Real-Time GraphQL Subscriptions: Push Updates to Clients

The Slack Notification Problem

You are building a chat app. Users need to see new messages instantly - not after polling every 2 seconds, not after refreshing the page. Slack, Discord, and every real-time app solve this with server push. GraphQL subscriptions give you this power using the same query language you already know for fetching data.

The Three GraphQL Operations

GraphQL Operation Types:

  query {                           # READ - one-time fetch
    user(id: 42) { name, email }
  }

  mutation {                        # WRITE - change data
    sendMessage(chatId: 'g-42', text: 'Hi') {
      id, sentAt
    }
  }

  subscription {                    # STREAM - real-time push
    messageReceived(chatId: 'g-42') {
      id
      text
      sender { name, avatar }
      sentAt
    }
  }

  Query:        Client asks, server responds once.
  Mutation:     Client sends change, server responds once.
  Subscription: Client subscribes, server pushes MANY times.

How Subscriptions Work Under the Hood

Subscription Lifecycle:

  [Client]                          [GraphQL Server]
     |                                    |
     |  1. WS handshake (HTTP upgrade)    |
     |----------------------------------->|
     |                                    |
     |  2. Subscribe: messageReceived     |
     |----------------------------------->|
     |                         [registers subscription]
     |                         [maps to event source]
     |                                    |
     |        ... time passes ...         |
     |                                    |
     |                  [User B sends a message]
     |                         [event fires]
     |                         [server evaluates query]
     |                                    |
     |  3. Push: { sender: 'Bob',         |
     |            text: 'Hey!' }          |
     |<-----------------------------------|
     |                                    |
     |        ... more messages ...        |
     |                                    |
     |  4. Unsubscribe or disconnect      |
     |----------------------------------->|
     |                  [cleans up subscription]

Real Code: Setting Up Subscriptions

# Server (Python + Strawberry GraphQL):
import strawberry
from strawberry.types import Info
import asyncio
from typing import AsyncGenerator

@strawberry.type
class Subscription:
    @strawberry.subscription
    async def message_received(
        self, chat_id: str
    ) -> AsyncGenerator[Message, None]:
        # Subscribe to Redis pub/sub channel
        async for event in redis.subscribe(f'chat:{chat_id}'):
            yield Message(
                id=event['id'],
                text=event['text'],
                sender=event['sender'],
                sent_at=event['sent_at']
            )

# Client (JavaScript):
const subscription = graphql`
  subscription OnMessage($chatId: String!) {
    messageReceived(chatId: $chatId) {
      id, text, sender { name, avatar }, sentAt
    }
  }
`;
// Client gets EXACTLY the fields it asked for - no more, no less

Scaling Subscriptions Across Servers

Single server: simple
  [Client] <--WS--> [Server with all subscriptions in memory]

Multi-server: needs pub/sub backbone

  [Client A]       [Client B]       [Client C]
      |                |                |
  [Server 1]       [Server 2]       [Server 3]
      |                |                |
      +-------[Redis Pub/Sub]----------+

  Problem: User A is on Server 1, User B on Server 2.
  User B sends message -> mutation hits Server 2.
  Server 2 publishes event to Redis channel.
  Server 1 receives event from Redis.
  Server 1 pushes to Client A's WebSocket.

  Memory per subscription:  ~1-5 KB
  100K active subscribers:  ~100-500 MB
  Redis pub/sub latency:    ~1-5 ms added

Subscriptions vs Polling vs SSE

Approach        Latency    Efficiency   Complexity   Best For
--------------  ---------  ----------   ----------   ----------------
Polling         1-10s      Low (waste)  Simple       Legacy clients
Long Polling    ~instant   Medium       Medium       Simple real-time
SSE             ~instant   High         Medium       One-way streams
WebSocket       ~instant   High         High         Bidirectional
GQL Sub (WS)    ~instant   High         Medium       Type-safe push

Interview Tip

When designing a real-time feature with a GraphQL API, say: 'I would use GraphQL subscriptions over WebSocket for push updates. The client subscribes using the same query language as reads, requesting exactly the fields it needs. For multi-server scaling, I would add Redis Pub/Sub as a backbone so events published on any server reach the correct subscriber. I would filter events server-side to avoid waking every subscription on every event - only subscribers interested in the specific chat room receive the push.'

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Key Takeaway

GraphQL subscriptions provide real-time push updates using the same query language as the rest of your API. They run over WebSockets and need pub/sub infrastructure for multi-server deployment. Use them for chat, live feeds, and dashboards where polling is too slow.