ReaderWriter

The reader-writer problem, also called the readers-writers problem, is a classical synchronization challenge in concurrent programming. It concerns coordinating access to a shared data structure so that multiple readers can access the data simultaneously, but writers require exclusive access. The goal is to maximize concurrent reads while ensuring data integrity when writes occur, preventing data races and inconsistencies.

Solutions to the problem typically involve read-write locks or similar synchronization primitives. Three common scheduling policies are described: readers-preferential, writers-preferential, and fair (or balanced). In a readers-preferential scheme, readers may repeatedly acquire the lock, potentially starving writers. In a writers-preferential scheme, a writer waiting for the lock will be given priority when no readers are active, reducing writer starvation but potentially delaying readers. Fair policies aim to alternate access to prevent starvation for both sides.

A typical implementation uses a counter for the number of active readers, guarded by a mutex, plus a separate lock for the shared resource. When the first reader enters, it locks the resource to prevent writers; subsequent readers simply increment the counter and proceed. When a reader finishes, the counter is decremented, and the last reader releases the resource lock. Writers acquire the resource lock exclusively, blocking new readers and other writers until the write completes.

In modern programming, read-write locks are provided as built-in primitives in many languages and libraries (for example, pthreads, Java's ReentrantReadWriteLock, and .NET's ReaderWriterLockSlim). They are widely used in databases, file systems, caches, and in-memory data structures to optimize read-heavy workloads while maintaining data integrity.