fix(Kafka): Default to KRaft for Confluent Platform 8.x images

[arnelirobles]

What does this PR do?

Picks the default consensus protocol from the image, as suggested on the issue. IKafkaVendorConfiguration.ConsensusProtocol becomes GetConsensusProtocol(IImage image). Confluent returns KRaft for 8.0.0 and later and for any tag starting with latest (Docker Hub also publishes latest-ubi9, latest.arm64 and latest.amd64, all 8.x), and ZooKeeper otherwise. Apache still always returns KRaft.

ConfluentConfiguration.Validate now also rejects ZooKeeper on those tags:

ZooKeeper is not supported for Confluent Platform images with versions 8.0.0 and later. Use KRaft instead.

Build() now validates kafkaBuilder.DockerResourceConfiguration instead of DockerResourceConfiguration, so a protocol filled in by default goes through the vendor check too. Before, only a protocol the user set explicitly was validated. No existing configuration starts throwing: Apache defaults to KRaft, and Confluent 7.x and earlier still default to ZooKeeper, which neither guard rejects there.

Why is it important?

Confluent Platform 8.0.0 removed ZooKeeper from the image, so new KafkaBuilder("confluentinc/cp-kafka:8.2.3").Build() with no other configuration fails. The startup script backgrounds a zookeeper-server-start that does not exist, configure then exits on the missing KAFKA_PROCESS_ROLES, and the caller sees ContainerNotRunningException with no mention of ZooKeeper.

Related issues

• Closes [Bug]: KafkaBuilder cannot start Confluent images 8.0.0 and later, which removed ZooKeeper #1773
• Relates [Bug]: KafkaBuilder fails to start container with Confluent Kafka image 8.2.3 #1771

How to test this PR

dotnet test tests/Testcontainers.Kafka.Tests

19/19 pass locally (macOS arm64). The two tests using cp-kafka:6.1.9 need the amd64 images pulled explicitly, since 6.1.9 has no arm64 build.

New tests:

• ConfluentKafkaV8DefaultConfiguration starts cp-kafka:8.2.4 with no protocol set. On develop it fails with environment variable "KAFKA_PROCESS_ROLES" is not set.
• ZooKeeperWithConfluent8ThrowsArgumentException and DefaultWithConfluent8DoesNotThrow, each on 8.0.0, latest, latest-ubi9 and latest.arm64.

Each test fails if the part it covers is reverted: forcing the Confluent default back to ZooKeeper fails the container test and the default theory, removing the guard fails the guard theory, and matching only the exact latest tag fails the latest-ubi9 and latest.arm64 cases.

Follow-ups

• A custom image with WithVendor(KafkaVendor.Confluent) and a tag that reads as 8.x or latest (for example myorg/kafka:latest) now defaults to KRaft, whatever Confluent version it was built on. On a 7.x base KRaft starts fine. On a pre-7 base it would not, and WithZooKeeper() is the workaround. I kept the default independent of IsImageFromVendor so custom images built on 8.x work out of the box, but I can match the guard instead if you prefer.
• Digest-only references (confluentinc/cp-kafka@sha256:...) have no tag, so they still default to ZooKeeper.
• Once Confluent Platform 7.9 is out of support (Feb 19, 2027), the ZooKeeper path in ConfluentConfiguration can go.

Summary by CodeRabbit

• New Features
  • Kafka containers now select a default consensus protocol based on the chosen image. Apache Kafka images use KRaft, while Confluent images that do not support ZooKeeper—including version 8 and newer and latest tags—use KRaft automatically.
• Bug Fixes
  • Validation now flags incompatible protocol choices for Confluent images: KRaft is unavailable before version 7, and ZooKeeper is unavailable for images that no longer support it. Error messages guide you toward the compatible protocol.

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📒 Files selected for processing (5)

• src/Testcontainers.Kafka/ApacheConfiguration.cs
• src/Testcontainers.Kafka/ConfluentConfiguration.cs
• src/Testcontainers.Kafka/KafkaBuilder.cs
• tests/Testcontainers.Kafka.Tests/KafkaBuilderTest.cs
• tests/Testcontainers.Kafka.Tests/KafkaContainerTest.cs

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Walkthrough

Kafka vendor configurations now select default consensus protocols based on the configured image. KafkaBuilder applies the selected default and validates the resulting configuration. Confluent images identified as lacking ZooKeeper default to KRaft and reject explicit ZooKeeper configuration.

Changes

Kafka consensus configuration

Layer / File(s)	Summary
Image-specific protocol defaults src/Testcontainers.Kafka/IKafkaVendorConfiguration.cs, src/Testcontainers.Kafka/ApacheConfiguration.cs, src/Testcontainers.Kafka/ConfluentConfiguration.cs, src/Testcontainers.Kafka/KafkaBuilder.cs	Vendor configurations provide an image-specific default. Apache configuration returns KRaft. Confluent configuration returns ZooKeeper for parseable image versions below 8 and KRaft otherwise. KafkaBuilder.Build() applies the default when no protocol is configured.
Protocol validation and tests src/Testcontainers.Kafka/ConfluentConfiguration.cs, src/Testcontainers.Kafka/KafkaBuilder.cs, tests/Testcontainers.Kafka.Tests/KafkaBuilderTest.cs, tests/Testcontainers.Kafka.Tests/KafkaContainerTest.cs	Confluent validation rejects KRaft before version 7 and ZooKeeper for version 8 or later or tags beginning with latest. Tests cover explicit ZooKeeper selection and default builds for four Confluent image tags. The Confluent 8.2.4 container test no longer explicitly selects KRaft.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix · Severity of issue fixed: Medium

Sequence Diagram(s)

sequenceDiagram
  participant KafkaBuilder
  participant ConfluentConfiguration
  participant KafkaConfiguration
  KafkaBuilder->>ConfluentConfiguration: GetConsensusProtocol(configured image)
  ConfluentConfiguration-->>KafkaBuilder: Return default protocol
  KafkaBuilder->>KafkaConfiguration: Apply selected protocol
  KafkaBuilder->>ConfluentConfiguration: Validate selected configuration
  ConfluentConfiguration-->>KafkaBuilder: Accept or throw ArgumentException

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Merge Risk: 🟡 Moderate · up to 57ea6

Custom Confluent-based 8.x images can still be configured with ZooKeeper. Confirm the intended custom-image exception or close this validation gap before merging.

Security Architecture Review

Security architecture risk: 🟡 Moderate · up to 57ea6

The change fixes the default for Confluent 8.x images, but a pinned image without a recognizable version can receive KRaft even when its contents require ZooKeeper. That can turn a previously working container into a startup failure. No new credential or listener access was identified.

Retained concerns

• Medium · reliability · inferred: Images without a parseable version now default to KRaft, while version-based validation cannot establish whether that protocol is supported. An older digest-pinned image that previously used ZooKeeper may consequently fail after container creation; cleanup then depends on the caller.

Security review details

Security Blast Radius

• inferred — The changed default affects containers built from the Kafka module rather than adding a new service or credential path. Common mapped broker ports predate the protocol choice; KRaft does not add a mapped controller port in these builder methods.

Trust Boundaries and Controls

• observed — Vendor validation receives the computed configuration before container construction and rejects incompatible explicit protocols for recognized versioned Confluent images. It cannot verify compatibility when the image version is absent or unparseable.

Resilience and Maintainability Implications

• inferred — An incompatible inferred protocol can fail only after a container has been created and started, making caller-managed disposal relevant to failure containment. Evidence does not establish that a failed broker accepts connections.

Hardening Proposals

• proposed — Define an explicit compatibility policy for images whose version cannot be inferred, such as requiring callers to choose a protocol for digest-only references, rather than treating an unknown version as KRaft-compatible.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 62.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 16 functions across 6 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main change: defaulting Confluent Platform 8.x images to KRaft.
Description check	✅ Passed	The description covers the required changes, rationale, related issues, testing steps, results, and follow-ups. It directly explains the behavior for Confluent and Apache images and the validation cha…
Linked Issues check	✅ Passed	The PR meets the coding requirements in issue [#1773]. ConfluentConfiguration.GetConsensusProtocol(IImage) selects ZooKeeper only for parseable versions below 8 and selects KRaft for version 8 or la…
Out of Scope Changes check	✅ Passed	The changes stay within issue [#1773]. The vendor interface update, image-specific default selection, builder validation order, focused unit tests, and removal of redundant explicit KRaft setup suppor…

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_{A rabbit checks the Kafka tag,
Then picks the protocol for the stack.
KRaft hops in where ZooKeeper’s gone,
The builder validates before moving on.
The rabbit thumps: the tests pass along.}

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

🤖 Prompt to fix review comments

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Inline comments:
In `@src/Testcontainers.Kafka/ConfluentConfiguration.cs`:
- Around line 49-50: Update the isUnsupportedZooKeeperImage predicate in
ConfluentConfiguration to reject ZooKeeper whenever IsZooKeeperRemoved
identifies the image tag as unsupported, regardless of repository name; remove
the IsImageFromVendor condition while preserving the existing consensus-protocol
check.

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📒 Files selected for processing (6)

• src/Testcontainers.Kafka/ApacheConfiguration.cs
• src/Testcontainers.Kafka/ConfluentConfiguration.cs
• src/Testcontainers.Kafka/IKafkaVendorConfiguration.cs
• src/Testcontainers.Kafka/KafkaBuilder.cs
• tests/Testcontainers.Kafka.Tests/KafkaBuilderTest.cs
• tests/Testcontainers.Kafka.Tests/KafkaContainerTest.cs

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[HofmeisterAn]

👍 Thanks.