Confusing Error `Can Not Coerce Json Type To Json`

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Introduction

When working with data pipelines, it's not uncommon to encounter errors that can be frustrating to debug. In this article, we'll explore a specific error that occurs when trying to coerce a JSON type to JSON in GreptimeDB. We'll delve into the details of the error, provide a minimal reproducible step, and offer insights into how to resolve the issue.

What type of bug is this?

This bug falls under the category of User Experience (UX) issues. It's a frontend-related problem that affects the user's interaction with the system.

What subsystems are affected?

The error is primarily affecting the frontend and standalone mode of GreptimeDB.

Minimal reproduce step

To reproduce the error, you can pipe the following input:

{"host":"sita","message":"{\"ts\":1744882691116184,\"data\":{\"stream\":\"bnbusdc@depth@100ms\",\"data\":{\"e\":\"depthUpdate\",\"E\":1744882691008,\"T\":1744882690907,\"s\":\"BNBUSDC\",\"U\":7294819079480,\"u\":7294819079480,\"pu\":7294819074877,\"b\":[[\"583.770\",\"2.57\"],[\"585.250\",\"13.46\"]],\"a\":[]}}}","source_type":"stdin","timestamp":1744963375149882}

with the following pipeline:

transform:
  # Timestamp field
  - fields:
      - ts
    type: epoch, us
    index: timestamp
  # fhid/xch/stream (tags), i.e. identifying only one time series.
  - fields:
      - fhid
    type: uint8
    tag: true
    on_failure: default
    default: 0
  - fields:
      - xch
    type: string
    tag: true
    index: inverted
  - fields:
      - stream
    type: string
    tag: true
    index: inverted
  # Payload
  - fields:
      - message
    type: json

What did you expect to see?

The pipeline was expected to assume that the message was just the field message where Vector added the other fields.

What did you see instead?

However, the actual output was a confusing error:

Can not coerce json type to json, at src/pipeline/src/etl/transform/transformer/greptime/coerce.rs:442:10

What operating system did you use?

The error was encountered on Archlinux.

What version of GreptimeDB did you use?

The version of GreptimeDB used was 0.14.

Relevant log output and stack trace

# No log output or stack trace provided

Analysis

The error occurs because GreptimeDB is trying to coerce a JSON type to JSON, which is not possible. The issue arises from the fact that the message field is being treated as a string instead of a JSON object.

Solution

To resolve the issue, you can modify the pipeline to correctly handle the message field as a JSON object. This can be achieved by changing the type of the message field to json and ensuring that the input data is in the correct format.

Conclusion

In conclusion, the error Can not coerce json type to json is a frustrating issue that can be resolved by correctly handling the message field as a JSON object in the pipeline. By following the steps outlined in this article, you should be able to resolve the issue and ensure that your data pipeline is working correctly.

Additional Tips

  • Always ensure that the input data is in the correct format before processing it.
  • Use the correct data types for each field in the pipeline.
  • Test your pipeline thoroughly to catch any errors or issues.

Related Issues

References

Introduction

In our previous article, we explored a confusing error that occurs when trying to coerce a JSON type to JSON in GreptimeDB. In this article, we'll provide a Q&A section to help you better understand the issue and its resolution.

Q: What is the cause of the error Can not coerce json type to json?

A: The error occurs because GreptimeDB is trying to coerce a JSON type to JSON, which is not possible. The issue arises from the fact that the message field is being treated as a string instead of a JSON object.

Q: What is the correct way to handle the message field in the pipeline?

A: To resolve the issue, you can modify the pipeline to correctly handle the message field as a JSON object. This can be achieved by changing the type of the message field to json and ensuring that the input data is in the correct format.

Q: How can I ensure that the input data is in the correct format?

A: You can ensure that the input data is in the correct format by checking the data before processing it. You can use tools like jq to validate the JSON data and ensure that it conforms to the expected format.

Q: What are some common mistakes that can lead to this error?

A: Some common mistakes that can lead to this error include:

  • Using the wrong data type for a field in the pipeline
  • Not properly handling JSON data in the pipeline
  • Not validating the input data before processing it

Q: How can I prevent this error from occurring in the future?

A: To prevent this error from occurring in the future, you can:

  • Always ensure that the input data is in the correct format before processing it
  • Use the correct data types for each field in the pipeline
  • Test your pipeline thoroughly to catch any errors or issues

Q: What are some best practices for handling JSON data in GreptimeDB?

A: Some best practices for handling JSON data in GreptimeDB include:

  • Using the json data type for fields that contain JSON data
  • Properly handling JSON data in the pipeline, including parsing and validating it
  • Using tools like jq to validate and manipulate JSON data

Q: Can you provide an example of how to handle JSON data in GreptimeDB?

A: Here is an example of how to handle JSON data in GreptimeDB:

transform:
  - fields:
      - message
    type: json

This will properly handle the message field as a JSON object and allow you to access its contents in the pipeline.

Q: What are some resources for learning more about GreptimeDB and its features?

A: Some resources for learning more about GreptimeDB and its features include:

Q: Can you provide a summary of the key takeaways from this article?

A: The key takeaways from this article are:

  • The error Can not coerce json type to json occurs when trying to coerce a JSON type to JSON in GreptimeDB
  • The issue arises from the fact that the message field is being treated as a string instead of a JSON object
  • To resolve the issue, you can modify the pipeline to correctly handle the message field as a JSON object
  • You can ensure that the input data is in the correct format by checking it before processing it
  • You can prevent this error from occurring in the future by following best practices for handling JSON data in GreptimeDB.