|
| 1 | +"""Simplest possible FastAPI chat demo using PostgreSQL LISTEN/NOTIFY. |
| 2 | +
|
| 3 | +The goal of this file is to show the basic flow end to end: |
| 4 | +
|
| 5 | +1. A user submits a chat message with ``POST /messages``. |
| 6 | +2. The API stores the message in PostgreSQL and issues ``pg_notify``. |
| 7 | +3. Every running API instance listens for that notification and forwards it |
| 8 | + to connected WebSocket clients. |
| 9 | +4. The browser keeps a WebSocket open so it can display new messages instantly. |
| 10 | +""" |
| 11 | + |
| 12 | +from __future__ import annotations |
| 13 | + |
| 14 | +import asyncio |
| 15 | +import json |
| 16 | +import logging |
| 17 | +import os |
| 18 | +from contextlib import asynccontextmanager |
| 19 | +from datetime import datetime, timezone |
| 20 | +from pathlib import Path |
| 21 | +from typing import Any |
| 22 | + |
| 23 | +import asyncpg |
| 24 | +from fastapi import FastAPI, HTTPException, WebSocket, WebSocketDisconnect |
| 25 | +from fastapi.responses import HTMLResponse |
| 26 | +from pydantic import BaseModel, Field |
| 27 | + |
| 28 | +logger = logging.getLogger("chat") |
| 29 | +logging.basicConfig(level=logging.INFO) |
| 30 | + |
| 31 | +CHAT_CHANNEL = "chat_messages" |
| 32 | +MIGRATION_LOCK_ID = 101 |
| 33 | +TEMPLATE_PATH = Path(__file__).resolve().parent / "templates" / "index.html" |
| 34 | + |
| 35 | + |
| 36 | +class MessageCreate(BaseModel): |
| 37 | + """Request body for ``POST /messages``.""" |
| 38 | + |
| 39 | + message: str = Field(..., min_length=1, max_length=500) |
| 40 | + |
| 41 | + |
| 42 | +class WebSocketRegistry: |
| 43 | + """Keeps track of the browsers that are connected via WebSocket.""" |
| 44 | + |
| 45 | + def __init__(self) -> None: |
| 46 | + self._sockets: set[WebSocket] = set() |
| 47 | + self._lock = asyncio.Lock() |
| 48 | + |
| 49 | + async def add(self, socket: WebSocket) -> None: |
| 50 | + async with self._lock: |
| 51 | + self._sockets.add(socket) |
| 52 | + |
| 53 | + async def remove(self, socket: WebSocket) -> None: |
| 54 | + async with self._lock: |
| 55 | + self._sockets.discard(socket) |
| 56 | + |
| 57 | + async def broadcast(self, payload: dict[str, Any]) -> None: |
| 58 | + async with self._lock: |
| 59 | + sockets = list(self._sockets) |
| 60 | + for socket in sockets: |
| 61 | + try: |
| 62 | + await socket.send_json(payload) |
| 63 | + except Exception: |
| 64 | + await self.remove(socket) |
| 65 | + |
| 66 | + |
| 67 | +def build_database_dsn() -> str: |
| 68 | + """Build a PostgreSQL connection string from environment variables.""" |
| 69 | + |
| 70 | + url = os.getenv("DATABASE_URL") |
| 71 | + if url: |
| 72 | + return url |
| 73 | + raise ValueError("DATABASE_URL environment variable is not set") |
| 74 | + |
| 75 | +async def create_tables(pool: asyncpg.Pool) -> None: |
| 76 | + """Make sure the ``messages`` table exists.""" |
| 77 | + |
| 78 | + async with pool.acquire() as conn: |
| 79 | + await conn.execute("SELECT pg_advisory_lock($1)", MIGRATION_LOCK_ID) |
| 80 | + try: |
| 81 | + await conn.execute( |
| 82 | + """ |
| 83 | + CREATE TABLE IF NOT EXISTS messages ( |
| 84 | + id BIGSERIAL PRIMARY KEY, |
| 85 | + message TEXT NOT NULL, |
| 86 | + created_at TIMESTAMPTZ NOT NULL DEFAULT NOW() |
| 87 | + ); |
| 88 | + """ |
| 89 | + ) |
| 90 | + finally: |
| 91 | + await conn.execute("SELECT pg_advisory_unlock($1)", MIGRATION_LOCK_ID) |
| 92 | + |
| 93 | + |
| 94 | +async def fetch_recent_messages(pool: asyncpg.Pool, limit: int = 50) -> list[dict[str, Any]]: |
| 95 | + """Return the most recent chat messages.""" |
| 96 | + |
| 97 | + async with pool.acquire() as conn: |
| 98 | + rows = await conn.fetch( |
| 99 | + """ |
| 100 | + SELECT id, message, created_at |
| 101 | + FROM messages |
| 102 | + ORDER BY created_at ASC |
| 103 | + LIMIT $1 |
| 104 | + """, |
| 105 | + limit, |
| 106 | + ) |
| 107 | + return [ |
| 108 | + { |
| 109 | + "id": row["id"], |
| 110 | + "message": row["message"], |
| 111 | + "created_at": row["created_at"].isoformat(), |
| 112 | + } |
| 113 | + for row in rows |
| 114 | + ] |
| 115 | + |
| 116 | + |
| 117 | +async def save_message(pool: asyncpg.Pool, text: str) -> dict[str, Any]: |
| 118 | + """Insert a message and notify other app instances about it.""" |
| 119 | + |
| 120 | + message = text.strip() |
| 121 | + if not message: |
| 122 | + raise HTTPException(status_code=400, detail="Message must not be empty") |
| 123 | + |
| 124 | + async with pool.acquire() as conn: |
| 125 | + async with conn.transaction(): |
| 126 | + record = await conn.fetchrow( |
| 127 | + """ |
| 128 | + INSERT INTO messages (message) |
| 129 | + VALUES ($1) |
| 130 | + RETURNING id, message, created_at |
| 131 | + """, |
| 132 | + message, |
| 133 | + ) |
| 134 | + body = { |
| 135 | + "type": "message", |
| 136 | + "data": { |
| 137 | + "id": record["id"], |
| 138 | + "message": record["message"], |
| 139 | + "created_at": record["created_at"].isoformat(), |
| 140 | + }, |
| 141 | + } |
| 142 | + await conn.execute("SELECT pg_notify($1, $2)", CHAT_CHANNEL, json.dumps(body)) |
| 143 | + return body["data"] |
| 144 | + |
| 145 | + |
| 146 | +async def forward_notification(payload: str) -> None: |
| 147 | + """Handle the JSON payload coming from PostgreSQL.""" |
| 148 | + |
| 149 | + try: |
| 150 | + data = json.loads(payload) |
| 151 | + except json.JSONDecodeError: |
| 152 | + logger.warning("Ignoring unexpected payload: %s", payload) |
| 153 | + return |
| 154 | + await app.state.connections.broadcast(data) |
| 155 | + |
| 156 | + |
| 157 | +async def notification_listener(stop_event: asyncio.Event) -> None: |
| 158 | + """Listen for ``pg_notify`` events until the app shuts down.""" |
| 159 | + |
| 160 | + loop = asyncio.get_running_loop() |
| 161 | + database_dsn = build_database_dsn() |
| 162 | + conn = await asyncpg.connect(dsn=database_dsn) |
| 163 | + |
| 164 | + def _listener(_connection: asyncpg.Connection, _pid: int, _channel: str, payload: str) -> None: |
| 165 | + loop.create_task(forward_notification(payload)) |
| 166 | + |
| 167 | + await conn.add_listener(CHAT_CHANNEL, _listener) |
| 168 | + try: |
| 169 | + await stop_event.wait() |
| 170 | + finally: |
| 171 | + await conn.remove_listener(CHAT_CHANNEL, _listener) |
| 172 | + await conn.close() |
| 173 | + |
| 174 | + |
| 175 | +@asynccontextmanager |
| 176 | +async def lifespan(app: FastAPI): |
| 177 | + """Manage application lifespan events.""" |
| 178 | + # Startup |
| 179 | + database_dsn = build_database_dsn() |
| 180 | + pool = await asyncpg.create_pool(dsn=database_dsn, min_size=1, max_size=5) |
| 181 | + app.state.db_pool = pool |
| 182 | + await create_tables(pool) |
| 183 | + |
| 184 | + stop_event = asyncio.Event() |
| 185 | + app.state.listener_stop = stop_event |
| 186 | + app.state.listener_task = asyncio.create_task(notification_listener(stop_event)) |
| 187 | + |
| 188 | + yield |
| 189 | + |
| 190 | + # Shutdown |
| 191 | + stop_event = app.state.listener_stop |
| 192 | + if stop_event: |
| 193 | + stop_event.set() |
| 194 | + listener_task = app.state.listener_task |
| 195 | + if listener_task: |
| 196 | + await listener_task |
| 197 | + |
| 198 | + pool = app.state.db_pool |
| 199 | + if pool: |
| 200 | + await pool.close() |
| 201 | + |
| 202 | + |
| 203 | +app = FastAPI(title="FastAPI Postgres Pub/Sub Chat", lifespan=lifespan) |
| 204 | +app.state.connections = WebSocketRegistry() |
| 205 | +app.state.listener_stop: asyncio.Event | None = None |
| 206 | +app.state.listener_task: asyncio.Task | None = None |
| 207 | +app.state.db_pool: asyncpg.Pool | None = None |
| 208 | + |
| 209 | + |
| 210 | +def get_pool() -> asyncpg.Pool: |
| 211 | + pool = app.state.db_pool |
| 212 | + if pool is None: |
| 213 | + raise HTTPException(status_code=500, detail="Database connection not ready") |
| 214 | + return pool |
| 215 | + |
| 216 | + |
| 217 | + |
| 218 | + |
| 219 | +@app.get("/", response_class=HTMLResponse) |
| 220 | +async def index() -> HTMLResponse: |
| 221 | + if not TEMPLATE_PATH.exists(): |
| 222 | + raise HTTPException(status_code=500, detail="Template not found") |
| 223 | + return HTMLResponse(TEMPLATE_PATH.read_text(encoding="utf-8")) |
| 224 | + |
| 225 | + |
| 226 | +@app.post("/messages") |
| 227 | +async def create_message(payload: MessageCreate) -> dict[str, Any]: |
| 228 | + pool = get_pool() |
| 229 | + message = await save_message(pool, payload.message) |
| 230 | + return message |
| 231 | + |
| 232 | + |
| 233 | +@app.websocket("/ws") |
| 234 | +async def websocket_endpoint(websocket: WebSocket) -> None: |
| 235 | + pool = get_pool() |
| 236 | + await websocket.accept() |
| 237 | + await app.state.connections.add(websocket) |
| 238 | + |
| 239 | + try: |
| 240 | + history = await fetch_recent_messages(pool) |
| 241 | + await websocket.send_json({"type": "history", "messages": history}) |
| 242 | + |
| 243 | + while True: |
| 244 | + await websocket.receive_text() |
| 245 | + except WebSocketDisconnect: |
| 246 | + pass |
| 247 | + finally: |
| 248 | + await app.state.connections.remove(websocket) |
| 249 | + |
| 250 | + |
| 251 | +@app.get("/healthz") |
| 252 | +async def healthcheck() -> dict[str, str]: |
| 253 | + pool = get_pool() |
| 254 | + async with pool.acquire() as conn: |
| 255 | + await conn.execute("SELECT 1") |
| 256 | + return {"status": "ok", "timestamp": datetime.now(timezone.utc).isoformat()} |
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