# Expect

The *expect* filter plugin lets you validate that records match certain criteria in their structure, like validating that a key exists or it has a specific value.

For a detailed explanation of its usage and use cases, see [Validating and your Data and Structure](https://docs.fluentbit.io/manual/4.1/local-testing/validating-your-data-and-structure).

## Configuration parameters

The plugin supports the following configuration parameters:

| Property              | Description                                                                                                                                                                                                                                                                                                             |
| --------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| `key_exists`          | Check if a key with a given name exists in the record.                                                                                                                                                                                                                                                                  |
| `key_not_exists`      | Check if a key doesn't exist in the record.                                                                                                                                                                                                                                                                             |
| `key_val_is_null`     | Check that the value of the key is `NULL`.                                                                                                                                                                                                                                                                              |
| `key_val_is_not_null` | Check that the value of the key is `NOT NULL`.                                                                                                                                                                                                                                                                          |
| `key_val_eq`          | Check that the value of the key equals the given value in the configuration.                                                                                                                                                                                                                                            |
| `action`              | Action to take when a rule doesn't match. Available options are `warn` , `exit` or `result_key`. On `warn`, a warning message is sent to the logging layer when a mismatch of the `key*` rules is found. Using `exit` makes Fluent Bit exit with status code `255`. `result_key` adds a matching result to each record. |
| `result_key`          | Specify a key name for the matching result added when `action` is set to `result_key`.                                                                                                                                                                                                                                  |