Deadlock in DBMS: Types, Handling, Applications

Deadlock occurs when two or more transactions cannot proceed because each is waiting for the other to release resources. This results in a cycle of dependencies where no transaction can be completed. Deadlocks typically happen when transactions hold some resources and wait for others, leading to a halt. DBMS uses deadlock detection and resolution techniques, like timeouts or transaction aborts, to break the cycle and restore progress.

There are two types of deadlocks in Database management system such as:

• Resource Deadlocks
• Communication Deadlocks

1. Resource Deadlocks

Resource Deadlocks occur when multiple processes require access to resources that are held by other processes, leading to a cycle of waiting. 

For example, if Process A holds Resource 1 and waits for Resource 2, while Process B holds Resource 2 and waits for Resource 1, a deadlock situation arises.

2. Communication Deadlocks

Communication Deadlocks are less common but can occur in distributed systems where processes communicate through messages.

For example, process A waits for a signal from B, B waits for a signal from C, and C waits for a signal from A. As each process depends on another to proceed, none can move forward, creating a deadlock where all processes are stuck, and unable to progress.

Deadlocks are characterized by four necessary conditions, commonly known as Coffman Conditions. These conditions provide a theoretical framework for understanding deadlocks in DBMS such as:

1. Mutual Exclusion

The Mutual Exclusion condition states that only one process can use a resource at a time. If multiple processes are attempting to access the same resource, the resource must be locked by one process, preventing others from accessing it at the same time.

2. Hold and Wait

The Hold and Wait condition occurs when a process is using one resource and is waiting for additional resources held by other processes. This creates a cycle where each process is waiting for resources that are locked by others.

3.No Preemption

The no preemption is the condition where resources cannot be taken from a process by force. A process can only release a resource voluntarily after it has completed its task.

4. Circular Wait 

It is the condition where a set of processes are waiting for each other in a circular chain. For example, Process A waits for Resource 1, Process B waits for Resource 2, and Process C waits for Resource 3, but ultimately, they all depend on each other, causing a circular wait.

In DBMS, it is the process of managing and understanding deadlocks to prevent them from impacting the system's performance and reliability. There are several strategies for handling deadlocks effectively such as:

1, Deadlock Avoidance 

This is a technique in a database management system (DBMS) that prevents deadlocks from occurring by monitoring the system's state and making decisions to keep processes from getting stuck. It's a proactive strategy that's often better than recovering from a deadlock, which can waste time and resources.

2. Deadlock Detection

Deadlock detection is a process that identifies if any processes in a system are stuck waiting for each other, preventing them from moving forward. This can be done by using a Wait-for Graph, where the system monitors the relationships between processes and resources. If the graph contains a cycle, a deadlock is detected, and necessary actions are taken.

3. Deadlock Prevention

Deadlock prevention is a method to ensure that processes do not get stuck waiting for each other and cannot move forward. It involves establishing rules to manage resource usage so that processes do not get into a deadlock situation.

4. Deadlock Recovery

This is the process of breaking a deadlock, which is when two or more transactions cannot proceed because they are waiting for resources held by other transactions.

Here are some applications of deadlock in database management systems such as:

• It ensures data consistency and proper execution of multiple concurrent transactions through deadlock detection and resolution.
• Deadlocks prevent transactions from waiting indefinitely for locks. When a transaction times out, it can be forced to release its current resources and try again later.
• A deadlock creates a cycle of dependencies where no transaction can continue.
• Deadlocks can significantly affect how well a system performs
• Allocates resources in a specific order to prevent circular waits.

Here are some drawbacks of deadlocks in database management systems such as:

• Deadlocks can cause the system to stop working, which can result in a loss of revenue and productivity for businesses that use the DBMS. 
• When transactions are blocked, the resources they require remain unused, resulting in a drop in system efficiency and wasted resources.
• Deadlocks can lead to a decrease in system concurrency, which can result in slower transaction processing and reduced throughput. 
• Resolving a deadlock can be a complex and time-consuming process that requires system administrators to manually get involved. 
• In some cases, recovery algorithms may require rolling back the state of one or more processes, which can lead to data loss or corruption.

A Wait-for Graph (WFG) is a directed graph used for deadlock detection in operating systems and relational database systems. It represents the dependencies among processes and resources in a system, helping to identify potential deadlocks.

Construction of the Wait-for Graph

In a WFG, processes are represented as nodes, and edges indicate the waiting relationship between processes. An edge from process Pj to Pk represents that Pj is waiting for Pk to release a lock on a resource. If a process is waiting for more than one resource to become available, multiple edges may represent a conjunctive (and) or disjunctive (or) set of different resources.

Detection of Deadlocks

The possibility of a deadlock is implied by graph cycles in the conjunctive case, and by knots in the disjunctive case. A cycle in the WFG indicates that a process is waiting for another process to release a resource, which in turn is waiting for a third process to release a resource, and so on. This creates a circular dependency, leading to a deadlock.

Here are some best practices for avoiding deadlocks in DBMS:

• Use timeouts: When acquiring resources, use timeouts to avoid waiting indefinitely. If a resource is not available within a certain time frame, the thread should release all acquired resources and try again later.
• Lock ordering: Establish a consistent order for acquiring locks. For example, if two transactions need to access tables A and B, make sure they both lock A before B or vice versa.‍
• Avoid nested transactions: Minimize transactions that require access to multiple resources simultaneously.

In conclusion, deadlocks in DBMS can severely impact performance and reliability. Understanding the causes, conditions, and methods of deadlock handling including deadlock detection, deadlock prevention, and deadlock recovery is essential for maintaining efficient database systems. By following best practices and understanding the implications of deadlocks, database administrators can ensure smooth operations and minimize risks associated with deadlock situations.

1. What are the types of deadlocks in DBMS?

There are 2 types of deadlock namely:

• Resource Deadlock: Transactions wait on resources (e.g., data rows, tables).
• Communication Deadlock: Processes are waiting for messages from each other.

2. Define the wait-for graph.

A wait-for graph is a directed graph used to detect deadlocks. Nodes represent transactions, and edges indicate waiting relationships (e.g., a transaction waiting for a resource held by another). A cycle in this graph signifies a deadlock.

3. What are the applications of deadlock in DBMS?

Deadlock management is critical in multi-user environments such as banking, online reservations, and e-commerce to ensure smooth database operations and prevent system failures.