As i read and understood from official cassandra document and from other posts here when we configure system_auth replication factor is 1.
But i would like to understood, how the system_auth replication works if i configure value as system_auth replication = 2?
which two nodes will maintain replicas?
There will be two copies of the system_auth keyspace spread across ALL of your nodes. That way, if one goes down, the data is still available on another node. Different entries to system_auth may be stored on different nodes, but there will always be two copies.
If your replication factor = the number of nodes, then each node will hold all the system_auth data. If your replication factor > number of nodes, you are gaining nothing, since all nodes already have a full copy of the data, no extra safety here. If your replication factor < number of nodes, no node will hold a complete copy of the data, but it will hold a portion of it.
Here system_auth replication = 2 means data of system_auth will be replicated on 2 nodes(total 2 copy of data) on cluster. if one node goes down then you can also able to login and authenticate the node.
you may increase the replication factor as well.
Related
Lets say I have two Data Centers(DC1, DC2) in a Single Cassandra cluster.
DC1 - 4 nodes.
DC2 - 4 nodes.
Initially i have set the replication factor for all the keyspaces to be {DC1:2 , DC2:2}.(Network topology strategy)
But After some time lets say I alter the keyspace and change the replication factor to {DC:2} for all the keyspaces.(removing DC1).No replication factor for DC1.
So now what will happen? Will DC1 get any data written into it in the future?
Will all the token ranges be assigned to only DC2?
If you exclude DC1 - it won't get data written for that keyspace, nor data will be read from the DC1. Before switching off DC1, make sure that you perform nodetool repair on the serves in DC2, to make sure that you have all data synchronized. After changing RF, you
When you change RF for specific keyspace, drivers and Cassandra itself recalculate the token ranges assignments taking into account information about data centers.
I am newbie to cassandra.
What exactly replication factor in cassandra means?
For example,
I have 3 node cluster(node1,node2,node3) and If I create keyspace with replication factor 1,and insert data through node1,Can I read the data from other 2 nodes?
Or It will store the data in node1. Is data available in other 2 nodes for read/write operations?
The total number of replicas across the cluster is referred to as the replication factor. A replication factor of 1 means that there is only one copy of each row on one node. You should be able to read/write data from the other two nodes, depending on ports and firewalls between nodes.
Now presently we are having a Dc with 3 nodes and with a replication of 3, i am planning to decommission a node do i need to decrease my replication to 2 or just decommissioning node will adjust the data among the two nodes with a replication of 3??
Decommissioning a node will not cause your Cassandra cluster to break necessarily, but it will make it so that a few things will stop working.
A few things that will happen if you decommission the node but don't adjust the replication factor:
First, nothing about your replication factor will be changed just because you decommission a node. To do otherwise would cause chaos.
Queries (both read and write) that attempt to use ConsistencyLevel.ALL will fail, because they will not be able to get 3 machines to participate
Queries with ConsistencyLevel.QUORUM will be less available, because BOTH remaining machines will need to respond to queries to meet quorum.
Because you have 3 machines and a RF of 3, that means that every machine has a complete copy of the data. Decommission the node, update your replication factor, and then run nodetool repair on the remaining two nodes. After you do that, you should be good to go.
My 2 cents: I would suggest you to first change your replication to 2, run a repair on all nodes and then issue "nodetool decommission" from the node you want to decommission. There will be data moving around, but by doing it this way nothing should stop working.
I have a 3 nodes Cassandra cluster with 2 keyspaces. One of them has replication factor 1 and the other, replication factor 2. I want to reduce the cluster, using nodetool decommission, to remove 2 nodes and leave only one (single node cluster).
So, what must I do with the replication factor? I think both keyspaces must have replication factor 1, but when must I modify it? Before decommission?
Thanks a lot!
You must reduce the RF to 1 before the decommissioning and you must also run a repair on the keyspace you are reducing the RF on to be on the safe side. Then you can proceed with the decommissioning in a sequential manner.
You will need to reduce the Replication Factor to 1 and you should do this before you decommission the 2 nodes.
I am new to Cassandra and at work I have a 4 node cluster.
nodetool gossipinfo tells me that there are one datacentre, 2 racks and 2 nodes in each rack. Replication factor is defined as 2. nodetool ring tell me that each node has 50% ownership. There are 2 seed nodes in our config. Each rack has 1 seed node.
Does this mean that for each rack, there is one seed node and its replicated node. If that is the case then why is datasize not the same for seed node and its replicated node.
what happens if one node goes down. Will it have any impact on the data availability of the cluster.
Seeds
Seeds nodes are only special in the way that new nodes that join the cluster contact the seed nodes to find out about other nodes and the topology of the ring. But in Cassandra, all nodes are the same, i.e. there are no master or slave, no primary or secondary node. Because of this, you can elect any (or all) node as the seed.
Since seeds only relate to gossip information, it does not have anything to do with replicated data.
Size
In relation to data size, each node will never be exactly the same since each partition/row size is never the same. If you look at the nodetool cfstats output, you will see that there is a big range between minimum and maximum sizes.
Availability
If the reads are done with a consistency level CL=ONE, then if a node is down the other replica will continue to serve requests. But if reads are done with a higher consistency, then reads will fail since it needs 2 nodes to be available, i.e. CL=LOCAL_QUORUM requires [ RF/2 + 1 ] nodes to respond.
EDIT: Response to:
Shouldn't each node own 25%?
Ownership
In Cassandra, data is not "distributed" across ALL nodes in ALL DCs. In fact, a DC is a copy of another DC depending on the replication factor.
To illustrate, consider the following keyspace definition:
CREATE KEYSPACE "myKS"
WITH REPLICATION = {
'class' : 'NetworkTopologyStrategy',
'DC1' : 2,
'DC2' : 2};
Based on this definition, it means that the myKS keyspace has 2 replicas in DC1 and 2 replicas in DC2. Since each of your data centres only have 2 nodes, this effectively means that each DC is a copy of each other.
Following from that, since the tokens are split between 2 nodes, each node owns half of the data which is 50%. So in DC1, each node owns 50% and in DC2 (which is a copy of DC1) each node also owns 50%.