Wait-for graph

A Wait-For Graph in computer science is a directed graph used for deadlock detection in operating systems and relational database systems.

In Computer Science, a system that allows concurrent operation of multiple processes and locking of resources and which does not provide mechanisms to avoid or prevent deadlock must support a mechanism to detect deadlocks and an algorithm for recovering from them.

One such deadlock detection algorithm makes use of a Wait-For Graph to track which other processes a process is currently blocking on. In a Wait-for Graph, processes are represented as nodes, and an edge from process $P_i$ to $P_j$ implies $P_j$ is holding a resource that $P_i$ needs and thus $P_i$ is waiting for $P_j$ to release its lock on that resource. A deadlock exists if the graph contains any cycles. The wait for graph scheme is applicable to a resource allocation system with multiple instances of each resource type.

References

Silberschatz, Abraham; Peter Galvin, Greg Gagne (2003). Operating System Concepts. John Wiley & Sons, INC.. pp. 260. ISBN 0-471-25060-0.