Cassandra data replication failure

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Symptom

When replicating data during a multi-region expansion, the CassandraDataReplication status may show an error state and data replication may fail.

Error message

When you use kubectl to view the rebuild status:

  kubectl -n apigee get apigeeds \
  -o jsonpath="{.items[].status.cassandraDataReplication}{'\n'}"

You see that one or more Cassandra pods show an error state and a message stating the rebuild has failed. For example:

{
  "rebuildDetails": {
    "apigee-cassandra-default-0": {
      "message": "failed to rebuild from us-west1: java.lang.IllegalStateException : Unable to find sufficient sources for streaming range (-8567285182390470134,-8567154549835592965] in keyspace system_distributed",
      "state": "error",
      "updated": 1641581899
    },
    …
  }
}

Possible causes

Cause	Description	Troubleshooting instructions applicable for
Incorrect source region	An incorrect value was specified for the `source.region` within the Cassandra data replication YAML file.	Apigee hybrid
Network connectivity issues	There could be network connectivity issues between Cassandra pods in different data centers.	Apigee hybrid

Common diagnosis steps

Fetch the status of the data replication:

kubectl -n apigee get apigeeds \
-o jsonpath="{.items[].status.cassandraDataReplication}{'\n'}"

If you see an error with a message similar to the one specified in Error message, then it indicates that you are observing this issue.

Cause: Incorrect source region

If you specify a source region (datacenter) in your data replication YAML file that is different from the actual source datacenter, then the data replication will fail. Perform the steps in Diagnosis to analyze this scenario and perform the steps in Resolution to correct it.

Diagnosis

List all the Cassandra pods in the source region:

kubectl -n apigee get pods -l app=apigee-cassandra

Get the actual datacenter value from any one of the Cassandra pods returned in step 1:

kubectl -n apigee exec -it apigee-cassandra-default-0 -- \
nodetool -u JMX_user -pw JMX_password status

Get the value used for source.region in the Cassandra data replication custom resource (YAML) file you created in Multi-region deployment. If you are using the example file name found in the Multi-region deployment documentation, then the file should be named datareplication.yaml.
```
cat datareplication.yaml
```
Example results:
```
apiVersion: apigee.cloud.google.com/v1alpha1
kind: CassandraDataReplication
metadata:
  name: region-expansion
  namespace: apigee
spec:
  organizationRef: apigee-hybrid-example-org
  force: false
  source:
    region: "us-west1"
```

Check the result of the nodetool status output and verify whether or not the datacenter value matches the source.region value:

kubectl -n apigee exec -it apigee-cassandra-default-0 -- \
nodetool -u jmxuser -pw iloveapis123 status

Example results:

Datacenter: dc-1
================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
--  Address      Load       Tokens       Owns (effective)  Host ID                               Rack
UN  10.104.13.2  491.84 KiB  256          100.0%            7254711c-fe0a-4b34-b50f-861109f97936  ra-1
UN  10.104.11.3  527.36 KiB  256          100.0%            5ec389f0-fd67-4de6-9f21-172d5899ff78  ra-1
UN  10.104.12.7  838.46 KiB  256          100.0%            7a88be82-1f81-4117-86e3-2cda434c0878  ra-1

Note that the source.region (us-west1) from the datareplication.yaml file does not match the actual datacenter value (dc-1) from the nodetool status output. Follow the steps in Resolution to correct the configuration.

Resolution

To correct the data replication, you'll need to delete the data replication job and create it with the correct datacenter name. Perform the following steps:

Delete the current data replication process. If using the example file name found in the Multi-region deployment documentation, then the file should be named datareplication.yaml.
```
kubectl delete -f datareplication.yaml
```

Update the region name in the YAML file to the correct datacenter value, e.g. dc-1:

apiVersion: apigee.cloud.google.com/v1alpha1
kind: CassandraDataReplication
metadata:
  name: region-expansion
  namespace: apigee
spec:
  organizationRef: apigee-hybrid-example-org
  force: false
  source:
    region: "dc-1"

Apply the updated data replication:
```
kubectl apply -f datareplication.yaml
```
Check the rebuild status using the following command and verify that you no longer see the error state previously reported:
```
  kubectl -n apigee get apigeeds \
  -o jsonpath="{.items[].status.cassandraDataReplication}{'\n'}"
```
If the problem still persists, proceed to Cause: Network connectivity issues.

Cause: Network connectivity issues

The data replication error may also be the result of connectivity issues between Cassandra nodes.

Diagnosis

Perform the following steps to analyze this scenario:

List all the Cassandra pods:

# list cassandra pods
kubectl -n=apigee get pods -l app=apigee-cassandra

Execute the following curl command and telnet to the first Cassandra pod in the second data center (dc-2) from the first Cassandra pod in the first data center (dc-1) using port 7001:
```
kubectl -n apigee exec -it apigee-cassandra-default-0 bash -- curl -v telnet://DC_2_APIGEE_CASSANDRA_DEFAULT_0_POD_IP:7001
```

If telnet was successful, an output similar to following is displayed:

* Rebuilt URL to: telnet://10.0.4.10:7001/
*   Trying 10.0.4.10...
* TCP_NODELAY set
* Connected to 10.0.4.10 (10.0.4.10) port 7001 (#0)

Otherwise, an error similar to following is displayed:

* Rebuilt URL to: telnet://10.0.4.10:7001/
*   Trying 10.0.4.10...
* TCP_NODELAY set
* connect to 10.0.4.10 port 7001 failed: Connection refused
* Failed to connect to 10.0.4.10 port 7001: Connection refused
* Closing connection 0
curl: (7) Failed to connect to 10.0.4.10 port 7001: Connection refused

The connectivity failure from the Cassandra pod in one data center to the Cassandra pod in another data center indicates that there must be a firewall restriction or some sort of network connectivity issue.

Resolution

If this Apigee hybrid deployment is on GKE, check to see if any firewall rules are set that block the traffic from one data center to another and analyze the network connectivity problem by referring to VPC firewall rules overview.
If this Apigee hybrid deployment is on GKE-on-prem, work with the relevant networking team and analyze the network connectivity problem.

If the problem still persists, go to Must gather diagnostic information.

Must gather diagnostic information

If the problem persists even after following the above instructions, gather the following diagnostic information and then contact Apigee Support:

The Google Cloud Project ID.
The Apigee hybrid organization.
The overrides.yaml files from both source and new regions, masking any sensitive information.
The CassandraDataReplication YAML file.

Cassandra nodetool status output:

kubectl -n apigee exec -it apigee-cassandra-default-0 -- \
nodetool -u JMX_user -pw JMX_password status

Cassandra nodetool describecluster output:

kubectl -n apigee exec -it apigee-cassandra-default-0 -- \
nodetool -u JMX_user -pw JMX_password describecluster