SOLR-18130: rename parameter "zkHost" to "solrCloud" in "solrj-streaming"

[vyatkinv]

https://issues.apache.org/jira/browse/SOLR-18130

Description

This PR updates the solrj-streaming module by replacing all usages of zkHost with solrCloud to enable support for HTTP-based quorum configurations.

Solution

Parameters, fields and variables in solrj-streaming, named as zkHost renamed to solrCloud
For backward compatibility, specifying zkHost is still supported.
A shared method has been introduced in an abstract class to resolve the effective solrCloud value using a priority-based approach (e.g., explicit parameter → legacy zkHost → default Zookeeper host).

Tests

To Be Done

Checklist

Please review the following and check all that apply:

• I have reviewed the guidelines for How to Contribute and my code conforms to the standards described there to the best of my ability.
• I have created a Jira issue and added the issue ID to my pull request title.
• I have given Solr maintainers access to contribute to my PR branch. (optional but recommended, not available for branches on forks living under an organisation)
• I have developed this patch against the main branch.
• I have run ./gradlew check.
• I have added tests for my changes.
• I have added documentation for the Reference Guide
• I have added a changelog entry for my change

Planned follow-ups:

• Perform more thorough testing of solrj-streaming with both Zookeeper-based and HTTP-based quorum configurations
• Add unit tests for HTTP quorum support
• Review and improve error messages and java-doc, where zookeeper still mentioned
• Add documentation and changelog entry

Open Questions / Discussion Points

I would appreciate feedback on the following topics (also noted as TODOs in the code):

1. Mandatory solrCloud parameter

I made solrCloud required in all stream handlers (an error is thrown if it cannot be resolved from parameters or the default Zookeeper host).
This caused two tests to fail:

• StreamExpressionTest#testCloudSolrStream

• StreamExpressionTest#testStatsStream

These tests did not provide any valid host.
I fixed them by adding .withDefaultZkHost(cluster.getZkServer().getZkAddress())

Question:

• Is this a test issue or a flaw in the implementation?

• Can such a situation occur in real-world usage?

2. toExpression behavior and backward compatibility

Currently, toExpression reconstructs the expression and always includes solrCloud, even if the user originally provided the legacy zkHost parameter.
As a result, a user who runs an expression with zkHost and then calls explain() will see solrCloud in the output.

Question:

• Should the original parameter name (zkHost) be preserved?
• Or is it acceptable to normalize everything to solrCloud?

3. Potential issue with gather parameter
In:
org/apache/solr/client/solrj/io/graph/GatherNodesStream.java:392
there is the following line:
expression.addParameter(new StreamExpressionNamedParameter("gather", solrCloud));

Question:

• Is this a bug?
• Or is solrCloud (previously zkHost) actually intended to be passed as gather?

UPDATE: I looked into the context more deeply and yes, it looks like this is a defect that affects explain(). I fixed it as part of my pull request.

4. Inconsistent parameter handling
Some stream handlers use Map<String, String> for parameters, while others use ModifiableSolrParams.

Question:

Would it make sense to standardize on a single approach?

5. ScoreNodesStream limitation
ScoreNodesStream can only obtain zkHost / solrCloud via: factory.getDefaultZkHost();

Question:

• Should this be improved as part of this PR?
• Or would it be better to handle it in a separate issue?

[epugh]

Shouldn't solrCloud be connectionString? I was hoping down the road we could put more into the connection string like username password or other details that are needed....