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Version: 1.2

Putting it all together

Β Example (Click Here)

As software gets more complex, we resort to modularity and composition to keep it manageable. We can do the same with configs. Suppose we want our working example to support multiple databases, with multiple schemas per database, and different UIs. We wouldn't write a separate class for each permutation of db, schema and UI, so we shouldn't write separate configs either. We use the same solution in configuration as in writing the underlying software: composition.

To do this in Hydra, we first add a schema and a ui config group:

Directory layout
β”œβ”€β”€ conf
β”‚ β”œβ”€β”€ config.yaml
β”‚ β”œβ”€β”€ db
β”‚ β”‚ β”œβ”€β”€ mysql.yaml
β”‚ β”‚ └── postgresql.yaml
β”‚ β”œβ”€β”€ schema
β”‚ β”‚ β”œβ”€β”€ school.yaml
β”‚ β”‚ β”œβ”€β”€ support.yaml
β”‚ β”‚ └── warehouse.yaml
β”‚ └── ui
β”‚ β”œβ”€β”€ full.yaml
β”‚ └── view.yaml

With these configs, we already have 12 possible combinations. Without composition, we would need 12 separate configs. A single change, such as renaming db.user to db.username, requires editing all 12 of them. This is a maintenance nightmare.

Composition can come to the rescue. Instead of creating 12 different config files, that fully specify each config, create a single config that specifies the different configuration dimensions, and the default for each.

- db: mysql
- ui: full
- schema: school

The resulting configuration is a composition of the mysql database, the full ui, and the school schema (which we are seeing for the first time here):

$ python
driver: mysql
user: omry
pass: secret
create_db: true
view: true
database: school
- name: students
- name: string
- class: int
- name: exams
- profession: string
- time: data
- class: int

Stay tuned to see how to run all of the combinations automatically (Multi-run).


  • The addition of each new db, schema, or ui only requires a single file.
  • Each config group can have a default specified in the Defaults List.
  • Any combination can be composed by selecting the desired option from each config group in the Defaults List or the command line.