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Synchronous Generator Subscriptions

Ariadne supports both asynchronous and synchronous generators as subscription sources. Synchronous generators are automatically executed in worker threads to avoid blocking the event loop, making it easy to integrate blocking I/O operations (like database queries, file operations, or third-party APIs) into GraphQL subscriptions.

Table of Contents​

Quick Start​

Basic Synchronous Generator​

from ariadne import SubscriptionType, make_executable_schema

subscription = SubscriptionType()

@subscription.source("messages")
def message_source(*_, channel: str = "default"):
    # This is a synchronous generator - no async/await needed!
    for message in get_messages_from_database(channel):  # Blocking DB call
        yield {"text": message.text, "author": message.author}

schema = make_executable_schema(
    """
    type Query {
        _: Boolean
    }
    type Subscription {
        messages(channel: String): Message!
    }
    type Message {
        text: String!
        author: String!
    }
    """,
    subscription,
)

Comparison: Async vs Sync​

Async Generator (existing approach):

@subscription.source("messages")
async def message_source(*_, channel: str = "default"):
    async for message in async_db_client.stream_messages(channel):
        yield {"text": message.text, "author": message.author}

Sync Generator (new approach):

@subscription.source("messages")
def message_source(*_, channel: str = "default"):
    for message in sync_db_client.get_messages(channel):  # Blocking call
        yield {"text": message.text, "author": message.author}

Both work identically from the client's perspective!

How It Works​

Automatic Detection​

When you register a subscription source, Ariadne automatically detects whether it's synchronous or asynchronous:

  • Async generators: Functions defined with async def that use yield
  • Synchronous generators: Functions defined with def that use yield

Thread Offloading​

Synchronous generators are automatically wrapped in an async generator that:

  1. Creates the sync generator when the subscription starts
  2. Executes next(gen) calls in a worker thread using anyio.to_thread.run_sync() (or asyncio.to_thread() as fallback)
  3. Yields values from the sync generator to the async stream
  4. Handles cleanup by calling gen.close() when the subscription ends or client disconnects

Flow Diagram​

Client Request
    ↓
GraphQL Subscription Execution
    ↓
Is source sync or async?
    ├─→ Async: Execute directly
    └─→ Sync: Wrap in async generator
              ↓
         Worker Thread Pool
              ↓
         Execute next(sync_gen)
              ↓
         Yield value to async stream
              ↓
         Client receives update

Use Cases​

1. Legacy Database Integrations​

When working with synchronous database libraries (like psycopg2, pymongo, or Django ORM):

from django.db import models

@subscription.source("posts")
def post_updates(*_, category: str = None):
    # Django ORM queries are synchronous
    queryset = models.Post.objects.filter(category=category)
    
    for post in queryset.iterator():
        yield {
            "id": post.id,
            "title": post.title,
            "content": post.content,
        }

2. File System Monitoring​

import os
import time

@subscription.source("fileChanges")
def watch_file(*_, filepath: str):
    last_modified = 0
    
    while True:
        try:
            current_modified = os.path.getmtime(filepath)
            if current_modified > last_modified:
                last_modified = current_modified
                with open(filepath, 'r') as f:
                    yield {"content": f.read(), "modified": current_modified}
            time.sleep(1)  # Blocking sleep is OK!
        except FileNotFoundError:
            break

3. Third-Party API Polling​

import requests

@subscription.source("apiUpdates")
def poll_api(*_, endpoint: str):
    while True:
        response = requests.get(endpoint)  # Blocking HTTP call
        yield {"data": response.json(), "status": response.status_code}
        time.sleep(5)  # Poll every 5 seconds

4. Message Queue Consumers​

import pika  # RabbitMQ synchronous client

@subscription.source("queueMessages")
def consume_queue(*_, queue_name: str):
    connection = pika.BlockingConnection(pika.ConnectionParameters('localhost'))
    channel = connection.channel()
    
    try:
        for method, properties, body in channel.consume(queue_name):
            if body:
                yield {"message": body.decode(), "routing_key": method.routing_key}
                channel.basic_ack(method.delivery_tag)
    finally:
        connection.close()  # Cleanup happens automatically!

Examples​

Example 1: Database Query Stream​

from ariadne import SubscriptionType, make_executable_schema
import sqlite3

subscription = SubscriptionType()

@subscription.source("users")
def user_stream(*_, active: bool = True):
    conn = sqlite3.connect('users.db')
    cursor = conn.cursor()
    
    try:
        query = "SELECT id, name, email FROM users WHERE active = ?"
        cursor.execute(query, (active,))
        
        for row in cursor:
            yield {
                "id": row[0],
                "name": row[1],
                "email": row[2],
            }
    finally:
        conn.close()  # Cleanup in finally block

@subscription.field("users")
def resolve_user(message, *_):
    return message  # Pass through the yielded data

schema = make_executable_schema(
    """
    type Query {
        _: Boolean
    }
    type Subscription {
        users(active: Boolean): User!
    }
    type User {
        id: ID!
        name: String!
        email: String!
    }
    """,
    subscription,
)

Example 2: Time-Based Events​

import time
from datetime import datetime

@subscription.source("timeUpdates")
def time_stream(*_, interval: int = 1):
    """Emit current time at specified intervals."""
    while True:
        yield {
            "timestamp": datetime.now().isoformat(),
            "unix_time": int(time.time()),
        }
        time.sleep(interval)  # Blocking sleep

@subscription.field("timeUpdates")
def resolve_time(message, *_):
    return message

Example 3: Mixed Sync/Async Sources​

You can mix synchronous and asynchronous generators in the same subscription type:

subscription = SubscriptionType()

# Sync source for blocking operations
@subscription.source("syncData")
def sync_source(*_):
    # Blocking I/O
    data = requests.get("https://api.example.com/data").json()
    yield data

# Async source for async operations
@subscription.source("asyncData")
async def async_source(*_):
    # Non-blocking I/O
    async with aiohttp.ClientSession() as session:
        async with session.get("https://api.example.com/data") as resp:
            data = await resp.json()
            yield data

Best Practices​

1. Resource Cleanup​

Always use try...finally blocks for cleanup in synchronous generators:

@subscription.source("data")
def data_source(*_):
    connection = create_connection()
    try:
        for item in connection.stream():
            yield item
    finally:
        connection.close()  # Always cleanup

Note: Ariadne automatically calls gen.close() when the subscription ends, which triggers the finally block.

2. Error Handling​

Handle exceptions appropriately:

@subscription.source("data")
def data_source(*_):
    try:
        for item in get_data():
            yield item
    except ConnectionError as e:
        # Log error, yield error message, or re-raise
        yield {"error": str(e)}
        raise  # Re-raise to propagate to GraphQL error handling

3. Generator Exhaustion​

Synchronous generators can exhaust naturally:

@subscription.source("limited")
def limited_source(*_, count: int = 10):
    for i in range(count):
        yield {"value": i}
    # Generator ends naturally - no need to raise StopIteration

4. Performance Considerations​

  • Use sync generators for: Legacy libraries, blocking I/O, CPU-bound operations
  • Use async generators for: Modern async libraries, high-concurrency scenarios
  • Thread pool: Sync generators share a thread pool, so avoid creating too many concurrent subscriptions with blocking operations

5. Scalability Considerations​

Important: Synchronous generators are implemented using thread-based execution (anyio.to_thread.run_sync or asyncio.to_thread). This approach has scalability implications that should be understood:

Thread Pool Limitations​

  • Limited concurrent subscriptions: The number of concurrent synchronous subscriptions is bounded by the thread pool size (typically 32-40 threads by default, depending on Python version and runtime)
  • Thread pool exhaustion: If you exceed the thread pool capacity, new subscription requests will wait for available threads, potentially causing delays or timeouts

Memory Overhead​

  • Higher memory usage: Each synchronous subscription consumes:
    • Thread stack space (~8MB per thread on most systems)
    • Generator state and context
    • Any resources held by the synchronous generator (database connections, file handles, etc.)
  • Memory scaling: Memory usage scales linearly with the number of concurrent sync subscriptions, unlike async generators which share the event loop

Performance Overhead​

  • Thread context switching: Each next() call requires:
    • Switching from async context to thread context
    • Executing in a worker thread
    • Switching back to async context
  • Overhead per yield: This context switching adds latency compared to async generators, which execute directly in the event loop
  • Blocking operations: While blocking operations don't block the event loop, they still consume thread resources and can impact overall throughput

Recommendations​

  • For high-concurrency scenarios: Prefer async generators when possible
  • For legacy integrations: Sync generators are acceptable for occasional or low-volume subscriptions
  • Monitor thread pool usage: Watch for thread pool exhaustion warnings in production
  • Consider connection pooling: If using sync generators with databases, use connection pooling to limit resource consumption
  • Batch operations: When possible, batch operations in sync generators to reduce the number of thread switches

When to Use Each Approach​

Use Async Generators when:

  • You need high concurrency (hundreds or thousands of concurrent subscriptions)
  • You have async libraries available (aiohttp, asyncpg, etc.)
  • Low latency is critical
  • You're building new features from scratch

Use Sync Generators when:

  • You're integrating with legacy/synchronous libraries
  • You have a small number of concurrent subscriptions (< 50)
  • Blocking I/O is unavoidable
  • You're migrating existing sync code to GraphQL subscriptions
  • The convenience of sync code outweighs scalability concerns

6. Testing​

Test synchronous generators like any other subscription:

import pytest
from ariadne.graphql import subscribe

@pytest.mark.asyncio
async def test_sync_subscription(schema):
    success, result = await subscribe(
        schema, {"query": "subscription { messages { text } }"}
    )
    assert success
    
    # Consume the async generator
    item = await result.__anext__()
    assert item.data["messages"]["text"] == "Hello"

Technical Details​

Thread Safety​

  • Each synchronous generator runs in its own worker thread
  • The next() calls are serialized per generator (one at a time)
  • Multiple subscriptions can run concurrently in different threads

Cleanup Mechanism​

When a subscription ends (client disconnects, generator exhausts, or error occurs):

  1. The async wrapper's finally block executes
  2. gen.close() is called on the synchronous generator in a worker thread
  3. This triggers the generator's finally block (if present)
  4. Resources are cleaned up properly

Exception Propagation​

Exceptions raised in synchronous generators:

  1. Are caught by the thread executor
  2. Propagated back to the async wrapper
  3. Raised in the async generator
  4. Handled by GraphQL's error reporting mechanism

Implementation Details​

The wrapping logic:

# Simplified version of what happens internally
async def async_wrapper(*args, **kwargs):
    sync_gen = sync_generator_function(*args, **kwargs)
    
    try:
        while True:
            try:
                # Run next() in worker thread
                value = await to_thread.run_sync(next, sync_gen)
                yield value
            except StopIteration:
                break
    finally:
        # Cleanup: close the sync generator
        if hasattr(sync_gen, "close"):
            await to_thread.run_sync(sync_gen.close)

Compatibility​

  • Python: 3.10+ (uses asyncio.to_thread or anyio.to_thread.run_sync)
  • Frameworks: Works with Starlette, FastAPI, and any ASGI-compatible framework
  • Backward Compatible: Existing async generators continue to work unchanged

Migration Guide​

From Async to Sync (when needed)​

If you have blocking I/O and want to simplify:

Before:

@subscription.source("data")
async def data_source(*_):
    # Had to wrap sync calls in run_in_executor
    loop = asyncio.get_event_loop()
    data = await loop.run_in_executor(None, blocking_function)
    yield data

After:

@subscription.source("data")
def data_source(*_):
    # Direct blocking call - Ariadne handles threading
    data = blocking_function()
    yield data

When to Use Each​

Use Async Generators when:

  • You have async libraries available (aiohttp, asyncpg, etc.)
  • You need high concurrency
  • You're already in an async context

Use Sync Generators when:

  • You have legacy/synchronous libraries
  • Blocking I/O is unavoidable
  • You want simpler code without async/await complexity
  • You're migrating existing sync code to GraphQL subscriptions

Troubleshooting​

Generator Not Closing​

If your generator's cleanup code isn't running:

  1. Ensure you're using try...finally blocks
  2. Check that gen.close() is being called (Ariadne does this automatically)
  3. Verify the subscription is properly ending (not hanging)

Thread Pool Exhaustion​

If you see thread pool errors:

  • Reduce the number of concurrent sync subscriptions
  • Consider converting some to async generators
  • Increase thread pool size (framework-dependent)

Performance Issues​

If sync subscriptions are slow:

  • Verify blocking operations are necessary
  • Consider caching or batching
  • Profile to identify bottlenecks
  • Consider async alternatives if available

Summary​

Synchronous generator subscriptions provide a simple way to integrate blocking I/O operations into GraphQL subscriptions without requiring async/await complexity. Ariadne automatically handles thread offloading, cleanup, and error propagation, making it easy to work with legacy libraries and blocking operations while maintaining the benefits of async GraphQL subscriptions.