriate "read" code.
Because BASIC+ is independent of filing systems, the call arguments for filing system subroutines use a standard protocol. BASIC+ produces the same subroutine call for all filing systems, always using the same arguments. A
READ statement, for example, always produces the same BFS call, regardless of what filing system is actually being accessed by the read logic. By extension, all MFSs must use the same calling conventions.
MFS "Trapping" Mechanism
If an MFS has been installed onto the file being accessed, the MFS "traps" or "interrupts" the call to the underlying filing system. Instead of calling the BFS, OpenInsight calls the MFS first, passing it the same arguments that would otherwise have been passed to the BFS.
Because the full list of arguments is available to the MFS, including the name of the operation to be performed, the MFS can examine and manipulate any of the arguments.
BFS/MFS Independence
When creating an MFS, a programmer must consider carefully the environment and purpose of the MFS. Developers should remember that an MFS can (and should) function generically within the OpenInsight system.
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