In database management systems (DBMS), the deadlock problem can be an important factor that considerably hampers the productivity and performance of a system. It describes a scenario in which two or more transactions are in a blockage situation because they need to wait for each other to release resources. This chain of dependence will result in no transaction being completed, which thus causes the system to be stopped.
Deadlock is the central issue in database management systems from its generation and detection to its proper resolution and prevention, therefore only by understanding it can we ensure streamlined and efficient database operations.
In this blog, we will go deep into the subject of deadlocks in DBMS, their reasons for appearance, techniques of detection, strategies of prevention, and multiple solutions. We intend to be of help in understanding some of the issues related to deadlocks.
What is Deadlock in DBMS?
Deadlocks in a DBMS occur when two or more transactions are in a situation of not being able to move forward, where a resource is required by the transaction that the other transaction holds. The above situation happens as a result of the too-solid interdependence between them. A circular dependency among joining processes then appears. This implies that none of the transactions signals the resources it holds because it is still waiting to receive the resources owned by the other transaction. This leads to a standstill condition between the two transactions.
The occurrence of deadlocks is a special kind of problem of concurrency in which multi-user environments are particularly susceptible, especially when several transactions share the same pool of resources. When it comes to DBMS, the resources that are usually mentioned are locks on data, memory, or input/output devices.
Characteristics of Deadlock
For a deadlock to occur, all four conditions have to be fulfilled at the same time in the DBMS known as the necessary conditions for deadlock.
- Mutual Exclusion: The resource cannot be shared by more than one transaction at the same time (mutual exclusion).
- Hold and Wait: A transaction that is already in control of one resource is waiting for the additional resources that are being held by other transactions.
- Lack of preemption: Once a resource is allocated to a transaction, it cannot be forcibly taken away from it at all unless that transaction relinquishes control over it voluntarily.
- Circular wait: There is a closed loop of transactions with each transaction waiting for the resource held by the next transaction in line.
When these four conditions are satisfied, a deadlock will occur, and the system will be in a starving state.
Causes of Deadlock in DBMS
There can be several reasons that can lead to deadlocks in the DBMS, mostly related to the way transactions acquire and release resources. Some common causes of a deadlock are as follows:
1. Resource Contention
When two or more transactions request the same resources locks on data items-and if these resources are not available simultaneously, then deadlock can occur. For instance, if Transaction A locks Resource X and requests Resource Y, while Transaction B holds a lock on Resource Y and requests Resource X - a deadlock situation arises.
2. Improper Locking Mechanisms
An inadequate or incorrect locking mechanism can also give rise to the occurrence of deadlock. For example, if a DBMS allows transactions to acquire locks in arbitrary order, then a circular wait condition can arise.
3. Long Transactions
Long-running transactions put the deadlock probability at risk. The longer a transaction uses a resource, the higher the chance it will wait for a resource assigned to another transaction, yielding a circular wait.
4. Concurrency Issues
High concurrency in multi-user environments often results in different transactions competing for the same resources; lack of effective management can also easily give rise to deadlock.
Deadlock Detection
Deadlock detection means determining when a deadlock has occurred in a DBMS. A few algorithms can be used for deadlock detection, and the choice of a suitable deadlock detection strategy depends on system needs.
1. Wait-for Graph
The wait-for graph is a directed graph that could be used to detect deadlocks in DBMS. In this graph, each transaction is represented by a node, and a directed edge from one transaction to another denotes that the former is waiting for some resource being held by the latter. In that way, if the cycle is detected in the graph, it detects deadlock since there exists a particular order that has occurred in the transactions, each waiting on the other to release some resource.
Example of a Wait-for Graph:
- Transaction A waits for Resource 1, which is held by Transaction B.
- Transaction B waits for Resource 2, which is held by Transaction A.
This forms a cycle in the wait-for graph, indicating a deadlock.
2. Resource Allocation Graph (RAG)
Another technique used to detect deadlock is the resource allocation graph (RAG). It is a directed graph containing nodes representing transactions and resources. The edges between the nodes indicate the following:
- An edge from a transaction to a resource is an indication that the transaction is requesting that resource.
- A directed edge from a resource to a transaction indicates that the resource is allocated to that transaction.
If there exists any cycle in the Resource Allocation Graph - a deadlock would be detected.
Deadlock Prevention
Deadlocks can be prevented if either at least one of the four conditions is avoided or an appropriate mechanism is employed for locking the shared resources. The design of the system should be such that, eventually, deadlock cannot occur.
1. Eliminating Circular Wait
One can develop a deadlock-free algorithm by enforcing that circular wait never occurs in the system. For example, in this process, locks for transactions are enforced in a strict unidirectional way. For example, transactions may be required to acquire resources sequentially, i.e., Resource 1 could be acquired before Resource 2.
2. Preemption
Preemption is the act of taking resources away from one transaction to break the deadlock cycle. For example, if transaction A is waiting for resource X, which is held by transaction B, the DBMS could preemptively release resource X from transaction B to transaction A if transaction A had previously requested it.
3. Avoiding Hold and Wait
One approach to eliminate deadlocks is to enforce that transactions shall request all the resources required at once. If the transaction cannot obtain all the resources requested, it will relinquish the ones it holds and will need to wait for its turn to request resources later. As a result, a transaction cannot end up in a situation where it holds on to some resources and waits indefinitely for others.
4. Priority-Based Scheduling
In some systems, deadlock can be resolved by employing priority-based scheduling. The transactions with higher priority are permitted to proceed and those with lower priority may either be aborted or delayed. This approach ensures that the critical transactions can keep executing while the others may roll back.
Conclusion
Deadlock poses a major problem for the DBMS in which the repercussions on system performance can be fatal. To maintain a smooth and effective database environment, it is imperative to understand the causes of deadlock, its detection, and the various approaches to deadlock prevention and resolution.
Although not all deadlocks are avoidable, deadlocks can be minimized and eliminated through effective transaction management, monitoring, and the appropriate detection and prevention schemes to provide for the continued functioning of a database system.