Question 1

What is a deadlock, and how do you avoid one?

Accepted Answer

A **deadlock** is a state in which two or more processes are each waiting for a resource held by another, so none can proceed and none will ever proceed without intervention.

**The classic shape.** Thread A holds lock 1 and wants lock 2. Thread B holds lock 2 and wants lock 1. Neither will release what it holds until it gets what it wants. They wait permanently.

**The four Coffman conditions**, which must **all** hold for deadlock to be possible — and this is the useful part, because breaking any one of them prevents it:

**Mutual exclusion** — at least one resource is held exclusively.

**Hold and wait** — a process holding a resource requests another.

**No preemption** — resources cannot be forcibly taken back.

**Circular wait** — a closed chain of processes each waiting on the next.

**Prevention strategies, in rough order of practicality:**

**Lock ordering.** Establish a global order for acquiring locks and always acquire in that order. This breaks circular wait and is by far the most widely used technique, because it requires no runtime machinery — only discipline and documentation.

**Lock timeouts.** Attempt acquisition with a deadline, and back off and retry on failure. Breaks hold-and-wait. Introduces the possibility of **livelock**, where processes repeatedly retry in lockstep and still make no progress — random backoff addresses that.

**Acquire everything at once**, or nothing.

**Reduce lock scope and duration**, and prefer **lock-free structures** or message passing where the problem allows. The most reliable way to avoid deadlock is to hold fewer locks.

**Detection and recovery** — periodically check for cycles in the wait-for graph and abort a participant. Databases do this routinely, which is why applications must be prepared to retry a transaction aborted as a deadlock victim.

**Where they actually appear in practice:** database transactions updating rows in different orders, nested synchronised methods, mixing synchronous and asynchronous code, and connection pool exhaustion — where a task holds a connection while waiting for another connection from the same pool.

Question 2

What are mutexes, semaphores and atomic operations?

Accepted Answer

Three mechanisms for coordinating concurrent access to shared state, at different levels of abstraction — and choosing the wrong one is how concurrency bugs are introduced. Atomic operations. A

Question 3

What is the difference between a process and a thread?

Accepted Answer

A process is an independent running program with its own memory space. A thread is a unit of execution inside a process, sharing that process's memory with its sibling threads. What a process owns:

Question 4

What is the difference between concurrency and parallelism?

Accepted Answer

Concurrency is about structure; parallelism is about execution. A concurrent program is composed of tasks that can make progress independently; a parallel program actually runs them at the same

Question 5

What is the difference between synchronous and asynchronous code?

Accepted Answer

Synchronous code runs one statement after another, and each waits for the previous to finish. Asynchronous code starts an operation and continues without waiting, handling the result when it arrives.

Question 6

What is a circuit breaker, and how do systems fail gracefully?

Accepted Answer

A circuit breaker stops calling a failing dependency for a while, so a slow or broken service does not take down everything that depends on it. It exists because the default behaviour — retrying

Question 7

How to handle exceptions in Java?

Accepted Answer

Handling exceptions in Java is essential for robust applications. Use try-catch blocks, multiple catches, finally blocks, and custom exceptions to manage errors effectively.

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