State of a Resource Request Queue

The composite state of a resource request queue is determined by evaluating all resource requests on the queue. A resource request queue can be in one of three composite states — free, shared, or exclusive.

Free — There are currently no owners or waiters (requests to own) the specified resource.
(You are a resource owner if you have been granted access to the resource. You are a waiter if your request has not yet been granted.)
Shared — All owners and waiters for the resource are in the shared state.
Exclusive — There is at least one owner or waiter for the specified resource in the exclusive state.

Since each individual resource request indicates the state in which the resource is requested, XES is able to maintain the composite state of the entire resource request queue.

When a new request for a resource is received, XES determines the compatibility of the request before adding it to the resource request queue. Figure 1 illustrates the compatibility rules used by XES and the resultant state of the resource request queue. A “C” indicates a compatible state and an “X” indicates an incompatible state.

Figure 1. XES Compatibility Rules

Figure 2 depicts two resource request queues — for resource XYZ and resource JKL.

Resource Request Queue for Resource XYZ
The composite state (C/S) of the resource request queue for XYZ is shared and all entries on the queue are compatible; both User A and User B have requested the resource in a shared state. The entry for both User A and User B show that the resource is held and that they have each specified user data associated with the request.
Resource Request Queue for Resource JKL
The composite state of the resource request queue for JKL is exclusive; User C has been granted exclusive use of the resource, while User A has requested shared use. The request queue is also said to be incompatible because it contains entries requesting access to the resource in conflicting states. The entry for User C shows that the resource is held in an exclusive state and that user data is associated with the resource request. The entry for User A shows that User A's request is pending and that user data is associated with the request.

Figure 2. Resource Request Queue Compatibility