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How to handle heterogeneous JSON with ksqlDB

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How to handle heterogeneous JSON with ksqlDB

Suppose you have a topic with records formatted in JSON, but not all the records have the same structure and value types. In this tutorial, we'll demonstrate how to work with JSON of different structures.

Set Up

For context, imagine you have three different JSON formats in a Kafka topic:

  "JSONType1": {
    "fieldA": "some data",
    "numberField": 1.001,
    "oneOnlyField": "more data", 
    "randomField": "random data"
  }
  "JSONType2": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 98.6 
  }
  "JSONType3": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 98.6,
    "fieldC": "data",
    "fieldD": "D-data"    
  }

From these three different JSON structures you want to extract oneOnlyField, numberField, and fieldD from JSONType, JSONType2, and JSONType3 respectively.

Your first step is to create a stream and use a VARCHAR keyword to define the outermost element of the JSON types.

CREATE STREAM data_stream (
    JSONType1 VARCHAR,
    JSONType2 VARCHAR,
    JSONType3 VARCHAR
) WITH (KAFKA_TOPIC='data_stream',
        VALUE_FORMAT='JSON',
        PARTITIONS=1);

Then you can access the fields using the EXTRACTJSONFIELD keyword and cast into the appropriate types by selecting from data_stream:

SELECT EXTRACTJSONFIELD (JSONType1, '$.oneOnlyField') AS special_info,
       CAST(EXTRACTJSONFIELD (JSONType2, '$.numberField') AS DOUBLE) AS runfld,
       EXTRACTJSONFIELD (JSONType3, '$.fieldD') AS description
FROM data_stream
EMIT CHANGES;

Running the example

You can run the example backing this tutorial in one of two ways: locally with the ksql CLI against Kafka and ksqlDB running in Docker, or with Confluent Cloud.

Local With Docker

Prerequisites

Run the commands

Clone the confluentinc/tutorials GitHub repository (if you haven't already) and navigate to the tutorials directory:

git clone git@github.com:confluentinc/tutorials.git
cd tutorials

Start ksqlDB and Kafka:

docker compose -f ./docker/docker-compose-ksqldb.yml up -d

Create the data_stream topic:

docker exec -it broker kafka-topics --bootstrap-server localhost:29092 --create --topic data_stream

Open a console producer:

docker exec -it broker kafka-console-producer --bootstrap-server localhost:29092 --topic data_stream

Ever the following four events at the prompt:

{ "JSONType1": { "fieldA": "some data", "numberField": 1.001, "oneOnlyField": "more data", "randomField": "random data" }, "JSONType2": { "fieldA": "data", "fieldB": "b-data", "numberField": 98.6 }, "JSONType3": { "fieldA": "data", "fieldB": "b-data", "numberField": 98.6, "fieldC": "data", "fieldD": "D-data" }}
{ "JSONType1": { "fieldA": "some data", "numberField": 2.001, "oneOnlyField": "more data", "randomField": "random data" }, "JSONType2": { "fieldA": "data", "fieldB": "b-data", "numberField": 99.6 }, "JSONType3": { "fieldA": "data", "fieldB": "b-data", "numberField": 98.6, "fieldC": "data", "fieldD": "D-data-2" }}
{ "JSONType1": { "fieldA": "some data", "numberField": 3.001, "oneOnlyField": "more data", "randomField": "random data" }, "JSONType2": { "fieldA": "data", "fieldB": "b-data", "numberField": 100.6 }, "JSONType3": { "fieldA": "data", "fieldB": "b-data", "numberField": 98.6, "fieldC": "data", "fieldD": "D-data-3" }}
{ "JSONType1": { "fieldA": "some data", "numberField": 4.001, "oneOnlyField": "more data", "randomField": "random data" }, "JSONType2": { "fieldA": "data", "fieldB": "b-data", "numberField": 101.6 }, "JSONType3": { "fieldA": "data", "fieldB": "b-data", "numberField": 98.6, "fieldC": "data", "fieldD": "D-data-4" }}

Next, open the ksqlDB CLI:

docker exec -it ksqldb-cli ksql http://ksqldb-server:8088

Enter the following statement. This will create a stream backed by the data_stream topic.

CREATE STREAM data_stream (
    JSONType1 VARCHAR,
    JSONType2 VARCHAR,
    JSONType3 VARCHAR
) WITH (KAFKA_TOPIC='data_stream',
        VALUE_FORMAT='JSON',
        PARTITIONS=1);

Now you can access the fields using the EXTRACTJSONFIELD function. Note that we first tell ksqlDB to consume from the beginning of the stream.

SET 'auto.offset.reset'='earliest';

SELECT EXTRACTJSONFIELD (JSONType1, '$.oneOnlyField') AS special_info,
       CAST(EXTRACTJSONFIELD (JSONType2, '$.numberField') AS DOUBLE) AS runfld,
       EXTRACTJSONFIELD (JSONType3, '$.fieldD') AS description
FROM data_stream
EMIT CHANGES;

The query output should look like this:

+------------------------+------------------------+------------------------+
|SPECIAL_INFO            |RUNFLD                  |DESCRIPTION             |
+------------------------+------------------------+------------------------+
|more data               |98.6                    |D-data                  |
|more data               |99.6                    |D-data-2                |
|more data               |100.6                   |D-data-3                |
|more data               |101.6                   |D-data-4                |
+------------------------+------------------------+------------------------+

When you are finished, exit the ksqlDB CLI by entering CTRL-D and clean up the containers used for this tutorial by running:

docker compose -f ./docker/docker-compose-ksqldb.yml down
Confluent Cloud

Prerequisites

Create Confluent Cloud resources

Login to your Confluent Cloud account:

confluent login --prompt --save

Install a CLI plugin that will streamline the creation of resources in Confluent Cloud:

confluent plugin install confluent-cloud_kickstart

Run the following command to create a Confluent Cloud environment and Kafka cluster. This will create resources in AWS region us-west-2 by default, but you may override these choices by passing the --cloud argument with a value of aws, gcp, or azure, and the --region argument that is one of the cloud provider's supported regions, which you can list by running confluent kafka region list --cloud <CLOUD PROVIDER>

confluent cloud-kickstart --name ksqldb-tutorial \
  --environment-name ksqldb-tutorial \
  --output-format stdout

Now, create a ksqlDB cluster by first getting your user ID of the form u-123456 when you run this command:

confluent iam user list

And then create a ksqlDB cluster called ksqldb-tutorial with access linked to your user account:

confluent ksql cluster create ksqldb-tutorial \
  --credential-identity <USER ID>

Run the commands

Login to the Confluent Cloud Console. Select Environments in the lefthand navigation, and then click the ksqldb-tutorial environment tile. Click the ksqldb-tutorial Kafka cluster tile, and then select Topics in the lefthand navigation. Create a topic called data_stream with 1 partition, and in the Messages tab, produce the following four events as the Value, one at a time.

{
  "JSONType1": {
    "fieldA": "some data",
    "numberField": 1.001,
    "oneOnlyField": "more data",
    "randomField": "random data"
  },
  "JSONType2": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 98.6
  },
  "JSONType3": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 98.6,
    "fieldC": "data",
    "fieldD": "D-data"
  }
}
{
  "JSONType1": {
    "fieldA": "some data",
    "numberField": 2.001,
    "oneOnlyField": "more data",
    "randomField": "random data"
  },
  "JSONType2": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 99.6
  },
  "JSONType3": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 98.6,
    "fieldC": "data",
    "fieldD": "D-data-2"
  }
}
{
  "JSONType1": {
    "fieldA": "some data",
    "numberField": 3.001,
    "oneOnlyField": "more data",
    "randomField": "random data"
  },
  "JSONType2": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 100.6
  },
  "JSONType3": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 98.6,
    "fieldC": "data",
    "fieldD": "D-data-3"
  }
}
{
  "JSONType1": {
    "fieldA": "some data",
    "numberField": 4.001,
    "oneOnlyField": "more data",
    "randomField": "random data"
  },
  "JSONType2": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 101.6
  },
  "JSONType3": {
    "fieldA": "data",
    "fieldB": "b-data",
    "numberField": 98.6,
    "fieldC": "data",
    "fieldD": "D-data-4"
  }
}

Next, select ksqlDB in the lefthand navigation.

The cluster may take a few minutes to be provisioned. Once its status is Up, click the cluster name and scroll down to the editor.

In the query properties section at the bottom, change the value for auto.offset.reset to Earliest so that ksqlDB will consume from the beginning of the stream we create.

Enter the following statement in the editor and click Run query. This will create a stream backed by the data_stream topic.

CREATE STREAM data_stream (
    JSONType1 VARCHAR,
    JSONType2 VARCHAR,
    JSONType3 VARCHAR
) WITH (KAFKA_TOPIC='data_stream',
        VALUE_FORMAT='JSON',
        PARTITIONS=1);

Now you can access the fields using the EXTRACTJSONFIELD function:

SELECT EXTRACTJSONFIELD (JSONType1, '$.oneOnlyField') AS special_info,
       CAST(EXTRACTJSONFIELD (JSONType2, '$.numberField') AS DOUBLE) AS runfld,
       EXTRACTJSONFIELD (JSONType3, '$.fieldD') AS description
FROM data_stream
EMIT CHANGES;

The query output should look like this:

+------------------------+------------------------+------------------------+
|SPECIAL_INFO            |RUNFLD                  |DESCRIPTION             |
+------------------------+------------------------+------------------------+
|more data               |98.6                    |D-data                  |
|more data               |99.6                    |D-data-2                |
|more data               |100.6                   |D-data-3                |
|more data               |101.6                   |D-data-4                |
+------------------------+------------------------+------------------------+

Clean up

When you are finished, delete the ksqldb-tutorial environment by first getting the environment ID of the form env-123456 corresponding to it:

confluent environment list

Delete the environment, including all resources created for this tutorial:

confluent environment delete <ENVIRONMENT ID>
Do you have questions or comments? Join us in the #confluent-developer community Slack channel to engage in discussions with the creators of this content.