How to dump avro data from Kafka topic and read it back in Java/Scala

Question

We need to export production data from a Kafka topic to use it for testing purposes: the data is written in Avro and the schema is placed on the Schema registry.

We tried the following strategies:

Using kafka-console-consumer with StringDeserializer or BinaryDeserializer. We were unable to obtain a file which we could parse in Java: we always got exceptions when parsing it, suggesting the file was in the wrong format.
Using kafka-avro-console-consumer: it generates a json which includes also some bytes, for example when deserializing BigDecimal. We didn't even know which parsing option to choose (it is not avro, it is not json)

Other unsuitable strategies:

deploying a special kafka consumer would require us to package and place that code in some production server, since we are talking about our production cluster. It is just too long. After all, isn't kafka console consumer already a consumer with configurable options?

Potentially suitable strategies

Using a kafka connect Sink. We didn't find a simple way to reset the consumer offset since apparently the connector created consumer is still active even when we delete the sink

Isn't there a simply, easy way to dump the content of the value (not the schema) of a Kafka topic containing avro data to a file so that it can be parsed? I expect this to be achievable using kafka-console-consumer with the right options, plus using the correct Java Api of Avro.

OneCricketeer OneCricketeer · Accepted Answer · 2018-03-03T15:47:55

for example, using kafka-console-consumer... We were unable to obtain a file which we could parse in Java: we always got exceptions when parsing it, suggesting the file was in the wrong format.

You wouldn't use regular console consumer. You would use kafka-avro-console-consumer which deserializes the binary avro data into json for you to read on the console. You can redirect > topic.txt to the console to read it.

If you did use the console consumer, you can't parse the Avro immediately because you still need to extract the schema ID from the data (4 bytes after the first "magic byte"), then use the schema registry client to retrieve the schema, and only then will you be able to deserialize the messages. Any Avro library you use to read this file as the console consumer writes it expects one entire schema to be placed at the header of the file, not only an ID pointing to anything in the registry at every line. (The basic Avro library doesn't know anything about the registry either)

The only thing configurable about the console consumer is the formatter and the registry. You can add decoders by additionally exporting them into the CLASSPATH

in such a format that you can re-read it from Java?

Why not just write a Kafka consumer in Java? See Schema Registry documentation

package and place that code in some production server

Not entirely sure why this is a problem. If you could SSH proxy or VPN into the production network, then you don't need to deploy anything there.

How do you export this data

Since you're using the Schema Registry, I would suggest using one of the Kafka Connect libraries

Included ones are for Hadoop, S3, Elasticsearch, and JDBC. I think there's a FileSink Connector as well

We didn't find a simple way to reset the consumer offset

The connector name controls if a new consumer group is formed in distributed mode. You only need a single consumer, so I would suggest standalone connector, where you can set offset.storage.file.filename property to control how the offsets are stored.

KIP-199 discusses reseting consumer offsets for Connect, but feature isn't implemented.

However, did you see Kafka 0.11 how to reset offsets?

Alternative options include Apache Nifi or Streamsets, both integrate into the Schema Registry and can parse Avro data to transport it to numerous systems

How to dump avro data from Kafka topic and read it back in Java/Scala

2 Answers