Deadlock in DBMS

Deadlock is a scenario in DBMS where two or more transactions are indefinitely blocked, each waiting for a resource that the other transaction holds. Essentially, it’s a state of impasse where no progress can be made, leading to a standstill in the execution of transactions.

In the below PDF we discuss about Deadlock in DBMS in detail in simple language, Hope this will help in better understanding.

Causes of Deadlock:

Deadlocks typically occur due to the following conditions, known as the “deadlock conditions”:

  1. Mutual Exclusion: Resources such as database rows or tables cannot be shared concurrently between transactions. If one transaction holds a resource, it may prevent others from accessing it until it releases it.
  2. Hold and Wait: Transactions acquire resources incrementally while still holding onto previously acquired resources. This condition can lead to situations where transactions wait indefinitely for resources held by others.
  3. No Preemption: Resources cannot be forcibly taken away from a transaction. If a transaction holds a resource and requires additional resources held by other transactions, it might wait indefinitely.
  4. Circular Wait: A cycle of transactions exists where each transaction in the cycle is waiting for a resource held by the next transaction in the cycle.

Deadlock Detection & Prevention in DBMS :

Deadlock detection is a mechanism employed in Database Management Systems (DBMS) to identify and resolve deadlocks that may occur in multi-user environments where transactions contend for shared resources. A deadlock arises when two or more transactions are waiting indefinitely for resources held by each other, preventing any of them from making progress.

The process of deadlock detection involves periodically examining the resource allocation graph or other data structures to identify cycles, which indicate potential deadlocks. Once a cycle is detected, the DBMS takes appropriate actions to resolve the deadlock and allow the affected transactions to proceed.

Key steps in deadlock detection in DBMS include:

  • Resource Allocation Graph: The DBMS maintains a resource allocation graph to represent the allocation of resources to transactions. Nodes in the graph represent transactions, and edges represent resource requests or allocations.
  • Cycle Detection: Periodically, the DBMS scans the resource allocation graph to detect cycles. A cycle in the graph indicates a potential deadlock situation, as it implies that transactions are waiting for resources held by each other in a circular dependency.
  • Transaction Abort: Upon detecting a deadlock, the DBMS selects one or more transactions involved in the deadlock and aborts them to break the cycle and release the resources they hold. The choice of transactions to abort may be based on various criteria, such as transaction priority, resource usage, or runtime.
  • Rollback and Recovery: Once a transaction is aborted, the DBMS rolls back its changes to maintain data consistency and integrity. Any changes made by the aborted transaction are undone, and the affected resources are released. The DBMS then allows the remaining transactions to proceed.


Deadlock Prevention:

By carefully managing resource allocation and transaction scheduling, deadlock can be prevented. Techniques such as strict two-phase locking and wound-wait protocol are commonly used for this purpose.


In Conclusion, Deadlock is a challenging issue in DBMS that can impede system performance and reliability. Understanding its causes and implications is crucial for database administrators and developers to implement effective prevention and resolution strategies. By employing a combination of prevention techniques, detection mechanisms, and resolution strategies, the impact of deadlock can be minimized, ensuring the smooth operation of database systems and the applications they support.

Related Question

A deadlock in a Database Management System (DBMS) occurs when two or more transactions are waiting indefinitely for each other to release resources, resulting in a standstill where none of the transactions can proceed.

Deadlocks require four conditions to be present simultaneously: mutual exclusion, hold and wait, no preemption, and circular wait.

Mutual exclusion means that only one transaction can use a resource at a time. If a transaction holds a resource and another transaction needs the same resource, it must wait, potentially leading to a deadlock if the waiting transaction holds resources needed by the first transaction.

Hold and wait condition implies that a transaction holding resources can request additional resources while waiting for others to be released. If the requested resources are held by another transaction, it may lead to a deadlock if neither transaction releases its resources.

In DBMS, preemption refers to forcibly taking resources away from a transaction. If preemption is not allowed, a transaction that is holding resources may continue to hold them indefinitely, potentially causing other transactions to wait indefinitely and leading to a deadlock.


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