问题
It is a follow-up question from High Availability in Cassandra
1) Let's say we have three nodes N1, N2 and N3, I have RF =3 and WC = 3 and RC = 1, then which means I cannot handle any node failure in case of write.
2) Let's say If the N3 (Imagine It holds the data) went down and as of now we will not be able to write the data with the consistency as '3'.
Question 1: Now If I bring a new Node N4 up and attach to the cluster, Still I will not be able to write to the cluster with consistency 3, So How can I make the node N4 act as the third node?
Question 2: I mean let's say we have 7 node cluster with RF = 3, then If any node holding the replica went down, Is there a way to make existing other nodes in the cluster to act as a node holding the partition?
回答1:
Look at the docs:
https://docs.datastax.com/en/cassandra/3.0/cassandra/operations/opsReplaceNode.html
You want to replace a dead node in your scenario. N3 should be removed from the ring and replaced by N4.
It should be easy to follow that instructions step by step. It is critial that if you installed the node via package mangement to stop it before configuring it new and to wipe out all existing data, caches and commitlogs from it (often found under /var/lib/cassandra/*).
Also it is possible to remove a dead node from the ring with nodetool removenode as described here http://docs.datastax.com/en/cassandra/2.1/cassandra/tools/toolsRemoveNode.html and here https://docs.datastax.com/en/cassandra/3.0/cassandra/operations/opsRemoveNode.html - this removes the node from your cluster (and you should ensure that it cant come back after that before wiping out its data).
Remember it only removes a dead node from the ring and assigns the token ranges to the remaining nodes, but no streaming will happen automatically. You will need to rum nodetool repair after removing a dead node.
If you want to remove a live node you can use nodetool decommission - but as above, ensure the node does not reenter the cluster by wiping out it's data.
Update:
Nodes in Cassandra are not "named" in that fashion N1, N2, etc. internally. The nodes have an uuid and they own so called token ranges which they are responsible for.
If a node is down - simply repair it if possible at all, bring it online again to join it your cluster - if that took less than the default 3 hours you are fine. Otherwise run nodetool repair.
But if the node is 'lost' completely and will never come back, run nodetool removenode on that dead node. This asks cassandra to assign the token ranges the dead node was responsible for to the remaining nodes. After that run nodetool repair so the nodes will stream the data which is missing. After that your cluster will now have one node less, so it will be six nodes.
回答2:
Suppose you have a 7 node cluster. N1, N2, N3, ... ,N7. Suppose you have a data . That has RF = 3, Write consistency = 2, Read consistency = 2. Let's say node N1,N2, N3 are holding the data. If any of this nodes goes down,the cluster will be completely fine and data read/write operation will not be affected as long as consistency level for read and write operation is satisfied.
Suppose you have a data . That has RF = 3, Write consistency = 3, Read consistency = 3. Let's say node N1,N2, N3 are holding the data. If any of this nodes goes down,the operations will fail as the consistency level is not satisfied.
Now you can do two things if any of the N1,N2,N3 goes down:
1) You can replace the node. In this case newly replaced node will act like old dead node.
2) You can manually add a new node N8 and remove the old dead node N3. In this case Cassandra will distribute it's partiotioner among the ring and resize partiotion.
来源：https://stackoverflow.com/questions/48059762/how-to-bring-up-the-new-node