Consider now that there is significant update activity in the database server and therefore, transactions need to acquire a database lock before they can update the database. Once a lock is acquired, a database transaction will either use the CPU or the disk while holding the database lock. Thus, two resources will be held by the transaction at the same time (i.e., a database lock and the CPU, or a database lock and the disk). This situation is called simultaneous resource possession (SRP). In an SRP situation, a customer in a QN is allowed to hold one or more resources at the same time. A slightly different, yet equivalent, view is that a customer is executing in parallel, both at the database server lock and at the resource (i.e., CPU or disk) server. Figure 2.10 shows three time axes: one for the CPU, one for the database lock, and another for the disk. The picture shows how a transaction spends time waiting and holding each of these resources.
Figure 2.10. Time axes illustration of SRP.
Figure 2.11 illustrates the QN with a queue for database locks. Dashed arrows from the database locks to the CPU and disk indicate that database locks are held simultaneously with these two resources. The probability that a lock requested by a transaction is being held by another transaction increases with the system load. See  for analytic models with database locking mechanisms. Queuing networks with SRP can also be used to model software contention. In this case, a software resource (e.g., a thread or a critical section) is being held simultaneously with another physical resource (e.g., a CPU or disk). Similarly, SRP can model other types of hardware contention (e.g., a bus in a shared-memory multiprocessor is held while a memory module is used).
Figure 2.11. QN with simultaneous resource possession. Database locks are held simultaneously with the CPU and with the disk.