Compare and contrast optimistic and pessimistic concurrency control strategies in EF Core. When would you choose one over the other?
.NET interview question for Advanced practice.
Answer
Optimistic Concurrency: Assumption: Assumes that conflicts are rare. Mechanism: It does not lock data. Instead, it checks if the data has been modified by another transaction before committing its own changes, typically using a version number or timestamp column. If a conflict is detected, EF Core throws a DbUpdateConcurrencyException. Pros: High concurrency, less database overhead, no risk of deadlocks from locking. Cons: Requires application-level logic to handle the DbUpdateConcurrencyException and resolve conflicts (e.g., retry, merge, notify user). It can result in 'lost work' if conflicts are frequent and resolution is simply 'last one wins'. Pessimistic Concurrency: Assumption: Assumes that conflicts are likely. Mechanism: It locks data when it is read to prevent other transactions from modifying it until the lock is released. This is typically done using database-specific SQL hints like FOR UPDATE. Pros: Guarantees that conflicts cannot happen, simplifying application logic as there is no need for conflict resolution. Cons: Significantly reduces concurrency as other users must wait for locks to be released. Increases the risk of deadlocks. Can lead to poor scalability, especially in high-throughput systems. When to Choose: Choose Optimistic Concurrency for most web and business applications where the likelihood of two users editing the exact same record at the exact same time is low. It offers better performance and scalability. Choose Pessimistic Concurrency for systems where conflicts are very frequent or the business cost of a conflict is extremely high (e.g., a reservation system for a single, unique item). It is more common in desktop or stateful applications than in web applications.
Explanation
Most web applications favor optimistic concurrency because connections are typically short-lived, and holding long-term database locks (pessimistic concurrency) is not scalable.