CODEWITHSUNDEEP
cassandra
Kshitij Mittal
Kshitij Mittal

Reputation: 2766

Cassandra not working when one of the nodes is down

I have a development cassandra cluster of two cassandra nodes [Let's call them NodeA and NodeB]. I also have a script that is continuously sending data on NodeA. I have created the database with the following parameters:

CREATE KEYSPACE test_database WITH replication = {'class': 'SimpleStrategy', 'replication_factor': '1'}  AND durable_writes = true;

Now, for some reason NodeB is stoping after some time. But the issue is, as soon as NodeB stops, the script that is sending data to NodeA starts giving data insertion error.

Can anyone point out a probable reason for the same.

Update: Both the nodes are seed nodes.

Upvotes: 2

Views: 3794

Answers (3)

ROHAN VADJE
ROHAN VADJE

Reputation: 233

If your table is not counter table , you can use consistency level of ANY which gives high availaiblity for write. Refer this to learn more about it => https://www.datastax.com/blog/2011/05/understanding-hinted-handoff-cassandra-08

Upvotes: 0

Whitefret
Whitefret

Reputation: 1067

How Cassandra handle data repartition

Each key in cassandra can be converted to a token. When you install your cluster, the nodes calculate what range of token they will accept.
Let's take a simple example:

You have two nodes, and a token that goes from 0 to 9. A simple repartition would be: node A stores every token between 0-4 and node B stores every token between 5-9.

How Cassandra works for write

You choose a Coordinator (in your case node A), that receive the data. This node will then calculate a token. As seen in the first example, every node has a range of token assigned to it. So imagine the key is converted to token 4, then the data goes to node A (here the coordinator). If the token is 8, the data will be sent to node B.

What is cassandra data replication factor

The replication factor is how many time your data will be stored on your cluster. For a single database with no racks (your case), the data is first send to the node who owns the token associated with the key, and the replicas are sent to the next node in the topology.
In case of failure of one node, the replicas will help the node to restore its data.
In your case, there are no replicas, and if a node is down, Cassandra can't store the data and throws an error. If you have replication factor 2, Cassandra should be able to store a replica on node A and not fail.

Upvotes: 2

Srinivasan MK
Srinivasan MK

Reputation: 801

Cassandra's Replication Factor: Lets say we have 'n' as replication factor which means given input data will be stored/retrieved from 'n' nodes. t If you mention the replication factor as '1' which means only one node will have the data.

Partitioning: Lets say we have 2 nodes, whenever you are inserting the data. Both these nodes will have some data, based on partitioning algorithm mentioned. For example: You are inserting 10 records, based on the hashing and partitioning algorithm, it chooses which node needs to be written for each record. Of-course the identification of node is done by the Coordinator :)

Durable Writes: By default, cassandra always write in commit-log before flushing to disk. If you set to false, it will bypass commit-log and write directly to disk(SSTable).

The problem you have mentioned, for example lets say you are inserting 10 rows. For simplicity, we can make the partitioning/hashing calculation as n/2.

So, Cassandra's Coordinator node splits up your data into two pieces(for simple calculation it will be 10/2) and tries to put 1st half in to 1st node and succeeds and tries to put the 2nd half into the second node(writing to commit-log), since it is unavailable it is throwing error.

Upvotes: 1

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