Transaction T1 holds a lock on resource A and requests a lock on resource
B. Transaction T2 holds a lock on resource B and requests a lock on resource
A. Both transactions remain blocked indefinitely, each waiting on the other. What condition does this represent, and what data structure does a DBMS commonly use to detect it?

This scenario is a textbook deadlock: each transaction holds a resource the other needs, and neither can proceed, resulting in indefinite blocking. DBMSs commonly detect this using a Wait-For Graph, where each transaction is a node, and a directed edge from Ti to Tj means Ti is waiting for a resource held by Tj. A cycle in this graph (here, T1→T2 and T2→T1, forming a 2-node cycle) indicates a deadlock, which the DBMS then resolves by choosing a "victim" transaction to abort and roll back, releasing its locks so the other can proceed.