Deadlock Situation in DBMS
Understand the Deadlock Situation in DBMS
Deadlock Situation in DBMS: A deadlock is a situation in computer science and operating systems where two or more processes are unable to proceed because each is waiting for the other to release a resource. This situation creates a cycle of dependencies where each process holds a resource and waits for another resource held by another process, leading to an indefinite halt in all involved processes.
Deadlock Conditions in DBMS
A deadlock occurs if the following four conditions hold simultaneously:
- Mutual Exclusion: At least one resource must be held in a non-shareable mode; that is, only one process can use the resource at any given time.
- Hold and Wait: A process holding at least one resource is waiting to acquire additional resources currently being held by other processes.
- No Preemption: Resources cannot be forcibly taken from processes; they must be released voluntarily by the process holding them.
- Circular Wait: A set of processes is waiting such that each process is waiting for a resource held by the next process in the set, forming a circular chain.
Deadlock Prevention in DBMS
Deadlock prevention strategies ensure that at least one of the necessary conditions for deadlock cannot hold:
- Mutual Exclusion: This condition cannot usually be prevented, as some resources are inherently non-shareable (e.g., a printer).
- Hold and Wait: Ensure that a process holds no resources when it requests additional resources. This can be achieved by requiring a process to request all the resources it needs at once or ensuring that it releases all its current resources before requesting additional ones.
- No Preemption: Allow preemption of resources. If a process holding some resources requests another resource that cannot be immediately allocated to it, all resources currently held are released. The preempted resources are added to the list of resources for which the process is waiting.
- Circular Wait: Impose a total ordering of all resource types and ensure that each process requests resources in an increasing order of enumeration.
Deadlock Avoidance
Deadlock avoidance requires that the operating system be given additional information in advance concerning which resources a process will request and use during its lifetime. The system must use this information to make decisions dynamically to ensure that a circular wait condition never occurs.
- Banker’s Algorithm: This is one of the most well-known deadlock avoidance algorithms. It allocates resources to processes only if the resulting state is safe, meaning there is a sequence of all processes such that each process can be allocated the maximum resources it may need without causing a deadlock.
Deadlock Detection and Recovery
If prevention and avoidance are not feasible or practical, the system must be able to detect and recover from deadlocks.
- Deadlock Detection: The system must have an algorithm that can examine the state of the system to determine whether a deadlock has occurred.
- Recovery from Deadlock: Once a deadlock is detected, the system can recover by:
- Process Termination: Terminate one or more processes involved in the deadlock to break the circular wait.
- Resource Preemption: Preempt some resources from processes and give them to other processes until the deadlock cycle is broken.
By implementing these strategies, systems can handle or prevent deadlocks, ensuring that processes can complete their tasks without being stuck indefinitely.
