feat(microservices): add topic-consumers option to server-kafka

[suuuuuuminnnnnn]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

ServerKafka uses a single KafkaJS consumer subscribed to all registered topics, so a slow handler on one topic can effectively block message processing on other topics (topic-level backpressure). Partition-level concurrency (partitionsConsumedConcurrently) does not address cross-topic blocking.

Issue Number: #14590

What is the new behavior?

Adds an opt-in topicConsumers: boolean option for Kafka microservices.

When topicConsumers: true:

• Creates a separate consumer per registered topic and subscribes each consumer to exactly one topic
• Uses {groupId}-{topic} as the consumer group id (independent offset tracking per topic)
• Disconnects all per-topic consumers in parallel on close()
• Logs a deduped warning when the composed group id appears invalid (whitespace or >255 chars)
• Avoids mutating options.run when building per-topic run() options

Default remains false, preserving the existing single-consumer behavior.

Does this PR introduce a breaking change?

• Yes
• No

Note: enabling topicConsumers: true creates new consumer groups ({groupId}-{topic}), so migrating an existing deployment may require offset management.

Other information

Tests

npm test -- packages/microservices/test/server/server-kafka.spec.ts

All tests pass, including new unit tests for topicConsumers mode.

[coveralls]

Pull Request Test Coverage Report for Build 157b7e61-c1ed-4ba3-99e3-02b51ffa7962

Details

• 46 of 50 (92.0%) changed or added relevant lines in 1 file are covered.
• 59 unchanged lines in 5 files lost coverage.
• Overall coverage decreased (-0.03%) to 89.815%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/microservices/server/server-kafka.ts	46	50	92.0%

Files with Coverage Reduction	New Missed Lines	%
packages/microservices/server/server-kafka.ts	2	86.21%
packages/microservices/server/server-grpc.ts	4	85.39%
packages/microservices/client/client-redis.ts	16	79.84%
packages/microservices/server/server-mqtt.ts	16	83.04%
packages/microservices/server/server-redis.ts	21	76.15%

Totals
Change from base Build 3000a4b9-5b8f-4abf-b1d5-fd69b0426434:	-0.03%
Covered Lines:	7487
Relevant Lines:	8336

---

💛 - Coveralls