Subscription syntax in GraphQL - Step-by-Step Execution

Concept Flow - Subscription syntax

Client sends subscription request

↓

Server registers subscription

↓

Event occurs on server

↓

Server sends data update to client

↓

Client receives real-time data

↓

Repeat on new events

The client subscribes to a data stream, the server listens for events, and sends updates to the client in real-time.

Execution Sample

GraphQL

subscription OnMessageAdded {
  messageAdded {
    id
    content
    author
  }
}

This subscription listens for new messages and receives their id, content, and author when added.

Execution Table

Step	Action	Server State	Client State	Data Sent
1	Client sends subscription request OnMessageAdded	Subscription registered for messageAdded	Waiting for data	None
2	New message event occurs on server	Detects new message event	Waiting for data	None
3	Server sends messageAdded data to client	Subscription active	Receives message with id, content, author	{"id": 1, "content": "Hello", "author": "Alice"}
4	Client processes received data	Subscription active	Data displayed to user	Displayed message data
5	Another message event occurs	Detects new message event	Waiting for data	None
6	Server sends new messageAdded data	Subscription active	Receives new message data	{"id": 2, "content": "Hi!", "author": "Bob"}
7	Client processes new data	Subscription active	Data updated on screen	Displayed new message data
8	Client unsubscribes or connection closes	Subscription removed	No longer receiving data	None

💡 Subscription ends when client unsubscribes or connection closes

Variable Tracker

Variable	Start	After Step 3	After Step 6	Final
subscriptionStatus	Not registered	Active	Active	Removed
receivedData	None	{"id":1, "content":"Hello", "author":"Alice"}	{"id":2, "content":"Hi!", "author":"Bob"}	None
clientDisplay	Empty	Shows first message	Shows updated messages	Cleared or static

Key Moments - 3 Insights

Why does the client keep receiving data after the initial subscription request?

Because the subscription stays active on the server, sending data whenever the event occurs, as shown in execution_table steps 3 and 6.

What happens if the client closes the connection?

The server removes the subscription and stops sending data, as shown in execution_table step 8.

Does the client need to request data repeatedly?

No, the server pushes updates automatically after the subscription is registered, shown by the continuous data sent in steps 3 and 6.

Visual Quiz - 3 Questions

Look at the execution_table, what is the subscriptionStatus after step 3?

A Not registered

B Removed

C Active

D Pending

💡 Hint

Check the variable_tracker row for subscriptionStatus at After Step 3

At which step does the client first receive data?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at execution_table Data Sent column to see when data is first sent

If the client unsubscribes earlier, which step would be skipped?

A Step 3

B Step 6

C Step 8

D Step 1

💡 Hint

Unsubscribing stops further data, so no new data after first send (step 6)

Concept Snapshot

GraphQL subscriptions let clients listen for real-time data.
Syntax starts with 'subscription' keyword.
Server pushes updates when events happen.
Client receives data automatically until unsubscribed.
Useful for live updates like chat messages or notifications.

Full Transcript

GraphQL subscriptions allow a client to open a real-time connection to the server. The client sends a subscription request specifying the event to listen for. The server registers this subscription and waits for relevant events. When an event occurs, the server sends the updated data to the client immediately. The client receives and processes this data, updating the display or state. This cycle repeats for each new event until the client unsubscribes or closes the connection. This mechanism enables live data updates without repeated requests.

Practice

1. What is the main purpose of a subscription in GraphQL?

easy

A. To delete data from the database

B. To receive real-time updates automatically when data changes

C. To fetch data only once from the server

D. To update data on the server

Solution

1. Step 1: Understand the role of subscription

   Subscriptions are designed to keep the client updated automatically when data changes, without asking repeatedly.

2. Step 2: Compare with other operations

   Queries fetch data once, mutations change data, but subscriptions listen for live updates.

3. Final Answer:

   To receive real-time updates automatically when data changes -> Option B

4. Quick Check:

   Subscription = live updates [OK]

Hint: Subscriptions push updates; queries pull once [OK]

Common Mistakes:

• Confusing subscription with query or mutation
• Thinking subscription fetches data only once
• Assuming subscription modifies data

2. Which of the following is the correct syntax to start a GraphQL subscription named newMessage requesting id and content fields?

easy

A. query { newMessage { id content } }

B. mutation { newMessage { id content } }

C. subscription { newMessage { id content } }

D. subscribe { newMessage { id content } }

Solution

1. Step 1: Identify the correct keyword for subscription

   The keyword to start a subscription is subscription, not query, mutation, or subscribe.

2. Step 2: Check the structure

   The subscription block should contain the event name and requested fields inside curly braces.

3. Final Answer:

   subscription { newMessage { id content } } -> Option C

4. Quick Check:

   Keyword subscription + fields = subscription { newMessage { id content } } [OK]

Hint: Use 'subscription' keyword, not 'query' or 'mutation' [OK]

Common Mistakes:

• Using 'query' or 'mutation' instead of 'subscription'
• Using 'subscribe' keyword which is invalid
• Missing curly braces around fields

3. Given the subscription below, what fields will the client receive when a new comment is added?

subscription { newComment { id text author { name } } }

medium

A. id, text, and author name of the new comment

B. Only id and text of the new comment

C. Only author name of the new comment

D. All comments in the database

Solution

1. Step 1: Analyze the subscription fields

   The subscription requests id, text, and nested author { name } fields for the newComment event.

2. Step 2: Understand what is sent on event

   When a new comment is added, the client receives exactly these fields for that comment, not all comments.

3. Final Answer:

   id, text, and author name of the new comment -> Option A

4. Quick Check:

   Requested fields = id, text, author name [OK]

Hint: Nested fields inside subscription are included in output [OK]

Common Mistakes:

• Assuming all comments are sent, not just new ones
• Ignoring nested author name field
• Thinking only scalar fields are sent

4. Identify the error in this subscription syntax:

subscription newMessage { id content }

medium

A. Missing curly braces around the subscription fields

B. Subscription name cannot be 'newMessage'

C. Subscription keyword should be 'query'

D. Fields 'id' and 'content' should be inside the event name braces

Solution

1. Step 1: Check subscription structure

   The subscription must specify the event name followed by its fields inside braces, e.g., subscription { newMessage { id content } }.

2. Step 2: Identify the error in given syntax

   The given syntax places fields directly after subscription name without nesting inside event braces.

3. Final Answer:

   Fields 'id' and 'content' should be inside the event name braces -> Option D

4. Quick Check:

   Fields must be nested inside event braces [OK]

Hint: Fields must be inside event braces, not directly after subscription name [OK]

Common Mistakes:

• Placing fields outside event braces
• Using 'query' instead of 'subscription'
• Omitting braces entirely

5. You want to subscribe to live updates of stock prices but only for stocks with price above 100. Which subscription syntax correctly applies this filter?

hard

A. subscription { stockPrice(price_gt: 100) { symbol price } }

B. subscription { stockPrice { symbol price where price > 100 } }

C. subscription { stockPrice { symbol price } filter: { price: { gt: 100 } } }

D. subscription { stockPrice { symbol price } if price > 100 }

Solution

1. Step 1: Understand argument filtering in subscriptions

   GraphQL subscriptions accept arguments like queries, so filters are passed as arguments inside parentheses after the event name.

2. Step 2: Identify correct filter syntax

   subscription { stockPrice(price_gt: 100) { symbol price } } uses price_gt: 100 as an argument, which is the standard way to filter for price greater than 100.

3. Final Answer:

   subscription { stockPrice(price_gt: 100) { symbol price } } -> Option A

4. Quick Check:

   Filters go as arguments inside parentheses [OK]

Hint: Use arguments in parentheses for filtering subscriptions [OK]

Common Mistakes:

• Trying to filter inside field braces
• Using invalid keywords like 'where' or 'if'
• Placing filters outside subscription block