qx-cqrs 0.2.0

Qx CQRS / Mediator: commands, queries, events, pipeline behaviors

qx-cqrs

CQRS and Mediator implementation for the Qx framework. Dispatches commands, queries, domain events, integration events, and notifications through a composable pipeline of behaviors.

What lives here

• qx.cqrs.Mediator — central dispatcher. Resolves handlers via the DI container; supports three registration modes: decorator scan, explicit register(), and type-based lookup.
• qx.cqrs.Command / Query — base Pydantic models for all messages. Command[TResult] mutates state; Query[TResult] is read-only.
• qx.cqrs.command_handler / query_handler / event_handler — decorators that bind a handler class to its message type.
• qx.cqrs.Behavior — single-method interface (handle(request, next)) for pipeline stages.
• Built-in behaviors: LoggingBehavior, TracingBehavior, MetricsBehavior, IdempotencyBehavior, ExceptionTranslationBehavior, TransactionBehavior, ValidationBehavior, RetryBehavior, AuthorizationBehavior.
• qx.cqrs.compose — assemble a BehaviorChain from an ordered list of behaviors.

Usage

from qx.cqrs import Command, Mediator, command_handler
from qx.core import Result

class CreateUserCommand(Command[UserDto]):
    email: str
    name: str

@command_handler(CreateUserCommand)
class CreateUserHandler:
    def __init__(self, repo: UserRepository, uow: UnitOfWork) -> None: ...

    async def handle(self, cmd: CreateUserCommand) -> Result[UserDto]:
        result = User.register(cmd.email, cmd.name)
        ...
        return Result.success(dto)

# Bootstrap
mediator = Mediator(container)
mediator.register_decorated("myapp.application")

# Dispatch (in a route or test)
result = await mediator.send(CreateUserCommand(email="[e-mail]", name="Ada"), scope=scope)

Pipeline composition

from qx.cqrs import compose, LoggingBehavior, TracingBehavior, ExceptionTranslationBehavior

pipeline = compose(
    LoggingBehavior(),
    TracingBehavior(tracer),
    ExceptionTranslationBehavior(),
)
mediator = Mediator(container, pipeline=pipeline)

Design rules

• Handlers return Result[T] — they never raise. ExceptionTranslationBehavior converts unexpected exceptions to InfrastructureError before they escape the pipeline.
• Commands and queries are plain Pydantic models; validation happens automatically before handle() is called.
• The mediator resolves handlers lazily per send() call so the DI scope for that request is used correctly.