However, in some cases, before a downstream consumer can process {prodname} change event messages, it requires additional information about field-level changes that result from the original database change.
To enhance event messages with details about how a database operation modifies fields in the source database, {prodname} provides the `ExtractChangedRecordState` single message transformation (SMT).
The complex format of the message in the preceding example provides detailed information about changes that occur in the source database.
However, the format might not be suitable for some downstream consumers.
Sink connectors, or other parts of the Kafka ecosystem might expect the message to explicitly identify the fields that a database operation changes or leaves unchanged.
The `ExtractChangedRecordState` SMT adds headers to the change event message to identify the fields that are modified by a database operation, and the fields that remain unchanged.
The transformation examines the `before` and `after` event state structures to identify the fields that are altered by an operation, and those that remain unchanged.
Depending on the connector configuration, the transformation then produces a modified event message that adds message headers to list the changed fields, the unchanged fields, or both.
* Identify the fields that are changed by an `UPDATE` event by listing them in the user-configured xref:extract-changes-header-changed-name[`header.changed.name`] header.
* Identify the fields that are not changed by an `UPDATE` event by listing them in the user-configured xref:extract-changes-header-unchanged-name[`header.unchanged.name`] header.
Configure the {prodname} event changes SMT in a Kafka Connect source or sink connector by adding the SMT configuration details to your connector's configuration.
As with any Kafka Connect connector configuration, you can set `transforms=` to multiple, comma-separated, SMT aliases in the order in which you want Kafka Connect to apply the SMTs.
`header.changed.name`:: The Kafka message header name to use for storing a comma-separated list of the fields that are changed by a database operation.
`header.unchanged.name`:: The Kafka message header name to use for storing a comma-separated list of the fields that remain unchanged after a database operation.
The connector might emit many types of event messages (heartbeat messages, tombstone messages, or metadata messages about transactions or schema changes).
To apply the transformation to a subset of events, you can define xref:options-for-applying-the-transformation-selectively[an SMT predicate statement that selectively applies the transformation] to specific events only.
== Options for applying the event changes transformation selectively
In addition to the change event messages that a {prodname} connector emits when a database change occurs, the connector also emits other types of messages, including heartbeat messages, and metadata messages about schema changes and transactions.
Because the structure of these other messages differs from the structure of the change event messages that the SMT is designed to process, it's best to configure the connector to selectively apply the SMT, so that it processes only the intended data change messages.
For more information about how to apply the SMT selectively, see xref:{link-smt-predicates}#applying-transformations-selectively[Configure an SMT predicate for the transformation].
[id="configuration-options"]
== Configuration options
The following table describes the options that you can specify to configure the event changes SMT.
.Descriptions of event changes SMT configuration options