# Step 4: Connection Checking

The second operation in the Airbyte Protocol that we'll implement is the `check` operation.

This operation verifies that the input configuration supplied by the user can be used to connect to the underlying data source. Note that this user-supplied configuration has the values described in the `spec.json` filled in. In other words if the `spec.json` said that the source requires a `username` and `password` the config object might be `{ "username": "airbyte", "password": "password123" }`. You should then implement something that returns a json object reporting, given the credentials in the config, whether we were able to connect to the source.

In our case, this is a fairly trivial check since the API requires no credentials. Instead, let's verify that the user-input `base` currency is a legitimate currency. In `source.py` we'll find the following autogenerated source:

```python
class SourcePythonHttpTutorial(AbstractSource):

    def check_connection(self, logger, config) -> Tuple[bool, any]:
        """
        TODO: Implement a connection check to validate that the user-provided config can be used to connect to the underlying API

        See https://github.com/airbytehq/airbyte/blob/master/airbyte-integrations/connectors/source-stripe/source_stripe/source.py#L232
        for an example.

        :param config:  the user-input config object conforming the connector's spec.json
        :param logger:  logger object
        :return Tuple[bool, any]: (True, None) if the input config can be used to connect to the API successfully, (False, error) otherwise.
        """
        return True, None

...
```

Following the docstring instructions, we'll change the implementation to verify that the input currency is a real currency:

```python
    def check_connection(self, logger, config) -> Tuple[bool, any]:
        accepted_currencies = {"USD", "JPY", "BGN", "CZK", "DKK"}  # assume these are the only allowed currencies
        input_currency = config['base']
        if input_currency not in accepted_currencies:
            return False, f"Input currency {input_currency} is invalid. Please input one of the following currencies: {accepted_currencies}"
        else:
            return True, None
```

Let's test out this implementation by creating two objects: a valid and an invalid config and attempt to give them as input to the connector

```text
echo '{"start_date": "2021-04-01", "base": "USD"}'  > sample_files/config.json
echo '{"start_date": "2021-04-01", "base": "BTC"}'  > sample_files/invalid_config.json
python main.py check --config sample_files/config.json
python main.py check --config sample_files/invalid_config.json
```

You should see output like the following:

```text
> python main.py check --config sample_files/config.json
{"type": "CONNECTION_STATUS", "connectionStatus": {"status": "SUCCEEDED"}}

> python main.py check --config sample_files/invalid_config.json
{"type": "CONNECTION_STATUS", "connectionStatus": {"status": "FAILED", "message": "Input currency BTC is invalid. Please input one of the following currencies: {'DKK', 'USD', 'CZK', 'BGN', 'JPY'}"}}
```

While developing, we recommend storing configs which contain secrets in `secrets/config.json` because the `secrets` directory is gitignored by default.