Which best distinguishes immediate consistency from transactional consistency in distributed systems?

• A. Immediate consistency ensures all reads see the latest writes; transactional consistency ensures ACID properties within a transaction.
• B. Immediate consistency guarantees no conflicting updates, while transactional consistency allows partial results.
• C. Immediate consistency focuses on latency, transactional consistency on durability.
• D. Immediate consistency implies eventual propagation, transactional consistency implies instant propagation.

Answer: (A)

Immediate consistency is about how writes become visible across the system. Once a write completes, every read should reflect that write, regardless of which node serves the read, effectively giving a real-time, global order of operations.

Transactional consistency, on the other hand, is about the guarantees inside a unit of work. All operations in a transaction succeed or fail together, preserving atomicity, isolation, and durability (the ACID properties) so the system moves from one consistent state to another without exposing partial results.

So the best answer captures these ideas: reads see the latest write immediately, whereas a transaction enforces ACID properties for all operations within it. The other statements mix up visibility guarantees, latency, or the notion of partial results, which aren’t the defining aspects of these concepts.