Resolving NULL Values in ksqlDB Streams: A Complete Guide to Proper Schema Definition

Discover how to fix NULL values in your ksqlDB streams by properly defining the schema according to your data structure in Kafka.
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This video is based on the question https://stackoverflow.com/q/68561744/ asked by the user 'DebianUser' ( https://stackoverflow.com/u/11841697/ ) and on the answer https://stackoverflow.com/a/68732316/ provided by the user 'Matthias J. Sax' ( https://stackoverflow.com/u/4953079/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Resolving NULL Values in ksqlDB Streams: A Complete Guide to Proper Schema Definition

If you're working with ksqlDB and facing the frustrating issue of NULL values appearing in your streamed data, you're not alone. This is a common challenge that many users encounter when establishing streams to process data from Kafka topics. Let’s dive deep into understanding this problem and explore how to effectively address it to ensure your queries return meaningful results rather than NULLs.

Understanding the Problem

In your scenario, you attempted to create a stream in ksqlDB to fetch data from a Kafka topic named public.location. Your initial stream definition appeared as follows:

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Upon trying to query the created stream, your output yielded nothing but NULL values across the board:

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While the Kafka topic itself contains valid data in JSON format, the mismatch between your schema definition and the actual structure of the data is likely causing this issue.

Analyzing the Data Structure

When you printed the messages in the Kafka topic, you observed the following structure:

[[See Video to Reveal this Text or Code Snippet]]

Noticeably, the fields id, name, and location are nested within the sourceTable object rather than being "top-level" attributes. This leads to your stream returning NULL values because the intended fields are not matched in the schema definition.

Defining the Correct Schema

To resolve this issue, you need to declare your schema in a way that correctly reflects the structure of your JSON data. Here’s how you can redefine your stream:

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Key Points to Consider:

Nested Structures: Understand that fields nested within an object must be represented as a STRUCT type in your ksqlDB stream definition.

Top-Level Fields: You can also include any top-level fields as is (e.g., ConnectorVersion).

With the new stream definition set up, you can access the individual fields through dot notation. For instance, you can retrieve id using sourceTable->id in your queries.

Querying the Data

After defining the corrected schema, you can perform queries to retrieve individual fields. For example:

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Creating an Unnested Stream

If you prefer to have a cleaner stream without the nesting for easier access, you can create a derived stream as follows:

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This process not only helps clarify your data structure but also populates a new topic with the un-nested data.

Conclusion

By adjusting your schema to accurately reflect the nested structure of your JSON data, you can resolve the issue of seeing NULL values in your ksqlDB streams. Remember that a well-defined schema is crucial for proper data retrieval and processing in ksqlDB. Now you can effectively query your data and have meaningful interactions with your streams!

Feel free to reach out if you have further questions or if you run into any issues while implementing these changes!