# Kafka Connect Drop SMT Usage Reference for Confluent Platform The Drop SMT (`io.confluent.connect.transforms.Drop`) sets the key or value of a Apache Kafka® record to null and lets you control how the corresponding schema is handled. ## Description The Drop SMT sets either the key or value of a message to null. You can also set the corresponding schema to null, make it optional, check that it is already optional, or keep it as-is. Use the transformation type designed for the record key (`io.confluent.connect.transforms.Drop$Key`) or value (`io.confluent.connect.transforms.Drop$Value`). ## Installation Confluent develops this transformation and does not ship it by default with Kafka or Confluent Platform. You can install this transformation using the [confluent connect plugin install](https://docs.confluent.io/confluent-cli/current/command-reference/connect/plugin/confluent_connect_plugin_install.html) command: ```properties confluent connect plugin install confluentinc/connect-transforms:latest ``` ## Properties | Name | Description | Type | Default | Valid values | Importance | |-------------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------|-----------|-------------------------------------------------------|--------------| | `schema.behavior` | How to handle non-null schemas:

- `nullify` (default): Sets the schema to null.
- `retain`: Keeps the schema regardless of optional status.
- `validate`: Checks that the schema is optional and throws an
exception if not.
- `force_optional`: Overwrites the schema to be optional if it is
not already. | string | nullify | `nullify`, `retain`, `validate`, and `force_optional` | medium | ## Examples The following examples show how to use and configure the `Drop` SMT. * Drop the key from the message. Because `schema.behavior` is not set, the SMT uses the default value `nullify`, which also sets the schema to null. ```json "transforms": "dropKeyExample", "dropValueAndForceOptionalSchemaExample", "transforms.dropKeyExample.type": "io.confluent.connect.transforms.Drop$Key" ``` * Drop the value from the message. If the schema for the value isn’t already optional, this transform updates it to be optional. ```json "transforms.dropValueAndForceOptionalSchemaExample.type": "io.confluent.connect.transforms.Drop$Value", "transforms.dropValueAndForceOptionalSchemaExample.schema.behavior": "force_optional" ``` ### Schema behavior options The following examples show how to use the `schema.behavior` property with the `Drop` SMT. * Record contents using `Drop$Key` with `schema.behavior` set to `nullify`: ```json "transforms.dropValueAndForceOptionalSchemaExample.type": "io.confluent.connect.transforms.Drop$Key", "transforms.dropValueAndForceOptionalSchemaExample.schema.behavior": "nullify" ``` * Before: `key: 24`, `schema: {"type": "integer"}` * After: `key: null`, `schema: null` The Drop SMT nullifies the key and the schema.
* Record contents using `Drop$Key` with `schema.behavior` set to `retain`: ```json "transforms.dropValueAndForceOptionalSchemaExample.type": "io.confluent.connect.transforms.Drop$Key", "transforms.dropValueAndForceOptionalSchemaExample.schema.behavior": "retain" ``` * Before: `key: 24`, `schema: {"type": "integer"}` * After: `key: null`, `schema: {"type": "integer"}` The Drop SMT nullifies the key and retains the schema.
* Record contents using `Drop$Key` with `schema.behavior` set to `validate` where the schema is not optional: ```json "transforms.dropValueAndForceOptionalSchemaExample.type": "io.confluent.connect.transforms.Drop$Key", "transforms.dropValueAndForceOptionalSchemaExample.schema.behavior": "validate" ``` * Before: `key: 24`, `schema: {"type": "integer"}`. * After: Throws exception because the schema is not optional.
* Record contents using `Drop$Key` with `schema.behavior` set to `validate`, where the schema is optional: ```json "transforms.dropValueAndForceOptionalSchemaExample.type": "io.confluent.connect.transforms.Drop$Key", "transforms.dropValueAndForceOptionalSchemaExample.schema.behavior": "validate" ``` * Before: `key: 24`, `schema: {"type": "integer", "optional": true}` * After: `key: null`, `schema: {"type": "integer", "optional": true}` The Drop SMT nullifies the key and retains the schema.
* Record contents using `Drop$Key` with `schema.behavior` set to `force_optional`, where the schema is not optional: ```json "transforms.dropValueAndForceOptionalSchemaExample.type": "io.confluent.connect.transforms.Drop$Key", "transforms.dropValueAndForceOptionalSchemaExample.schema.behavior": "force_optional" ``` * Before: `key: 24`, `schema: {"type": "integer"}` * After: `key: null`, `schema: {"type": "integer", "optional": "true"}` The Drop SMT nullifies the key and makes the schema optional.
* Record contents using `Drop$Key` with `schema.behavior` set to `force_optional`, where the schema is already optional: ```json "transforms.dropValueAndForceOptionalSchemaExample.type": "io.confluent.connect.transforms.Drop$Key", "transforms.dropValueAndForceOptionalSchemaExample.schema.behavior": "force_optional" ``` * Before: `key: 24`, `schema: {"type": "integer", "optional": true}` * After: `key: null`, `schema: {"type": "integer", "optional": true}` The Drop SMT nullifies the key and retains the schema. ## Predicates Configure transformations with predicates to ensure they process only records that satisfy a particular condition. You can also use predicates in a transformation chain with the [Kafka Connect Filter (Kafka) SMT Usage Reference for Confluent Platform](filter-ak.md#ak-filter) to conditionally filter specific records. For more information, see [Predicates](filter-ak.md#predicates).