Thread safety in design in LLD - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Understanding Thread Safety in Shared Resource Access

In a multi-threaded system, what is the main reason to use locks when multiple threads access a shared resource?

A To prevent threads from reading stale data by ensuring exclusive access during updates

B To increase the speed of thread execution by allowing simultaneous writes

C To allow threads to skip resource access if another thread is using it

D To reduce memory usage by sharing locks among unrelated resources

💡 Hint

Think about what happens if two threads write to the same data at the same time.

Locks ensure that only one thread can modify or read the shared resource at a time, preventing inconsistent or corrupted data.

❓ Architecture

intermediate

Designing a Thread-Safe Counter

You need to design a thread-safe counter that multiple threads can increment concurrently without losing updates. Which design choice ensures correctness?

A Use an atomic increment operation provided by the hardware or language runtime

B Use a simple integer variable without synchronization

C Use a global lock but only lock during reads, not writes

D Use separate counters per thread without combining results

💡 Hint

Consider how to avoid race conditions when multiple threads update the same value.

Atomic operations guarantee that increments happen without interruption, preventing lost updates.

❓ scaling

advanced

Scaling a Thread-Safe Cache System

You have a cache accessed by many threads. Using a single global lock causes contention and slows down the system. Which design improves scalability while maintaining thread safety?

A Use a single lock but increase its timeout to reduce waiting

B Partition the cache into multiple segments, each with its own lock

C Remove all locks and rely on eventual consistency

D Use a global lock but only lock during cache misses

💡 Hint

Think about dividing work to reduce contention.

Segmenting the cache reduces lock contention by allowing parallel access to different parts.

❓ tradeoff

advanced

Choosing Between Locking and Lock-Free Designs

Which is a key tradeoff when choosing a lock-free data structure over a lock-based one?

A Lock-free designs guarantee better performance in all scenarios

B Lock-free designs always use more memory and are slower than lock-based ones

C Lock-based designs never cause deadlocks or performance issues

D Lock-free designs reduce blocking but are often more complex to implement and debug

💡 Hint

Consider complexity and debugging challenges.

Lock-free designs avoid blocking but require careful design to avoid subtle bugs and are harder to maintain.

❓ estimation

expert

Estimating Maximum Throughput with Locks

A system has 8 threads accessing a shared resource protected by a single lock. Each thread holds the lock for 5 milliseconds per operation. Assuming no other delays, what is the maximum number of operations per second the system can perform?

A 400 operations per second

B 1600 operations per second

C 200 operations per second

D 800 operations per second

💡 Hint

Calculate how many operations fit in one second considering the lock is exclusive.

Since the lock is exclusive, only one thread can operate at a time. Each operation takes 5 ms, so 1000 ms / 5 ms = 200 operations per second.