The value of x starts at 1. A write of x = 2 finishes. Then a read of x starts. No other operation runs. What must the read return?

• 1, the value from before
• 2
• Either 1 or 2

Each operation appears to take effect at one instant. Where must that instant sit?

• Somewhere between its call and its return
• At the moment the server first hears about it
• Just after its return, once the client has the answer

A write of x = 4 finishes. Then two reads start one after the other, and both return 4. This history is linearizable.

A write of x = 1 starts and finishes. Then a write of x = 2 starts and finishes. Then a read starts and returns 1. What is the verdict?

• Not linearizable: the read missed the latest write
• Linearizable, because the read is allowed to be stale
• Linearizable, if the two writes are swapped in order

A write of x = 1 and a write of x = 2 overlap in time. A read that overlaps both returns 1. Is there a single-object explanation?

• No, overlapping writes never allow any valid order
• No, a read that overlaps a write must return 2
• Yes: write 1, then the read, then write 2

The client times and the server's private log disagree about which operation ran first. What decides the check?

• The order the server wrote the operations to its log
• The order the replicas applied them to disk
• The call and return times the clients saw

Write A finishes. Then write B starts. A read that overlaps write B returns B's value. Which order explains it?

• B, then A, then the read
• A, then B, then the read
• A, then the read, then B

A database runs every transaction in some valid order. But a read that starts after a write finishes sometimes returns the old value. What is true?

• It is linearizable, since the order is valid
• It can be serializable but not linearizable
• It is neither, because stale reads break every model