Event-Driven Architecture with Modern Patterns

This skill provides comprehensive patterns for implementing event-driven microservices using modern messaging systems, distributed runtimes, and cloud-native patterns. It's designed to be framework-agnostic and applicable to any domain requiring event-driven capabilities.

When to Use This Skill

Use this skill when you need to:

• Build event-driven microservices architecture
• Implement pub/sub patterns with Kafka, RabbitMQ, or cloud services
• Use Dapr for distributed application patterns
• Implement real-time notifications and workflows
• Build recurring task and reminder systems
• Create audit trails and activity logs
• Implement distributed state management
• Build serverless event workflows
• Handle event sourcing and CQRS patterns

1. Core Event Architecture

Base Event Schema

# events/core.py
from pydantic import BaseModel, Field, validator
from datetime import datetime
from typing import Optional, Dict, Any, Union, List
from enum import Enum
from abc import ABC, abstractmethod
import uuid
import json

class EventVersion(str, Enum):
    """Event versioning support"""
    V1_0 = "1.0"
    V1_1 = "1.1"
    V2_0 = "2.0"

class EventPriority(str, Enum):
    """Event priority levels"""
    LOW = "low"
    NORMAL = "normal"
    HIGH = "high"
    CRITICAL = "critical"

class BaseEvent(BaseModel, ABC):
    """Base event schema with common fields"""

    # Core identification
    event_id: str = Field(default_factory=lambda: str(uuid.uuid4()))
    event_type: str = Field(..., description="Type of the event")
    event_version: EventVersion = Field(default=EventVersion.V1_0)

    # Metadata
    timestamp: datetime = Field(default_factory=datetime.utcnow)
    correlation_id: Optional[str] = None
    causation_id: Optional[str] = None  # Event that caused this one
    source: str = Field(..., description="Source service/identifier")

    # Context
    user_id: Optional[str] = None
    tenant_id: Optional[str] = None  # Multi-tenant support
    session_id: Optional[str] = None

    # Processing metadata
    priority: EventPriority = Field(default=EventPriority.NORMAL)
    retry_count: int = Field(default=0)
    max_retries: int = Field(default=3)
    delay_until: Optional[datetime] = None  # For delayed processing

    # Schema versioning
    schema_version: str = Field(default="1.0")

    class Config:
        # Allow additional fields for extensibility
        extra = "allow"
        # Use enum values
        use_enum_values = True

    @validator('event_type')
    def validate_event_type(cls, v):
        """Validate event type format"""
        if not v or '.' not in v:
            raise ValueError('event_type must be in format: domain.event_name')
        return v.lower()

    @validator('correlation_id')
    def validate_correlation_id(cls, v, values):
        """Set correlation_id from causation_id if not provided"""
        if not v and 'causation_id' in values:
            return values['causation_id']
        return v

    def to_dict(self) -> Dict[str, Any]:
        """Convert event to dictionary for serialization"""
        return self.model_dump()

    @classmethod
    def from_dict(cls, data: Dict[str, Any]) -> 'BaseEvent':
        """Create event from dictionary"""
        return cls(**data)

    def with_context(self, **context) -> 'BaseEvent':
        """Add context to event"""
        for key, value in context.items():
            if hasattr(self, key):
                setattr(self, key, value)
        return self

class DomainEvent(BaseEvent):
    """Domain-specific event"""

    aggregate_id: str = Field(..., description="Aggregate root ID")
    aggregate_type: str = Field(..., description="Aggregate type")
    event_data: Dict[str, Any] = Field(default_factory=dict)

    @validator('aggregate_type')
    def validate_aggregate_type(cls, v):
        """Validate aggregate type"""
        if not v:
            raise ValueError('aggregate_type is required')
        return v.lower()

class IntegrationEvent(BaseEvent):
    """Integration event for cross-service communication"""

    target_services: List[str] = Field(default_factory=list)
    routing_key: Optional[str] = None
    message_format: str = Field(default="json")

    @validator('routing_key')
    def validate_routing_key(cls, v, values):
        """Generate routing key from event_type if not provided"""
        if not v and 'event_type' in values:
            return values['event_type'].replace('.', '/')
        return v

class CommandEvent(BaseEvent):
    """Command event representing an intent"""

    command_type: str = Field(..., description="Type of command")
    command_data: Dict[str, Any] = Field(default_factory=dict)
    expected_version: Optional[int] = None  # For optimistic concurrency

    @validator('command_type')
    def validate_command_type(cls, v):
        """Validate command type"""
        if not v.endswith('.command'):
            v = f"{v}.command"
        return v.lower()

class QueryEvent(BaseEvent):
    """Query event for data retrieval"""

    query_type: str = Field(..., description="Type of query")
    query_params: Dict[str, Any] = Field(default_factory=dict)
    result_topic: Optional[str] = None  # Where to send results

    @validator('query_type')
    def validate_query_type(cls, v):
        """Validate query type"""
        if not v.endswith('.query'):
            v = f"{v}.query"
        return v.lower()

Event Store Pattern

# events/store.py
import asyncio
from abc import ABC, abstractmethod
from typing import List, Optional, Dict, Any, AsyncIterator
from datetime import datetime, timedelta
import json
import uuid

class EventStore(ABC):
    """Abstract event store interface"""

    @abstractmethod
    async def save_event(self, event: BaseEvent, stream_id: str) -> None:
        """Save event to a stream"""
        pass

    @abstractmethod
    async def get_events(
        self,
        stream_id: str,
        from_version: Optional[int] = None,
        to_version: Optional[int] = None,
        limit: Optional[int] = None
    ) -> AsyncIterator[BaseEvent]:
        """Get events from a stream"""
        pass

    @abstractmethod
    async def get_event_by_id(self, event_id: str) -> Optional[BaseEvent]:
        """Get a specific event by ID"""
        pass

    @abstractmethod
    async def get_events_by_type(
        self,
        event_type: str,
        from_timestamp: Optional[datetime] = None,
        to_timestamp: Optional[datetime] = None
    ) -> AsyncIterator[BaseEvent]:
        """Get events by type"""
        pass

    @abstractmethod
    async def get_aggregate_snapshot(
        self,
        aggregate_id: str,
        aggregate_type: str
    ) -> Optional[Dict[str, Any]]:
        """Get aggregate snapshot"""
        pass

    @abstractmethod
    async def save_aggregate_snapshot(
        self,
        aggregate_id: str,
        aggregate_type: str,
        data: Dict[str, Any],
        version: int
    ) -> None:
        """Save aggregate snapshot"""
        pass

class InMemoryEventStore(EventStore):
    """In-memory event store for testing and development"""

    def __init__(self):
        self._events: Dict[str, List[BaseEvent]] = {}
        self._snapshots: Dict[str, Dict[str, Any]] = {}
        self._type_index: Dict[str, List[str]] = {}

    async def save_event(self, event: BaseEvent, stream_id: str) -> None:
        """Save event to memory"""
        if stream_id not in self._events:
            self._events[stream_id] = []

        self._events[stream_id].append(event)

        # Update type index
        if event.event_type not in self._type_index:
            self._type_index[event.event_type] = []
        self._type_index[event.event_type].append(event.event_id)

    async def get_events(
        self,
        stream_id: str,
        from_version: Optional[int] = None,
        to_version: Optional[int] = None,
        limit: Optio