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A healthy Kafka cluster does not prove that a particular event was acknowledged, committed, written to the topic your consumer reads, or processed by your application. Trace one uniquely identified event through the producer result, topic and partition, transaction state, consumer position, and downstream handling to find where it disappeared.
Start with one event and follow its checkpoints
Choose a unique event ID and trace that same record at each stage. A timestamp or similar-looking payload is weaker evidence: multiple records can share timestamps or content. Record the expected topic, key, headers, and serialized value when the application creates the event.
- Check what the producer was asked to send. Confirm the actual record’s topic, key, headers, and value. A serializer error or a producer interceptor that changes a record can make the sent data differ from what the application log suggests. See the KafkaProducer API, version 3.9.2.
- Prove the send completed. Kafka producer sends are asynchronous. Capture the callback or future result and surface exceptions; a log written immediately after
send()does not establish that the broker acknowledged the record. For a controlled diagnostic, wait for the send future or callflush(), which waits for earlier sends to complete with acknowledgement or error under the configuredackspolicy. - Identify the effective producer settings. Record the client version and resolved values for
acks,enable.idempotence, retries, delivery timeout, and any producer interceptor. Do not assume a default without checking the deployed client. - Search the intended topic and partitions. Match the event ID in the expected topic and relevant partitions. Kafka offsets are per partition, not a single global topic cursor. Ordering is guaranteed within a partition, not across all partitions. The Kafka 0.8 introduction explains these foundational concepts; consult documentation for your deployed version for current operational details.
- Check transaction completion and consumer visibility. If the producer uses transactions, confirm that
commitTransaction()completed successfully and that the code did not abort. A consumer configured to read only committed records will not expose uncommitted or aborted transactional data. - Compare the consumer’s assignment and position. Confirm that the affected group is subscribed to the intended topic, has the relevant partition assigned, and is positioned to read the record’s offset. Also verify that producer and consumer are connected to the expected cluster and environment.
- If the record was there before, check retention. Compare the partition’s available offset range with the event’s offset. Retention can remove older records; the Kafka 0.8 source describes the concept, but exact settings and procedures depend on the deployed broker version.
- If Kafka has the record, trace application handling. Follow it through deserialization, filters, transformations, routing, retries, and dead-letter handling. Cluster health cannot show whether business logic dropped or redirected an event.
What a successful producer acknowledgement proves
The acks setting changes what the send result tells you. Kafka 4.0’s producer configuration documents three acknowledgement levels:
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acks |
What the producer waits for | What it does not establish |
|---|---|---|
0 |
No broker acknowledgement. | That the server received the record. |
1 |
An acknowledgement from the leader. | That follower replicas received the record. |
all |
Acknowledgement from the full in-sync replica set, provided at least one in-sync replica remains alive. | That the record went to the intended topic or that a consumer processed it. |
Even a successful acknowledgement is evidence about the producer-to-broker step, not end-to-end business delivery. Preserve the configured acknowledgement policy when interpreting logs or comparing environments.
#1 Best Overall
Idempotence helps with duplicates, not every missing event
Producer idempotence addresses a particular risk: duplicate writes caused by retries within a producer session. It does not deduplicate application-level resends, prove that a send succeeded, or repair every event lost elsewhere in the flow. Kafka’s producer API cautions that its protection is session-scoped.
Defaults vary by client version. Kafka 4.0’s configuration documentation says enable.idempotence defaults to true unless conflicting settings are specified; Kafka 2.6’s documentation records a default of false. Inspect the actual client version and effective configuration rather than relying on a remembered default.
Transactions can make an accepted write invisible
A transactional write can be accepted and later aborted. A consumer using committed-only isolation will not expose that record. Check the producer’s transaction path for a successful commitTransaction(), and check the consumer’s isolation setting.
The Kafka 3.9.2 KafkaProducer API says transactional topics should be configured for durability, specifically discussing a replication factor of at least 3 and min.insync.replicas of 2. It also states that end-to-end transactional guarantees require consumers to read only committed messages. Treat those as the API’s guidance and verify them against the topology and Kafka version in use.
Rank #3
When the record exists but the event does not
If you can find the event ID in Kafka at the expected partition and offset, the investigation has crossed the broker-storage boundary. The remaining question is how the consumer application handled it. Inspect the deserializer and any validation, filter, transformation, routing, retry, and dead-letter paths. A consumer may successfully read a Kafka record while application logic prevents the corresponding business action.
Offsets are partition-specific, and a consumer group can have different positions across its assigned partitions. Compare the record’s partition and offset with the affected group’s assignment and position; a healthy cluster alone cannot tell you whether that group read or acted on it.
Quick Recap
Rank #4
- Metamorphosis: Franz Kafka (Little Clothbound Classics)
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