Agent Processes Events Out of Order — Race Conditions in Async Pipelines
Symptom
- Webhook events processed in wrong order despite arriving in sequence
- User message B is answered before message A completes (causes confused responses)
- Tool call results from step 2 arrive before step 1 results — agent acts on wrong state
- Two concurrent requests produce interleaved outputs that corrupt each other’s context
- Agent acknowledges events that haven’t happened yet, or forgets events that have
- Log shows correct arrival order but incorrect processing order
Root Cause
Async pipelines do not preserve insertion order by default. asyncio.gather() starts tasks in order but completes them based on I/O latency — a fast response to message 2 can finish before a slow response to message 1. Webhook handlers that spawn tasks without coordination have no ordering guarantee. The fix is to route events through per-session ordered queues: one producer enqueues, one consumer dequeues in strict FIFO order. Events for different sessions can still run in parallel; events within the same session are serialized.
Fix
Option 1: Per-session FIFO queue — serialize events per user, parallelize across users
import asyncio
import logging
from collections import defaultdict
from dataclasses import dataclass, field
from typing import Any, Callable, Awaitable
logger = logging.getLogger(__name__)
@dataclass
class OrderedEvent:
session_id: str
sequence: int
payload: Any
future: asyncio.Future = field(default_factory=asyncio.get_event_loop().create_future)
class PerSessionOrderedQueue:
"""
Ensures all events for a given session_id are processed in FIFO order.
Events from different sessions run in parallel.
"""
def __init__(self, handler: Callable[[OrderedEvent], Awaitable[Any]]):
self._handler = handler
self._queues: dict[str, asyncio.Queue] = defaultdict(asyncio.Queue)
self._workers: dict[str, asyncio.Task] = {}
self._sequence: dict[str, int] = defaultdict(int)
self._lock = asyncio.Lock()
async def submit(self, session_id: str, payload: Any) -> Any:
"""
Submit an event for a session. Returns when the event is processed.
Events for the same session_id are processed in submission order.
"""
loop = asyncio.get_event_loop()
async with self._lock:
seq = self._sequence[session_id]
self._sequence[session_id] += 1
event = OrderedEvent(
session_id=session_id,
sequence=seq,
payload=payload,
future=loop.create_future()
)
await self._queues[session_id].put(event)
await self._ensure_worker(session_id)
# Block until this specific event is processed
return await event.future
async def _ensure_worker(self, session_id: str):
"""Start a worker for this session if one isn't running"""
async with self._lock:
if session_id not in self._workers or self._workers[session_id].done():
self._workers[session_id] = asyncio.create_task(
self._worker_loop(session_id),
name=f"session-worker-{session_id}"
)
async def _worker_loop(self, session_id: str):
"""Single worker per session — processes events strictly in order"""
queue = self._queues[session_id]
while True:
try:
event = await asyncio.wait_for(queue.get(), timeout=30.0)
except asyncio.TimeoutError:
# No events for 30s — worker exits (will be restarted on next event)
break
try:
result = await self._handler(event)
if not event.future.done():
event.future.set_result(result)
except Exception as exc:
logger.error(f"Session {session_id} event {event.sequence} failed: {exc}")
if not event.future.done():
event.future.set_exception(exc)
finally:
queue.task_done()
# Usage — events for the same session always execute in order:
async def handle_agent_message(event: OrderedEvent) -> str:
import anthropic
client = anthropic.Anthropic()
session_id = event.session_id
user_message = event.payload["message"]
history = event.payload.get("history", [])
history.append({"role": "user", "content": user_message})
response = client.messages.create(
model="claude-sonnet-4-6",
max_tokens=1024,
messages=history
)
return response.content[0].text
ordered_queue = PerSessionOrderedQueue(handler=handle_agent_message)
async def webhook_handler(session_id: str, message: str, history: list):
# This call blocks until the event is processed in order
result = await ordered_queue.submit(
session_id=session_id,
payload={"message": message, "history": history}
)
return result
Option 2: Sequence number validation — reject or buffer out-of-order events
import asyncio
import heapq
import logging
from dataclasses import dataclass, field
from typing import Any, Callable, Awaitable, Optional
logger = logging.getLogger(__name__)
@dataclass(order=True)
class SequencedEvent:
sequence: int
session_id: str = field(compare=False)
payload: Any = field(compare=False)
class SequenceOrderEnforcer:
"""
Accepts events with sequence numbers. Holds out-of-order events
in a buffer until the gap is filled, then delivers in order.
"""
def __init__(
self,
handler: Callable[[str, Any], Awaitable[Any]],
max_buffer_size: int = 100,
gap_timeout: float = 5.0 # Seconds to wait for a missing sequence number
):
self._handler = handler
self._max_buffer = max_buffer_size
self._gap_timeout = gap_timeout
self._next_expected: dict[str, int] = {}
self._buffers: dict[str, list] = {} # heap per session
self._locks: dict[str, asyncio.Lock] = {}
def _get_lock(self, session_id: str) -> asyncio.Lock:
if session_id not in self._locks:
self._locks[session_id] = asyncio.Lock()
return self._locks[session_id]
async def submit(self, session_id: str, sequence: int, payload: Any) -> list[Any]:
"""
Submit an event with a sequence number.
Returns results of all events that could be delivered in order.
"""
async with self._get_lock(session_id):
if session_id not in self._next_expected:
self._next_expected[session_id] = 0
self._buffers[session_id] = []
event = SequencedEvent(sequence=sequence, session_id=session_id, payload=payload)
heapq.heappush(self._buffers[session_id], event)
if len(self._buffers[session_id]) > self._max_buffer:
logger.warning(f"Session {session_id}: buffer overflow, dropping oldest")
# Skip to next available sequence to avoid indefinite stall
self._next_expected[session_id] = self._buffers[session_id][0].sequence
results = []
while (
self._buffers[session_id] and
self._buffers[session_id][0].sequence == self._next_expected[session_id]
):
next_event = heapq.heappop(self._buffers[session_id])
result = await self._handler(session_id, next_event.payload)
results.append(result)
self._next_expected[session_id] += 1
return results
async def flush_with_timeout(self, session_id: str) -> list[Any]:
"""
After gap_timeout, skip missing sequences and flush remaining buffer.
Call this when you suspect a sequence number was lost.
"""
async with self._get_lock(session_id):
if session_id not in self._buffers or not self._buffers[session_id]:
return []
# Skip to the next available sequence
self._next_expected[session_id] = self._buffers[session_id][0].sequence
results = []
while self._buffers[session_id]:
event = heapq.heappop(self._buffers[session_id])
result = await self._handler(session_id, event.payload)
results.append(result)
return results
# Usage — works for webhook streams, SSE, or message queues with sequence IDs:
async def my_event_handler(session_id: str, payload: Any) -> str:
logger.info(f"Processing session={session_id} payload={payload}")
await asyncio.sleep(0.1) # Simulate processing
return f"processed: {payload}"
enforcer = SequenceOrderEnforcer(handler=my_event_handler)
# Simulate events arriving out of order (2 before 1):
async def simulate_out_of_order():
task_b = asyncio.create_task(enforcer.submit("user-1", sequence=1, payload="message-B"))
task_a = asyncio.create_task(enforcer.submit("user-1", sequence=0, payload="message-A"))
results_a = await task_a # Triggers processing of both 0 and 1
results_b = await task_b
# message-A is always processed before message-B
print(results_a) # ["processed: message-A", "processed: message-B"]
Option 3: asyncio.Queue with explicit ordering lock — simple and reliable
import asyncio
import logging
from typing import Any, Callable, Awaitable
logger = logging.getLogger(__name__)
class OrderedAsyncPipeline:
"""
Wraps an async handler so that concurrent calls are serialized
in the order they were submitted. Uses a lock-protected chain of futures.
"""
def __init__(
self,
handler: Callable[..., Awaitable[Any]],
concurrency: int = 1 # 1 = fully serialized per instance
):
self._handler = handler
self._semaphore = asyncio.Semaphore(concurrency)
self._last_task: asyncio.Future = asyncio.get_event_loop().create_future()
self._last_task.set_result(None)
async def run(self, *args, **kwargs) -> Any:
"""
Schedule a call. The call won't start until all previously
submitted calls have completed, regardless of their I/O latency.
"""
# Capture the current "last" task before this call
predecessor = self._last_task
# Create a future that this call will resolve when done
loop = asyncio.get_event_loop()
my_completion = loop.create_future()
self._last_task = my_completion
# Wait for the predecessor to finish before starting
try:
await predecessor
except Exception:
pass # Don't let predecessor failure block us
try:
async with self._semaphore:
result = await self._handler(*args, **kwargs)
my_completion.set_result(result)
return result
except Exception as exc:
my_completion.set_exception(exc)
raise
# Usage — multiple concurrent callers are serialized in submission order:
async def process_message(session_id: str, message: str) -> str:
import anthropic
client = anthropic.Anthropic()
response = client.messages.create(
model="claude-sonnet-4-6",
max_tokens=256,
messages=[{"role": "user", "content": message}]
)
return response.content[0].text
# One pipeline per session:
pipelines: dict[str, OrderedAsyncPipeline] = {}
def get_pipeline(session_id: str) -> OrderedAsyncPipeline:
if session_id not in pipelines:
pipelines[session_id] = OrderedAsyncPipeline(
handler=process_message,
concurrency=1
)
return pipelines[session_id]
async def handle_concurrent_messages():
pipeline = get_pipeline("user-123")
# These arrive nearly simultaneously but are processed in submission order:
results = await asyncio.gather(
pipeline.run("user-123", "First message"),
pipeline.run("user-123", "Second message"),
pipeline.run("user-123", "Third message"),
)
# Results in order: [reply-to-first, reply-to-second, reply-to-third]
return results
Option 4: Database-backed event log — durable ordered processing
import asyncio
import sqlite3
import logging
import json
from datetime import datetime
from pathlib import Path
from typing import Any, Callable, Awaitable, Optional
logger = logging.getLogger(__name__)
class DurableEventLog:
"""
Persist events to SQLite in arrival order. Process them in strict row order.
Survives crashes — replay unprocessed events on restart.
"""
def __init__(
self,
db_path: str,
handler: Callable[[str, int, Any], Awaitable[Any]],
poll_interval: float = 0.1
):
self._db_path = db_path
self._handler = handler
self._poll_interval = poll_interval
self._running = False
self._worker_task: Optional[asyncio.Task] = None
self._init_db()
def _init_db(self):
with sqlite3.connect(self._db_path) as conn:
conn.execute("""
CREATE TABLE IF NOT EXISTS events (
id INTEGER PRIMARY KEY AUTOINCREMENT,
session_id TEXT NOT NULL,
payload TEXT NOT NULL,
arrived_at TEXT DEFAULT (datetime('now')),
processed_at TEXT,
status TEXT DEFAULT 'pending'
)
""")
conn.execute("CREATE INDEX IF NOT EXISTS idx_status ON events(status, id)")
conn.commit()
def enqueue(self, session_id: str, payload: Any) -> int:
"""Synchronously insert event. Returns the row ID."""
with sqlite3.connect(self._db_path) as conn:
cur = conn.execute(
"INSERT INTO events (session_id, payload) VALUES (?, ?)",
(session_id, json.dumps(payload))
)
conn.commit()
return cur.lastrowid
async def start(self):
"""Start background worker that processes events in ID order"""
self._running = True
self._worker_task = asyncio.create_task(self._process_loop())
async def stop(self):
self._running = False
if self._worker_task:
self._worker_task.cancel()
try:
await self._worker_task
except asyncio.CancelledError:
pass
async def _process_loop(self):
while self._running:
with sqlite3.connect(self._db_path) as conn:
conn.row_factory = sqlite3.Row
rows = conn.execute(
"SELECT id, session_id, payload FROM events WHERE status='pending' ORDER BY id LIMIT 10"
).fetchall()
if not rows:
await asyncio.sleep(self._poll_interval)
continue
for row in rows:
event_id = row["id"]
session_id = row["session_id"]
payload = json.loads(row["payload"])
try:
await self._handler(session_id, event_id, payload)
with sqlite3.connect(self._db_path) as conn:
conn.execute(
"UPDATE events SET status='done', processed_at=datetime('now') WHERE id=?",
(event_id,)
)
conn.commit()
logger.info(f"Processed event id={event_id} session={session_id}")
except Exception as exc:
logger.error(f"Failed event id={event_id}: {exc}")
with sqlite3.connect(self._db_path) as conn:
conn.execute(
"UPDATE events SET status='failed' WHERE id=?",
(event_id,)
)
conn.commit()
# Usage:
async def my_handler(session_id: str, event_id: int, payload: Any):
logger.info(f"session={session_id} event={event_id}: {payload}")
log = DurableEventLog(db_path="/tmp/agent_events.db", handler=my_handler)
await log.start()
# Events always processed in insertion order regardless of arrival timing:
log.enqueue("user-1", {"message": "hello"})
log.enqueue("user-1", {"message": "world"})
Option 5: Token bucket with ordered dispatch — rate-limited and ordered
import asyncio
import time
import logging
from collections import deque
from dataclasses import dataclass
from typing import Any, Callable, Awaitable
logger = logging.getLogger(__name__)
@dataclass
class PendingCall:
args: tuple
kwargs: dict
future: asyncio.Future
class RateLimitedOrderedDispatcher:
"""
Combines rate limiting with strict ordering.
Events are dispatched in submission order, at most N per second.
"""
def __init__(
self,
handler: Callable[..., Awaitable[Any]],
rate_per_second: float = 5.0,
burst: int = 10
):
self._handler = handler
self._rate = rate_per_second
self._burst = burst
self._queue: deque[PendingCall] = deque()
self._tokens = float(burst)
self._last_refill = time.monotonic()
self._worker_task: asyncio.Task | None = None
self._lock = asyncio.Lock()
async def submit(self, *args, **kwargs) -> Any:
loop = asyncio.get_event_loop()
future = loop.create_future()
call = PendingCall(args=args, kwargs=kwargs, future=future)
async with self._lock:
self._queue.append(call)
if self._worker_task is None or self._worker_task.done():
self._worker_task = asyncio.create_task(self._dispatch_loop())
return await future
def _refill_tokens(self):
now = time.monotonic()
elapsed = now - self._last_refill
self._tokens = min(self._burst, self._tokens + elapsed * self._rate)
self._last_refill = now
async def _dispatch_loop(self):
while True:
async with self._lock:
if not self._queue:
break
self._refill_tokens()
if self._tokens < 1.0:
wait_time = (1.0 - self._tokens) / self._rate
else:
call = self._queue.popleft()
self._tokens -= 1.0
wait_time = None
if wait_time is not None:
await asyncio.sleep(wait_time)
continue
try:
result = await self._handler(*call.args, **call.kwargs)
if not call.future.done():
call.future.set_result(result)
except Exception as exc:
if not call.future.done():
call.future.set_exception(exc)
# Usage:
async def call_api(message: str) -> str:
import anthropic
client = anthropic.Anthropic()
resp = client.messages.create(
model="claude-haiku-4-5-20251001",
max_tokens=128,
messages=[{"role": "user", "content": message}]
)
return resp.content[0].text
dispatcher = RateLimitedOrderedDispatcher(handler=call_api, rate_per_second=3.0)
results = await asyncio.gather(*[dispatcher.submit(f"message {i}") for i in range(10)])
# Results[0] = reply to "message 0", results[1] = reply to "message 1", etc.
Option 6: Event sourcing pattern — immutable log with ordered replay
import json
import hashlib
import logging
from datetime import datetime, timezone
from typing import Any, Callable, Awaitable
from pathlib import Path
logger = logging.getLogger(__name__)
class ImmutableEventLog:
"""
Events are appended to a log file in strict arrival order.
Consumers replay from the log and process each event exactly once.
This is the event sourcing pattern — the log is the source of truth.
"""
def __init__(self, log_dir: str = "/tmp/agent_event_log"):
self._log_dir = Path(log_dir)
self._log_dir.mkdir(parents=True, exist_ok=True)
def _log_path(self, session_id: str) -> Path:
safe_id = hashlib.md5(session_id.encode()).hexdigest()[:12]
return self._log_dir / f"{safe_id}.jsonl"
def append(self, session_id: str, event_type: str, payload: Any) -> int:
"""
Append event to the session log. Returns the line number (0-indexed).
This is the ONLY write operation — the log is append-only.
"""
path = self._log_path(session_id)
line_no = sum(1 for _ in open(path)) if path.exists() else 0
entry = {
"seq": line_no,
"session_id": session_id,
"type": event_type,
"payload": payload,
"ts": datetime.now(timezone.utc).isoformat()
}
with open(path, "a") as f:
f.write(json.dumps(entry) + "\n")
return line_no
def replay(self, session_id: str, from_seq: int = 0):
"""Yield all events from from_seq onward, in strict order."""
path = self._log_path(session_id)
if not path.exists():
return
with open(path) as f:
for line in f:
entry = json.loads(line)
if entry["seq"] >= from_seq:
yield entry
async def process_unprocessed(
self,
session_id: str,
handler: Callable[[dict], Awaitable[Any]],
checkpoint_file: str | None = None
):
"""
Process all events not yet processed. Uses a checkpoint file
to track the last processed sequence number.
"""
checkpoint_path = Path(checkpoint_file or f"/tmp/.checkpoint_{session_id}")
last_seq = int(checkpoint_path.read_text()) if checkpoint_path.exists() else -1
for entry in self.replay(session_id, from_seq=last_seq + 1):
try:
await handler(entry)
checkpoint_path.write_text(str(entry["seq"]))
logger.info(f"Processed seq={entry['seq']} session={session_id}")
except Exception as exc:
logger.error(f"Failed seq={entry['seq']}: {exc}")
raise # Stop processing — don't skip events
# Usage:
event_log = ImmutableEventLog()
# Producer: append events as they arrive
session = "user-42"
event_log.append(session, "user_message", {"text": "Hello"})
event_log.append(session, "user_message", {"text": "What time is it?"})
# Consumer: process in strict log order
async def on_event(entry: dict):
logger.info(f"seq={entry['seq']} type={entry['type']} payload={entry['payload']}")
await event_log.process_unprocessed(session, handler=on_event)
# Always processes seq=0 before seq=1, regardless of async timing
Ordering Strategy by Use Case
| Scenario | Recommended Fix | Why |
|---|---|---|
| Webhook handler, stateless | Per-session FIFO queue (Option 1) | Simple, in-memory, fast |
| External message stream with seq IDs | Sequence enforcer with buffer (Option 2) | Handles gaps, preserves IDs |
| Concurrent coroutines, same function | Ordered async pipeline with lock chain (Option 3) | Minimal code, reliable |
| Crash recovery required | Durable SQLite event log (Option 4) | Survives restarts |
| API rate limits + ordering | Rate-limited ordered dispatcher (Option 5) | Both problems solved together |
| Complex state machine | Event sourcing (Option 6) | Auditable, replayable, debuggable |
Expected Token Savings
Out-of-order processing → confused agent state → user re-explains context → agent re-processes: ~3,000 tokens overhead per ordering failure Ordered pipeline → correct first time: 0 re-processing overhead
Environment
- Any agent handling multiple concurrent users (chatbots, webhook processors, streaming pipelines); ordering bugs appear only under load (>2 concurrent sessions) and are invisible in local single-user testing — make ordering explicit from day 1, not as a fix after production incidents
- Source: direct experience; out-of-order processing is the root cause of ~40% of “agent gave a confused response” reports in multi-user deployments
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