What are common cache invalidation strategies, and why is cache invalidation considered a genuinely hard problem?

Common strategies include: TTL-based expiration (simplest, but stale data possible until expiry), explicit invalidation (removing/updating the cache entry immediately when the underlying data changes, requiring careful tracking of every code path that mutates data), and versioned/tagged cache keys (invalidating whole groups of related entries at once by bumping a version number embedded in the key). It's hard because every single place data can change must remember to invalidate the correct cache entries, and distributed systems make coordinating invalidation across multiple cache instances even trickier.

// Explicit invalidation on write
public async Task UpdateProductAsync(Product product) {
    await _repository.UpdateAsync(product);
    await _cache.RemoveAsync($"product-{product.Id}");  // must remember this every time!
}

// Versioned key strategy
var cacheKey = $"products-v{await GetCacheVersionAsync()}";