What is replication an answer to?

• Clocks drifting apart so timestamps cannot be trusted
• Messages overtaking each other on different routes
• One machine failing and taking the only copy with it

You call a node and hear nothing. Which two states of the node itself look exactly this way from outside?

• A crash and mere slowness
• A healthy node and one that replies with an error
• Slowness and a full disk

A replica promises to serve only what it has caught up with. After rejoining with stale state, it may serve reads immediately.

Why do systems use timeouts if a timeout cannot prove a crash?

• Timeouts let the system act despite never knowing for sure
• Timeouts prove the node crashed
• Timeouts restart crashed disks

A node misses three writes while down, then rejoins. What are its correct steps?

• Serve reads at once, then copy the writes quietly
• Copy the three missed writes, then serve reads
• Delete its copy and stay silent forever

A replica returns a value that the primary never wrote. Which promise did the replica break?

• Answering every client request within a fixed time
• Keeping a durable copy of its data on local disk
• Serving only what it has copied from the primary

Replicas copy the primary's writes after a short delay. Two clients read the same key from two healthy replicas at the same moment and get different values. What is the most likely story?

• One replica is stale and has not yet caught up
• The primary wrote two values for the key at once
• One replica crashed and is answering from memory

A designer treats every silent node as crashed and deletes its data. One slow node returns to find its copy wiped. What was the design error?

• Timeouts were set too generously
• Silence was read as proof of crash, which it never is
• Replication was used where a single copy sufficed