Add: How to inspect Kafka messages using offsets

Closes #242

Author: github-actions[bot]

_questions/data-engineering-zoomcamp/module-7/033_c0e1671b46_inspect-kafka-offsets-messages.md

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+---
+id: c0e1671b46
+question: How to Inspect Messages in a Kafka Topic Using Offsets?
+sort_order: 33
+---
+
+An offset in Kafka is a per-partition sequence number that uniquely identifies messages within that partition. There is no global offset for the entire topic, and consumers use offsets to track what they have processed.
+
+Why inspecting offsets helps: when errors occur in a real-time stream, inspecting messages near a known offset helps you see what data caused the error, understand surrounding context, and reproduce the issue locally.
+
+Viewing offsets and consumer lag
+
+Use the Kafka CLI to see how far a consumer group has progressed:
+
+```bash
+kafka-consumer-groups \
+  --bootstrap-server localhost:9092 \
+  --describe \
+  --group rides-to-postgres
+```
+
+You can also run this via Docker:
+
+```bash
+docker run --rm -it --network pyflink_default confluentinc/cp-kafka:7.6.0 kafka-consumer-groups \
+--bootstrap-server redpanda:29092 \
+--describe \
+--group rides-to-postgres
+```
+
+Key fields to look at:
+
+- CURRENT-OFFSET: last offset processed by the consumer
+- LOG-END-OFFSET: last offset available in the topic
+- LAG: messages pending to be processed
+
+For more details, see the official docs: kafka-consumer-groups-sh
+
+Consuming messages from the beginning of a topic
+
+To inspect all messages in a topic, you can use kafka-console-consumer:
+
+```bash
+kafka-console-consumer \
+  --bootstrap-server localhost:9092 \
+  --topic rides \
+  --from-beginning
+```
+
+Or via Docker:
+
+```bash
+docker run --rm -it --network pyflink_default confluentinc/cp-kafka:7.6.0 kafka-console-consumer \
+--bootstrap-server redpanda:29092 \
+--topic rides \
+--from-beginning
+```
+
+Notes:
+- This is useful for basic exploration but does not allow jumping to a specific offset.
+
+Inspecting messages from a specific offset with kcat (formerly kafkacat)
+
+A very handy tool is kcat for reading messages starting from a given offset:
+
+```bash
+kcat -C \
+-b localhost:9092 \
+-t rides \
+-p 0 \
+-o 25 \
+-c 5
+```
+
+Docker usage:
+
+```bash
+docker run --network pyflink_default edenhill/kcat:1.7.1 -C \
+-b redpanda:29092 \
+-t rides \
+-p 0 \
+-o 25 \
+-c 5
+```
+
+Options explained:
+
+- consumer mode `-C`
+- broker `-b` (host:port)
+- topic `-t` the topic to read from
+- partition `-p` the partition
+- start at offset `-o` to begin reading
+- read up to `-c` messages
+
+This lets you see exactly what happens starting from a particular offset.
+
+Notes:
+- kcat is the successor to kafkacat; you can install or run it from Docker.
+- Replace broker address and topic/partition as per your environment.
+
+Conclusion
+
+Knowing how to inspect messages with specific offsets is a fundamental skill for Kafka debugging. Use these commands to locate the data around a known offset, monitor consumer lag, and reproduce issues locally when needed.