Decode Logical Decoding Message Content

The DecodeLogicalDecodingMessageContent SMT converts the binary content of a PostgreSQL logical decoding message to a structured form.

When the Debezium PostgreSQL connector captures logical decoding messages, it emits message event records to Kafka. By default, the content field in these message records contain encoded binary data. To facilitate processing of PostgreSQL event messages by other Kafka consumers, you can use the DecodeLogicalDecodingMessageContent SMT to decode the binary content of the original message, and convert it into a more consumable format.

You also use the SMT in conjunction with other SMTs, such as the Debezium Outbox Event Router.

Example

To enable a Debezium PostgreSQL connector to decode the binary content in message events, add the DecodeLogicalDecodingMessageContent SMT to the Kafka Connect configuration for the connector, as shown in the following example:

"connector.class": "io.debezium.connector.postgresql.PostgresConnector",
...
"transforms": "decodeLogicalDecodingMessageContent",
"transforms.decodeLogicalDecodingMessageContent.type": "io.debezium.connector.postgresql.transforms.DecodeLogicalDecodingMessageContent",
"key.converter": "org.apache.kafka.connect.json.JsonConverter",
"key.converter.schemas.enable": false,
"value.converter": "org.apache.kafka.connect.json.JsonConverter",
"value.converter.schemas.enable": "false",
...

The following example shows the key and value of an event record before and after the transformation is applied.

Example 1. Effect of applying the DecodeLogicalDecodingMessageContent SMT

Before processing

Event key before the SMT processes the record

{
    "prefix": "test-prefix"
}

Event value before the SMT processes the record

{
    "op": "m",
    "ts_ms": 1723115240065,
    "source": {
        "version": "3.0.0-SNAPSHOT",
        "connector": "postgresql",
        "name": "connector-name",
        "ts_ms": 1723115239782,
        "snapshot": "false",
        "db": "source-db",
        "sequence": "[\"26997744\",\"26997904\"]",
        "ts_us": 1723115239782690,
        "ts_ns": 1723115239782690000,
        "schema": "",
        "table": "",
        "txId": 756,
        "lsn": 26997904,
        "xmin": null
    },
    "message": {
        "prefix": "test-prefix",
        "content": "eyJpZCI6IDEsICJpdGVtIjogIkRlYmV6aXVtIGluIEFjdGlvbiIsICJzdGF0dXMiOiAiRU5URVJFRCIsICJxdWFudGl0eSI6IDIsICJ0b3RhbFByaWNlIjogMzkuOTh9"
    }
}

After processing

Event key after the SMT processes the record

null

Event value after the SMT processes the record

{
    "op": "c",
    "ts_ms": 1723115415729,
    "source": {
        "version": "3.0.0-SNAPSHOT",
        "connector": "postgresql",
        "name": "connector-name",
        "ts_ms": 1723115415640,
        "snapshot": "false",
        "db": "source-db",
        "sequence": "[\"26717416\",\"26717576\"]",
        "ts_us": 1723115415640161,
        "ts_ns": 1723115415640161000,
        "schema": "",
        "table": "",
        "txId": 745,
        "lsn": 26717576,
        "xmin": null
    },
    "after": {
        "id": 1,
        "item": "Debezium in Action",
        "status": "ENTERED",
        "quantity": 2,
        "totalPrice": 39.98
    }
}

In the preceding example, the SMT applies the following changes to the original event record:

• Removes the key that contained the prefix field in the original logical decoding message ("prefix": "test-prefix").
• Converts the value of the op field from m (message) to c (create), effectively changing the event type from message to INSERT.
• Replaces the message field with an after field that contains the decoded content of a logical decoding message.

After the SMT applies these changes, the record can be more easily processed by downstream consumers or by other SMTs, such as the Debezium Outbox Event Router.

Configuration options

The following table lists the configuration options that you can use with the DecodeLogicalDecodingMessageContent SMT.

Table 1. DecodeLogicalDecodingMessageContent SMT configuration options