Function Points FAQs
External Interface Files - Complexity
When Function Point counting an application that references data maintained by another application, we credit the application under study with an EIF.
How do we establish the logical file structure and hence the number and complexity of data files we are referencing?
The amount of interfacing between applications within most organisations is significant. Most applications reference data held on at least one other application.
To avoid loading the network with calls from Application X to Application Y for the purposes of referencing data, overnight batch jobs may be run to copy/download data from one application to temporary files on another.
IFPUG defines an External Interface File (EIF) as:
"a user identifiable group of logically related data or control information referenced by the application, but maintained within the boundary of another application. The primary intent of an EIF is to hold data referenced through one or more elementary processes within the boundary of the application being counted. This means an EIF counted for the application must be in an ILF in another application.".
The definition of an External Interface File requires the group of data or control information to be a user identifiable group of data that fulfils specific user requirements and that the data not be maintained by the application under study. The number of EIFs counted is dependent upon how the business identifies groups of logically related data.
Where data is copied into a temporary file in a receiving system, to facilitate referencing of this data, the external files from which the data was sourced should be counted as External Interface Files. (See Section 3.4.4, External Interface Files - Sample Scenarios.) In order to establish the identity and complexity of the source logical files (EIF's) an investigation of the source application is recommended. This approach requires considerable effort on the part of the FP Analyst and application expert. Whether this investigation is conducted may be a timing, and/or scheduling issue.
Note: The issue of identifying the logical structure of a referenced application becomes more complicated if the application is outside the scope of the organisation's count portfolio.
In situations where the counting schedule does not permit the FP Analyst to interrogate the referenced application, it is recommended that counting of the External Interface File/s be based upon the structure of the corresponding temporary files that are created during the download process.
The complexity of the EIF is always based on what the application being counted knows and uses.
Example: The Employee logical file is described in the following table along with its use by the Staffing, Payroll and Benefits applications.
For the Staffing Application, the Employee logical file is counted as an ILF with 2 RETs and 10 DETs. For the Payroll Application, the Employee logical file is counted as an EIF with 1 RET and 3 DETs. For the Benefits Application, the Employee logical file is an ILF with 2 RETs and 6 DETs.