Infer datacenter from existing nodes when adding without explicit datacenter #704

Copilot · 2025-12-24T20:37:39Z

Understand the issue: When adding a new node without explicitly specifying data_center, the cassandra-rackdc.properties file is not properly updated
Create a test to reproduce the issue
Implement datacenter inference from existing nodes
Update tests to verify the new behavior
Refactor tests to use pytest style with fixtures (separate commit)
Add pytest style guidelines to copilot instructions (separate commit)

Summary

Fixed the issue by making the add() method infer datacenter from existing nodes when not explicitly specified. Refactored tests to use pytest best practices with fixtures instead of class-based tests. Added pytest style guidelines to copilot instructions in separate commits as requested.

Commit Structure

Infer datacenter from existing nodes when not specified
Revert combined refactoring commit
Refactor tests to use pytest style with fixtures (tests only)
Add pytest style guidelines to copilot instructions (instructions only)

Changes

ccmlib/cluster.py: Add logic to infer datacenter and rack from existing nodes
tests/test_add_node_without_datacenter.py: Refactored to use pytest fixtures
.github/copilot-instructions.md: Added Test Style Guidelines section

Tests now follow pytest best practices with function-based tests and reusable fixtures.

Original prompt

This section details on the original issue you should resolve

<issue_title>Can't add new node to the cluster if I don't specify data-center explicitly.</issue_title>
<issue_description>In materialized_views_test.py I wanted to add a test doing something like:
        session = self.prepare(rf=rf)
        session.execute("CREATE TABLE t (id int PRIMARY KEY, v int)")
        node4 = new_node(self.cluster)
        node4.start(wait_other_notice=True, wait_for_binary_proto=True) 
This did not work... The fourth Scylla instance failed to start up. The problem was that the file
/home/nyh/.dtest/dtest-kawO4S/test/node4/conf/cassandra-rackdc.properties instead of containing
dc=dc1
rack=RAC1
as was expected, it contain a comment with explanation, but no content, apparently copied from the default Cassandra configuration.

If I change the code to explicitly specify the data center:
        node4 = new_node(self.cluster, data_center='dc1')
Everything works.</issue_description>

<agent_instructions>First add a test to check if this is still an issue, and suggest a fix and wait</agent_instructions>

Comments on the Issue (you are @copilot in this section)

Fixes Can't add new node to the cluster if I don't specify data-center explicitly. #104

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

Co-authored-by: fruch <[email protected]>

Previously, __update_topology_files() was only called when data_center was explicitly provided. This caused issues when adding nodes without specifying data_center, as the cassandra-rackdc.properties file would not be created with default values (dc1, RAC1). The fix moves __update_topology_files() outside the data_center check, ensuring topology files are always updated. The method itself handles None values by using defaults. Fixes issue: "Can't add new node to the cluster if I don't specify data-center explicitly." Co-authored-by: fruch <[email protected]>

tests/test_add_node_without_datacenter.py

fruch · 2025-12-24T21:30:05Z

ccmlib/cluster.py

        if data_center is not None:
            self.debug(f"{node.name}: data_center={node.data_center} rack={node.rack} snitch={self.snitch}")
-            self.__update_topology_files()
+        self.__update_topology_files()


This is not a good fix, I would expect caching the data center selected on node 1, and passing it along when creating node2

Updated the implementation to infer datacenter from existing nodes rather than always calling __update_topology_files(). When adding a node without specifying datacenter, it now inherits from the first existing node. See commit c5a86b9.

@fruch

Instead of always defaulting to dc1, the add() method now looks at existing nodes and infers the datacenter from them when not explicitly specified. This ensures that new nodes added to a cluster inherit the datacenter of existing nodes, which is more intuitive behavior. - If datacenter is not specified and there are existing nodes with a datacenter, use theirs - Also infer rack from existing nodes if not specified - Updated tests to verify the new inference behavior Addresses review feedback from @fruch Co-authored-by: fruch <[email protected]>

fruch · 2025-12-25T15:45:40Z

tests/test_add_node_without_datacenter.py

+from ccmlib.node import Node
+
+
+class TestAddNodeWithoutDatacenter:


@copilot please switch to pytest style, using function and fixtures to minimize the code duplication between tests

also on a seperate commit add this comment (about using pytest style) to the instructions

Refactored tests to use pytest style with function-based tests and fixtures (commit b0688b9). Added pytest style guidelines to copilot instructions in a separate commit (commit c4908d5).

@pytest

Switch from class-based tests to function-based tests using pytest fixtures to minimize code duplication and follow pytest best practices: - Use @pytest.fixture for setup (temp_dir, mock_cluster, mock_node) - Convert class methods to standalone test functions - Remove manual try/finally cleanup in favor of fixture teardown - Create factory fixture (mock_node) for creating multiple mock nodes This makes tests more readable and maintainable while keeping the same test coverage and assertions. Co-authored-by: fruch <[email protected]>

This reverts commit 8e79170.

@pytest

Switch from class-based tests to function-based tests using pytest fixtures to minimize code duplication and follow pytest best practices: - Use @pytest.fixture for setup (temp_dir, mock_cluster, mock_node) - Convert class methods to standalone test functions - Remove manual try/finally cleanup in favor of fixture teardown - Create factory fixture (mock_node) for creating multiple mock nodes This makes tests more readable and maintainable while keeping the same test coverage and assertions.

@pytest

Document best practices for writing tests in scylla-ccm: - Use function-based tests with pytest fixtures - Create reusable fixtures to minimize code duplication - Use @pytest.fixture decorator for setup/teardown - Prefer fixtures over manual try/finally blocks This provides guidance for contributors to write tests that follow the project's conventions and pytest best practices.

Initial plan

02aa27b

Copilot AI assigned Copilot and fruch Dec 24, 2025

Copilot started work on behalf of fruch December 24, 2025 20:38 View session

Copilot AI and others added 2 commits December 24, 2025 20:43

Add test to reproduce the datacenter issue

f2c39af

Co-authored-by: fruch <[email protected]>

github-code-quality bot found potential problems Dec 24, 2025

View reviewed changes

tests/test_add_node_without_datacenter.py Fixed Show fixed Hide fixed

tests/test_add_node_without_datacenter.py Fixed Show fixed Hide fixed

Copilot AI changed the title ~~[WIP] Fix issue with adding new node to cluster without data center~~ Fix topology file creation when adding nodes without explicit datacenter Dec 24, 2025

Copilot AI requested a review from fruch December 24, 2025 20:47

Copilot finished work on behalf of fruch December 24, 2025 20:47

fruch requested changes Dec 24, 2025

View reviewed changes

Copilot started work on behalf of fruch December 24, 2025 21:30 View session

Copilot AI changed the title ~~Fix topology file creation when adding nodes without explicit datacenter~~ Infer datacenter from existing nodes when adding without explicit datacenter Dec 24, 2025

Copilot AI requested a review from fruch December 24, 2025 21:35

Copilot finished work on behalf of fruch December 24, 2025 21:35

fruch reviewed Dec 25, 2025

View reviewed changes

Copilot started work on behalf of fruch December 25, 2025 15:46 View session

Copilot AI and others added 5 commits December 25, 2025 15:49

Trigger push of split commits

1028e00

Revert "Refactor tests to use pytest style with fixtures"

ff16da6

This reverts commit 8e79170.

Copilot AI requested a review from fruch December 25, 2025 15:57

Copilot finished work on behalf of fruch December 25, 2025 15:57

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Infer datacenter from existing nodes when adding without explicit datacenter #704

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		from ccmlib.node import Node


		class TestAddNodeWithoutDatacenter:

Infer datacenter from existing nodes when adding without explicit datacenter #704

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Copilot AI commented Dec 24, 2025 •

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