18.04.2024 Problem:
The Production (Prod) and Disaster Recovery (DR) environments for Cassandra databases exhibit different sizes, as observed from the nodetool status output.
The client needs to identify the reason behind this discrepancy and ensure the readiness and reliability of the Disaster Recovery environment for use in the event of a failure.
Process:
The client provided the output of the command run on a Cassandra cluster. This command displays the status of each node in the cluster, including information such as:
- Address: IP address of each node
- Load: Amount of data stored on each node
- Tokens: Number of tokens assigned to each node for data distribution
- Owns (effective): Percentage of data owned by each node (effective ownership after data distribution)
- Host ID: Unique identifier for each node
- Rack: Rack information where each node resides
This information is essential for investigating the health and distribution of data across the Cassandra cluster. It helps identify any nodes that might be experiencing issues, such as being down or leaving/joining the cluster, and provides insights into data distribution and load balancing within the cluster.
Solution:
The issue of a significant difference in data between two data centers was approached and resolved by following these steps:
- Checking the replication factor of keyspaces to ensure they are equal. This was done by entering cqlsh and running DESCRIBE KEYSPACE {KEYSPACE_NAME} and examining the replication settings.
- Verifying data consistency between the data centers by running “nodetool repair” and “nodetool cleanup” commands. These commands were executed on all nodes, one by one.
- After successful execution of the commands, nodetool status was used to ensure that the data usage in both data centers was similar.
If the replication factor on keyspaces for both data centers is equal, similar data usage should be observed. However, if different replication factors per data center exist, differences in data sizes are expected.
Conclusion:
The observed 624 GB data difference between the two data centers raises concerns about data consistency. The initial focus should be on verifying the replication factor of keyspaces. This can be accomplished by using cqlsh to run DESCRIBE KEYSPACE {KEYSPACE_NAME} and examining the replication settings. Subsequently, execute nodetool repair and nodetool cleanup on each node individually to ensure data consistency across data centers. A successful execution should reflect similar data usage in nodetool status, particularly if the replication factor for keyspaces across data centers is equal. However, it’s essential to acknowledge that variations in data sizes are expected if different replication factors exist per data center.