"""
Pattern 21.03: Event Sourcing
Event log as source of truth
"""
from typing import TypedDict
from langgraph.graph import StateGraph, END
from langchain_openai import ChatOpenAI
from langchain_core.messages import HumanMessage


class EventSourcingState(TypedDict):
    """State for Event Sourcing pattern."""
    input: str
    output: str
    metadata: dict


def agent_function(state: EventSourcingState) -> EventSourcingState:
    """Event Sourcing agent implementation."""
    llm = ChatOpenAI()
    prompt = f"Process using Event Sourcing: {state['input']}"
    response = llm.invoke([HumanMessage(content=prompt)])
    state["output"] = str(response.content)
    state["metadata"] = {"pattern": "Event Sourcing", "status": "completed"}
    return state


def create_event_sourcing():
    """Create Event Sourcing workflow."""
    workflow = StateGraph(EventSourcingState)
    workflow.add_node("agent", agent_function)
    workflow.set_entry_point("agent")
    workflow.add_edge("agent", END)
    return workflow.compile()


if __name__ == "__main__":
    workflow = create_event_sourcing()
    result = workflow.invoke({"input": "Test Event Sourcing", "output": "", "metadata": {}})
    print(f"\n=== Event Sourcing Pattern ===\nOutput: {result['output'][:200]}...")

    

Agentic Orchestration Pattern: Event Sourcing Pattern

03 Event Sourcing Pattern – Detailed Documentation

Overview

File: 21_advanced_coordination/03_event_sourcing.py
Pattern Category: 21 Advanced Coordination
Implementation: LangGraph/CrewAI

Pattern Description

Pattern 21.03: Event Sourcing Event log as source of truth

---

Code Structure

Imports

from typing import TypedDict
from langgraph.graph import StateGraph, END
from langchain_openai import ChatOpenAI
from langchain_core.messages import HumanMessage

Purpose of imports: – TypedDict: For type-safe state definitions – StateGraph: LangGraph’s graph-based workflow orchestration – ChatOpenAI: OpenAI LLM integration – HumanMessage: LangChain message formatting

State Definition

class EventSourcingState(TypedDict):
    """State for Event Sourcing pattern."""
    input: str
    output: str
    metadata: dict


def agent_function(state: EventSourcingState) -> EventSourcingState:

EventSourcingState: State for Event Sourcing pattern.

State Fields: – input: Stores input – output: Stores output – metadata: Stores metadata – def agent_function(state: Stores def agent function(state – prompt = f"Process using Event Sourcing: Stores prompt = f”Process using Event Sourcing – state["metadata"] = {"pattern": Stores state[“metadata”] = {“pattern”

---

Functions

agent_function()

Line: 18
Arguments: state

Event Sourcing agent implementation.

def agent_function(state: EventSourcingState) -> EventSourcingState:
    """Event Sourcing agent implementation."""
    llm = ChatOpenAI()
    prompt = f"Process using Event Sourcing: {state['input']}"
    response = llm.invoke([HumanMessage(content=prompt)])
    state["output"] = str(response.content)
    state["metadata"] = {"pattern": "Event Sourcing", "status": "completed"}
    return state


def create_event_sourcing():

create_event_sourcing()

Line: 28
Arguments: “

Create Event Sourcing workflow.

def create_event_sourcing():
    """Create Event Sourcing workflow."""
    workflow = StateGraph(EventSourcingState)
    workflow.add_node("agent", agent_function)
    workflow.set_entry_point("agent")
    workflow.add_edge("agent", END)
    return workflow.compile()


if __name__ == "__main__":

---

Line-by-Line Explanation

Line 5: Import statement

from typing import TypedDict

Imports TypedDict for creating type-safe state dictionaries.

Line 6: Import statement

from langgraph.graph import StateGraph, END

Imports StateGraph – the core LangGraph class for building workflows.

Line 7: Import statement

from langchain_openai import ChatOpenAI

Imports ChatOpenAI for LLM interactions.

Line 8: Import statement

from langchain_core.messages import HumanMessage

Imports HumanMessage for formatting prompts to the LLM.

Line 11: Class definition

class EventSourcingState(TypedDict):

Defines the state structure using TypedDict for type safety.

Line 12: Module docstring

    """State for Event Sourcing pattern."""

Describes the pattern and its purpose.

Line 18: Function agent_function definition

def agent_function(state: EventSourcingState) -> EventSourcingState:

Defines the agent_function function that processes the workflow state.

Line 19: Module docstring

    """Event Sourcing agent implementation."""

Describes the pattern and its purpose.

Line 28: Function create_event_sourcing definition

def create_event_sourcing():

Defines the create_event_sourcing function that processes the workflow state.

Line 29: Module docstring

    """Create Event Sourcing workflow."""

Describes the pattern and its purpose.

---

Usage Example

if __name__ == "__main__":
    workflow = create_event_sourcing()
    result = workflow.invoke({"input": "Test Event Sourcing", "output": "", "metadata": {}})
    print(f"\n=== Event Sourcing Pattern ===\nOutput: {result['output'][:200]}...")

---

Key Concepts

1. State Management: Uses TypedDict to maintain workflow state
2. Graph-Based Flow: LangGraph orchestrates execution flow
3. Agent Functions: Each node processes and updates state
4. LLM Integration: Uses ChatOpenAI for intelligent processing

Pattern Benefits

• ✅ Type-safe state management
• ✅ Clear workflow visualization
• ✅ Modular and reusable components
• ✅ Easy to test and debug
• ✅ Scalable architecture

Related Patterns

• See other patterns in the same category
• Explore combination possibilities
• Check the main documentation for pattern relationships