Version: 1.3

Structured Config schema

We have seen how to use Structured Configs as configuration, but they can also be used as a schema (i.e. validating configuration files). To achieve this, we will follow the common pattern of Extending Configs - but instead of extending another config file, we will extend a Structured Config.

This page shows how to validate the config files config.yaml, db/mysql.yaml and db/postgresql.yaml against a Structured Config schema.

Validating against a schema in the same config group

Given the config directory structure:

conf/
├── config.yaml
└── db
    ├── mysql.yaml
    └── postgresql.yaml

We will add Structured Config schema for each of the config files above and store in the Config Store as base_config, db/base_mysql and db/base_postgresql.

Then, we will use the Defaults List in each config to specify its base config as follows:

config.yaml
defaults:
  - base_config
  - db: mysql
  # See composition order note
  - _self_

debug: true

db/mysql.yaml
defaults:
  - base_mysql

user: omry
password: secret

db/postgresql.yaml
defaults:
  - base_postgresql

user: postgres_user
password: drowssap

One difference in the source code is that we have removed the Defaults List from the Config dataclass. The primary Defaults List will come from config.yaml.

my_app.py (Click to expand)

from dataclasses import dataclass

import hydra
from hydra.core.config_store import ConfigStore

@dataclass
class DBConfig:
    driver: str = MISSING
    host: str = "localhost"
    port: int = MISSING

@dataclass
class MySQLConfig(DBConfig):
    driver: str = "mysql"
    port: int = 3306
    user: str = MISSING
    password: str = MISSING

@dataclass
class PostGreSQLConfig(DBConfig):
    driver: str = "postgresql"
    user: str = MISSING
    port: int = 5432
    password: str = MISSING
    timeout: int = 10

@dataclass
class Config:
    db: DBConfig = MISSING
    debug: bool = False

cs = ConfigStore.instance()
cs.store(name="base_config", node=Config)
cs.store(group="db", name="base_mysql", node=MySQLConfig)
cs.store(group="db", name="base_postgresql", node=PostGreSQLConfig)

@hydra.main(version_base=None, config_path="conf", config_name="config")
def my_app(cfg: Config) -> None:
    print(OmegaConf.to_yaml(cfg))

if __name__ == "__main__":
    my_app()

When Hydra composes the final config object it will use the config schemas as specified in the Default Lists. Like before, Hydra will catch user errors in the command line:

$ python my_app.py db.port=fail
Error merging override db.port=fail
Value 'fail' could not be converted to Integer
        full_key: db.port
        object_type=MySQLConfig

Use --info commands to see how a config was composed (Expand)

$ python my_app.py --info defaults-tree

Defaults Tree
*************
<root>:
  hydra/config:
    hydra/output: default
    hydra/launcher: basic
    hydra/sweeper: basic
    hydra/help: default
    hydra/hydra_help: default
    hydra/hydra_logging: default
    hydra/job_logging: default
    _self_
  config:
    base_config
    db: mysql:
      db/base_mysql
      _self_
    _self_

$ python my_app.py --info defaults

Defaults List
*************
| Config path                 | Package             | _self_ | Parent       |
------------------------------------------------------------------------------
| hydra/output/default        | hydra               | False  | hydra/config |
| hydra/launcher/basic        | hydra.launcher      | False  | hydra/config |
| hydra/sweeper/basic         | hydra.sweeper       | False  | hydra/config |
| hydra/help/default          | hydra.help          | False  | hydra/config |
| hydra/hydra_help/default    | hydra.hydra_help    | False  | hydra/config |
| hydra/hydra_logging/default | hydra.hydra_logging | False  | hydra/config |
| hydra/job_logging/default   | hydra.job_logging   | False  | hydra/config |
| hydra/config                | hydra               | True   | <root>       |
| base_config                 |                     | False  | config       |
| db/base_mysql               | db                  | False  | db/mysql     |
| db/mysql                    | db                  | True   | config       |
| config                      |                     | True   | <root>       |
------------------------------------------------------------------------------

Validating against a schema from a different config group

In the above example, the schema we used was stored in the same config group. This is not always the case, for example - A library might provide schemas in its own config group.

Here is a modified version of the example above, where a mock database_lib is providing the schemas we want to validate against.

my_app.py
from dataclasses import dataclass

import hydra
from hydra.core.config_store import ConfigStore

import database_lib


@dataclass
class Config:
    db: database_lib.DBConfig = MISSING
    debug: bool = False

cs = ConfigStore.instance()
cs.store(name="base_config", node=Config)

# database_lib registers its configs
# in database_lib/db
database_lib.register_configs()


@hydra.main(
    version_base=None,
    config_path="conf",
    config_name="config",
)
def my_app(cfg: Config) -> None:
    print(OmegaConf.to_yaml(cfg))


if __name__ == "__main__":
    my_app()

database_lib.py
from dataclasses import dataclass

from hydra.core.config_store import ConfigStore

@dataclass
class DBConfig:
  ...

@dataclass
class MySQLConfig(DBConfig):
  ...

@dataclass
class PostGreSQLConfig(DBConfig):
  ...


def register_configs() -> None:
    cs = ConfigStore.instance()
    cs.store(
        group="database_lib/db",
        name="mysql",
        node=MySQLConfig,
    )
    cs.store(
        group="database_lib/db",
        name="postgresql",
        node=PostGreSQLConfig,
    )

The Defaults List entry for the base config is slightly different:

db/mysql.yaml
defaults:
  - /database_lib/db/mysql@_here_

user: omry
password: secret

db/postgresql.yaml
defaults:
  - /database_lib/db/postgresql@_here_
  # See composition order note
  - _self_

user: postgres_user
password: drowssap

We refer to the config with an absolute path because it is outside the subtree of the db config group.
we override the package to _here_ to ensure that the package of the schema is the same as the package of the config it's validating.

A Note about composition order

By default, Hydra 1.1 appends _self_ to the end of the Defaults List. This behavior is new in Hydra 1.1 and different from previous Hydra versions. As such Hydra 1.1 issues a warning if _self_ is not specified in the primary config, asking you to add _self_ and thus indicate the desired composition order. To address the warning while maintaining the new behavior, append _self_ to the end of the Defaults List. Note that in some cases it may instead be desirable to add _self_ directly after the schema and before other Defaults List elements.

See Composition Order for more information.

Validating against a schema in the same config group​

Validating against a schema from a different config group​

A Note about composition order​

Validating against a schema in the same config group

Validating against a schema from a different config group

A Note about composition order