Contention for VSAM data (the contents of a control interval) can lead to deadlocks, in which a processing program is prevented from continuing because its request for data cannot be satisfied. A and B can engage as contenders in four distinct ways:

1. A wants exclusive control, but B has exclusive control. VSAM refuses A's request: A must either do without the data or retry the request.
2. A wants exclusive control, but B is only willing to share. VSAM queues A's request (without notifying A of a wait) and gives A use of the data when B releases it.
3. A wants to share, but B has exclusive control. VSAM refuses A's request: A must either do without the data or retry the request.
4. A wants to share, and B is willing to share. VSAM gives A use of the data, along with B.

In 1 and 3 above, B is responsible for giving up exclusive control of a control interval through an ENDREQ, a MRKBFR with MARK=RLS, or a request for access to a different control interval. (The RPL that defines the ENDREQ, MRKBFR, or request is the one used to acquire exclusive control originally.)