Companion suite for the ankurm.com guide. Every test runs against an in-JVM KRaft broker
(spring-kafka-test), so there is no Docker requirement. Verified on Spring Boot 4.1.0,
Spring Kafka 4.1.0, kafka-clients 4.2.1, JDK 25.0.3. results/full-run.txt is unedited
output of mvn test: 15 tests, 0 failures.
Demonstrations
_01_idempotence verifies idempotence from the BROKER via Admin.describeProducers rather
than by reading config back: a default producer registers one producer
state, an acks=1 producer registers zero. Also covers the ConfigException
you get only when idempotence is requested explicitly.
_02_transactions transaction markers consuming offsets, aborted records surviving in the
log, read_committed vs read_uncommitted, and a correction: markers do NOT
inflate consumer lag (a drained consumer reaches zero), they break
counting records by offset arithmetic (measured 100% overstatement).
_03_read_process_write
the Spring loop with container-managed transactions; a listener that
fails after sending runs twice and is committed once.
_04_fencing shared transactional.id fences the zombie; a random per-startup id lets
both producers commit and silently defeats the guarantee entirely.
_05_database Kafka EOS does not extend to a database. Naive listener writes 2 rows for
1 message; the same listener keyed on topic-partition-offset writes 1.
_06_performance cost per transaction size, median of 5 interleaved rounds.
Docs
Seven chapters: Boot 4 setup (auto-configuration moved to spring-boot-starter-kafka),
idempotence, transactions and markers, fencing and KIP-890, the database boundary and the
outbox pattern, performance, and corner cases.
Measured highlights
acks=1 producer 0 broker-tracked producer states (idempotence silently off)
5 records in one tx endOffset 6; offset span overstates record count by 100%
1 record per tx 711 records/s
100 records per tx 78,770 records/s (~111x)
