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Scylla consistency quiz for administrators

Topic: Architecture and development

Learn: Understanding consistency in Scylla: a quiz

Audience: Scylla administrators

Q: When you run nodetool decommission to remove a node…

  1. Will the token range the node had be relocated to other nodes? What about vnodes?

  2. When and how will we know the “decommission” operation is finished?

A1. Yes. The node enters the state “STATE_LEAVING” while streaming its data to other nodes. Then it is in “STATE_LEFT” when the data has been streamed.

A2. Use nodetool netstats to check the state of a node.

Q: Let’s say I have a 16 node cluster using Network Topology Strategy across 2 data centers. The replication factor is TWO in each datacenter (DC1: 2, DC2: 2). If I write using a LOCAL_QUORUM, I will write the data to 4 nodes (2 in each data center) but when will the acknowledgement happen?

A: The acknowledgment will be sent to the user when two local replicas respond. In this case, floor(local_RF/2 +1) = 2 , so LOCAL_QUORUM is equal to RF per DC, which is why RF of 3 is usually better.

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