Initialize the domain

The domain is initialized by calling the init method.

domain.init()

A call to init does the following:

1. Traverse the domain model

By default, Protean traverses the directory structure under the domain file to discover domain elements. You can control this behavior with the traverse flag:

domain.init(traverse=False)

If you choose to not traverse, Protean will not be able to detect domain elements automatically. You are responsible for registering each element with the domain explicitly.

2. Construct the object graph

Protean constructs a graph of all elements registered with a domain and exposes them in a registry.

from protean import Domain
from protean.fields import Identifier, String

domain = Domain(__file__)


@domain.aggregate
class User:
    name = String(max_length=50)


@domain.entity(part_of=User)
class Credentials:
    email = String(max_length=254)
    password_hash = String(max_length=128)


@domain.event(part_of=User)
class Registered:
    id = Identifier()
    email = String()
    name = String()
    password_hash = String()


print(domain.registry.elements)
""" Output:
{
    'aggregates': [<class '__main__.User'>],
    'events': [<class '__main__.Registered'>],
    'entities': [<class '__main__.Credentials'>]
}
"""

3. Initialize dependencies

Calling domain.init() establishes connectivity with the underlying infra, testing access, and making them available for use by the rest of the system.

By default, a protean domain is configured to use in-memory replacements for infrastructure, like databases, brokers, and caches. They are useful for testing and prototyping. But for production purposes, you will want to choose a database that actually persists data.

from protean import Domain

domain = Domain(__file__)

domain.config["databases"]["default"] = {
    "provider": "sqlalchemy",
    "database": "sqlite",
    "database_uri": "sqlite:///test.db",
}

domain.init(traverse=False)

In the example above, the domain activates an SQLite database repository and makes it available for domain elements for further use.

Refer to Configuration handling to understand the different ways to configure the domain.

4. Validate Domain Model

In the final part of domain initialization, Protean performs additional setup tasks on domain elements and also conducts various checks to ensure the domain model is specified correctly.

Examples of checks include:

1. Resolving references that were specified as Strings, like:

   @domain.entity(part_of="User")
   class Account:
       ...
   

2. Setting up Aggregate clusters and their shared settings. The object graph constructed earlier is used to homogenize settings across all elements under an aggregate, like the stream category and database provider.

3. Constructing a map of command and event types to reference when processing incoming messages later.

4. Various checks and validations to ensure the domain structure is valid.