Software Design Designing the overall structure architecture of a software system Designing small pieces of computation Designing non automated processes Use DFDs as a starting point Structure Charts Used to create a high level or architectural design Shows the thread of control in a system as a whole Breaks up the system into modules Shows the order in which processing takes place Includes both data and processes but more emphasis on processes Structure Charts A B D Example Control starts in module A then passes to module B and from B to module C When C is done control is returned back to module B and then back to A Then A passes control to module D which passes it back to A when it s finished OR C Example Module A calls module B which in turn calls C C returns back to B which returns to A Then A calls D which returns back to A when it s finished Structure Charts A X Z Data couple represents data that is passed from one module to another usually smaller grained than a data flow Z B D X Y C Example Module A passes X to module B which passes it to module C Module C creates Y and passes it back to module B which creates Z and passes it back to A Module A then passes Z to module D Module D doesn t pass any data back to A Creating a Structure Chart Start with a level 0 DFD Determine if system is transform centered or transaction centered Find central transform or transaction center Create boss module Create first tier of processes Afferent module then central processes then efferent module Add data couples Continue refining Improve Transform vs Transaction Centered Efferent processes Afferent processes Transform centered many inputs few outputs central process es concerned with creating new data out of inputs Transaction centered few inputs many outputs central process es concerned with moving data to different places Finding the Central Process es Start at each external entity and follow data flows forward or backward until you find processes that represent the essence of the processing of the system where routing takes place transaction center where data is transformed and created central transform Can be more than one usually independent processes Appropriate numbers of inputs and outputs Creating the First Tier Boss Module Afferent Module Central Process Central Process Efferent Module Identifying Data Couples Data couples usually correspond to data elements not data flows Look at data flows going into and out of afferent central and efferent processes on the DFD Pick out the relevant data elements Put them in the appropriate place on the structure chart Refining a Structure Chart Boss Module Afferent Module Afferent Processes Afferent Processes Central Process Central Process From DFD Efferent Module Efferent Processes Efferent Processes Improving a Structure Chart Check modularity Take advantage of shared modules Decrease coupling Increase cohesion Consider alternative architectures Modularity Factoring breaking the system up into modules Span of control rule of thumb is that no module should control more than 7 subordinate modules Reasonable size a module should fit on a page or a screen Reuse At the lower levels of the structure chart there are often modules that performs the same function in different parts of the system These modules can be reused in different parts of the design Can make the arrows tangled up Coupling Interdependence of modules 5 types data communication only through well defined passing of specific data elements stamp data passed is of larger granularity control modules affect the control flow of other modules common modules use common data content modules modify each other Cohesion Degree to which a module performs a single cohesive function 7 types functional ideal sequential functions form a chain leading from input to output communicational functions use the same data procedural functions relate through flow of control temporal functions happen at same time logical functions related by some outside criterion coincidental functions not related Alternative Architectures Pipeline Hierarchical Layered DB Mediated Process or Program Design Start with one process on a DFD or module on a structure chart Continue decomposing Introduce concept of time process steps control flow Define all decision conditions Include error and exception conditions Control Flow Diagram Get Info Yes New Customer Iteration Take Order No Get Record Decision Task Control Flow Answer Call Other Notations for Process Design Decision tables Decision graphs Structured English pseudocode 4GL Specification Language Mathematical notation Pseudocode A little more structured than Structured English Pay close attention to data must reference by name specific data elements must be consistent with data flows and elements on the DFD Decision and iteration conditions should be precisely defined