InMemoryChatMessageHistory vs persistent storage

from langchain_core.chat_history import InMemoryChatMessageHistory
from langchain_core.messages import HumanMessage, AIMessage
from langchain_openai import ChatOpenAI
from langchain_core.prompts import ChatPromptTemplate, MessagesPlaceholder
from langchain_core.runnables.history import RunnableWithMessageHistory
import json
from datetime import datetime

print("=== InMemoryChatMessageHistory Demo ===")
memory = InMemoryChatMessageHistory()

memory.add_message(HumanMessage(content="What is the capital of France?"))
memory.add_message(AIMessage(content="The capital of France is Paris."))
memory.add_message(HumanMessage(content="And Germany?"))
memory.add_message(AIMessage(content="The capital of Germany is Berlin."))

print("\nMessages in memory:")
for msg in memory.messages:
    print(f"  {msg.__class__.__name__}: {msg.content}")

print(f"\nTotal messages: {len(memory.messages)}")
print(f"Memory object ID: {id(memory)} (will be same until process exits)")

print("\n=== Persistent Storage Simulation (SQLite-style) ===")

class SqliteChatHistory:
    """Simulates persistent storage behavior"""
    def __init__(self, filepath: str, session_id: str):
        self.filepath = filepath
        self.session_id = session_id
        self.messages = []
        self._load_from_disk()
    
    def _load_from_disk(self):
        """In real code, this reads from SQLite/PostgreSQL"""
        try:
            with open(self.filepath, 'r') as f:
                data = json.load(f)
                self.messages = data.get(self.session_id, [])
                print(f"Loaded {len(self.messages)} messages from disk for session {self.session_id}")
        except FileNotFoundError:
            print(f"No existing history file, starting fresh")
            self.messages = []
    
    def add_message(self, role: str, content: str):
        msg = {"role": role, "content": content, "timestamp": datetime.now().isoformat()}
        self.messages.append(msg)
        self._save_to_disk()
    
    def _save_to_disk(self):
        """In real code, this writes to SQLite/PostgreSQL"""
        try:
            with open(self.filepath, 'r') as f:
                all_data = json.load(f)
        except FileNotFoundError:
            all_data = {}
        
        all_data[self.session_id] = self.messages
        
        with open(self.filepath, 'w') as f:
            json.dump(all_data, f, indent=2)
    
    def get_messages(self):
        return self.messages

persistent = SqliteChatHistory(filepath="/tmp/chat_history.json", session_id="user_123")

persistent.add_message(role="user", content="What is Python?")
persistent.add_message(role="assistant", content="Python is a programming language.")
persistent.add_message(role="user", content="Is it good for AI?")
persistent.add_message(role="assistant", content="Yes, Python dominates AI/ML development.")

print("\nMessages in persistent storage:")
for msg in persistent.get_messages():
    print(f"  [{msg['role']}]: {msg['content']}")

print(f"\nAll messages saved to disk at: /tmp/chat_history.json")
print("If this process crashes, messages are still there.")

print("\n=== Key Differences ===")
print(f"InMemory - Lookup: O(1), Survives restart: No, Multi-process: No")
print(f"Persistent - Lookup: O(n), Survives restart: Yes, Multi-process: Yes")