 Directed acyclic graphs representations of partial orderings have many applications in scheduling for systems of tasks with ordering constraints . An important class of problems of this type concern collections of objects that need to be updated , such as the cells of a spreadsheet after one of the cells has been changed , or the object files of a piece of computer software after its source code has been changed . In this context , a dependency graph is a graph that has a vertex for each object to be updated , and an edge connecting two objects whenever one of them needs to be updated earlier than the other . A cycle in this graph is called a circular dependency , and is generally not allowed , because there would be no way to consistently schedule the tasks involved in the cycle . Dependency graphs without circular dependencies form DAGs . 
 Directed acyclic graphs representations of partial orderings <a href='got' title='1'><mark>have</mark></a> many applications in scheduling for systems of tasks with ordering constraints . An important class of problems of this type concern collections of objects <a href='null_relcl' title='2'><mark>that</mark></a>need to be updated , such as the cells of a spreadsheet after one of the cells <a href='been_done' title='3'><mark>has</mark></a> been changed , or the object files of a piece of computer software after its source code <a href='been_done' title='4'><mark>has</mark></a> been changed . In this context , a dependency graph <a href='drop_aux' title='5'><mark>is</mark></a>a graph <a href='null_relcl' title='6'><mark>that</mark></a><a href='got' title='7'><mark>has</mark></a> a vertex for each object to be updated , and an edge connecting two objects whenever one of them <a href='uninflect' title='8'><mark>needs</mark></a> to be updated earlier than the other . A cycle in this graph is called a circular dependency , and is generally not allowed , because <a href='dey_it' title='9'><mark>there</mark></a> would be no way to consistently schedule the tasks involved in the cycle . Dependency graphs without circular dependencies form DAGs . 
Directed acyclic graphs representations of partial orderings got many applications in scheduling fa systems of tasks with ordering constraints . An important class of problems of this type concern collections of objects gotta be updated , such as the cells of a spreadsheet after one of da cells done been changed , or the object files of a piece of computer software after its source code done been changed . In this context , a dependency graph a graph got a vertex for each object to b updated , and an edge connecting two objects whenever one of them gotta bbupdated earlier than the other . A cycle in this graph is called a circular dependency , and is generally not allowed , cuz it would be no way to consistently schedule the tasks involved in tdacycle . Dependency graphs w/o circular dependencies form DAGs .