What is deadlock in a distributed system?

Deadlock in a distributed system occurs when a group of processes becomes stuck, each waiting for resources held by others in the group, and none can proceed. It results from circular dependencies among processes in different parts of the system, leading to a standstill.

Key Characteristics of Deadlock in Distributed Systems

1. Distributed Nature: Processes involved in the deadlock may reside on different nodes across the network.
2. Resource Dependency: Each process holds one or more resources and is waiting for others held by another process.
3. No Preemption: Resources cannot be forcibly taken from a process; they must be released voluntarily.
4. Circular Wait: A closed chain of processes exists, where each process is waiting for a resource held by the next process.

Necessary Conditions for Deadlock

1. Mutual Exclusion: Resources are non-shareable and can only be held by one process at a time.
2. Hold and Wait: A process holding resources is waiting for additional resources held by others.
3. No Preemption: Allocated resources cannot be forcibly taken away.
4. Circular Wait: A cycle of processes exists, each waiting for a resource held by another process.

Example of Deadlock in a Distributed System

Consider two processes running on different servers:

• Process A holds Resource X and requests Resource Y.
• Process B holds Resource Y and requests Resource X.
  Both processes wait indefinitely, resulting in a deadlock.

Detection and Resolution in Distributed Systems

1. Deadlock Detection:

   • Global Wait-For Graphs: Combine local wait-for graphs from all nodes to detect cycles that indicate deadlocks.
   • Centralized Detection: A central coordinator collects and analyzes resource allocation states.
   • Distributed Detection: Nodes communicate to identify cycles in a distributed wait-for graph.

2. Deadlock Resolution:

   • Resource Preemption: Forcefully take resources from processes and reassign them.
   • Process Termination: Terminate one or more processes in the deadlock cycle.
   • Rollback: Roll back some processes to a safe state and release resources.

Prevention Strategies

• Avoid Circular Wait: Impose a global ordering of resources and require processes to request resources in that order.
• Avoid Hold and Wait: Require processes to request all needed resources at once.
• Timeout Mechanisms: Abort processes if they wait too long for resources.
• Distributed Algorithms: Use advanced distributed resource allocation protocols to avoid dependencies leading to deadlocks.

Deadlocks in distributed systems are more challenging to manage due to their decentralized nature and lack of global visibility, making robust detection and resolution strategies critical for maintaining system performance and reliability.