# Expect

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

The following page just describes the configuration properties available, for a detailed explanation of its usage and use cases, please refer the following page:

* [Validating and your Data and Structure](https://docs.fluentbit.io/manual/1.9/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 does not 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 does not match. The available options are `warn` , `exit` or "result\_key". On `warn`, a warning message is sent to the logging layer when a mismatch of the rules above is found; using `exit` makes Fluent Bit abort with status code `255`; `result_key` is to add a matching result to each record. |
| result\_key             | specify a key name of matching result. This key is to be used only when 'action' is 'result\_key'.                                                                                                                                                                                                                                 |

## Getting Started

As mentioned on top, refer to the following page for specific details of usage of this filter:

* [Validating and your Data and Structure](https://docs.fluentbit.io/manual/1.9/local-testing/validating-your-data-and-structure)