# Constraints

Constraints are a powerful feature that allow you to evaluate each value of a certain column and handle any failures. They can be specified using SQL-like syntax, separated by a `|` symbol. The advantage of using constraints is that data quality can be ensured *while* ingesting the data (at runtime), not way later in the pipeline.

```yaml
source: source_name
target: target_name

streams:
  my_stream:
    columns:
      # id values cannot be null
      id: bigint | value is not null

      # status values can only be active or inactive
      status: string | value in ('active', 'inactive')

      # col_1 value length can only be 6, 7 or 8
      col_1: string(8) | value_len > 5 and value_len <= 8
```

#### Handling failures

Sling looks for the environment variable `SLING_ON_CONSTRAINT_FAILURE` to know what to do.

Here are the values allowed in `SLING_ON_CONSTRAINT_FAILURE`:

* `warn`: This is the default. When using the Sling Platform, this will emit a warning status, which can notify you (Email, Slack, etc.)
* `skip`: Skip the record (do not ingest into target)
* `abort`: Will immediately abort the run, and fail/error. When using the Sling Platform, this can notify you (Email, Slack, etc.)

#### Supported Operators

* `is null` - Check if value is null
* `is not null` - Check if value is not null
* `==` - Equal to
* `!=` or `<>` - Not equal to
* `>` - Greater than
* `>=` - Greater than or equal to
* `<` - Less than
* `<=` - Less than or equal to
* `~` - Matches regex pattern
* `!~` - Does not match regex pattern
* `in` - Value matches any in list
* `not in` - Value does not match any in list
* `and` - Combine multiple conditions (all must be true)
* `or` - Combine multiple conditions (at least one must be true)

#### Special Variables

* `value` - Record value for respective column
* `value_len` - Length of record value for respective column

#### Using CLI Flags

```bash
# template. data type must be specified when using constraints
sling run --columns '{ "<column_name>": "<data_type> | <constraint expression>" }'

# values cannot null
sling run --columns '{ "my_column": "bigint | value is not null" }'

# cannot be zero
sling run --columns '{ "my_column": "bigint | value != 0" }'

```

#### Using YAML

Using the `defaults` and `streams` keys, you can specify different columns/constraints for each stream.

```yaml
source: source_name
target: target_name

defaults:
  # apply to all streams by default
  columns:
    id: bigint | value is not null  # value cannot be null

streams:
  # inherit defaults
  my_stream:

  my_other_stream:
    columns:
      # can evaluate value_len (number of digits)
      id: bigint | value_len > 0 or value = -10  

      # enum values
      my_column: string | value in ('first', 'second', 'third')

      # pattern matching or is null
      my_other_column: string | value ~ 'abc(d)' or value is null
```


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