Subscription filtering in GraphQL - Step-by-Step Execution

Concept Flow - Subscription filtering

Client subscribes with filter

↓

Server receives subscription request

↓

Server applies filter criteria

↓

Events generated on server

↓

Filter checks each event

↓

Send event

↓

Client receives filtered events

The client subscribes with a filter; the server applies it to events and sends only matching events to the client.

Execution Sample

GraphQL

subscription onNewMessage($userId: ID!) {
  messageAdded(filter: {recipientId: $userId}) {
    id
    content
    senderId
  }
}

Client subscribes to new messages only for a specific user ID.

Execution Table

Step	Event Generated	Filter Condition (recipientId == userId)	Pass Filter?	Action	Client Receives
1	{id: 101, content: 'Hi', senderId: 'A', recipientId: 'U1'}	U1 == U1	Yes	Send event	{id: 101, content: 'Hi', senderId: 'A'}
2	{id: 102, content: 'Hello', senderId: 'B', recipientId: 'U2'}	U2 == U1	No	Ignore event	None
3	{id: 103, content: 'Hey', senderId: 'C', recipientId: 'U1'}	U1 == U1	Yes	Send event	{id: 103, content: 'Hey', senderId: 'C'}
4	{id: 104, content: 'Yo', senderId: 'D', recipientId: 'U3'}	U3 == U1	No	Ignore event	None
5	No more events	-	-	-	-

💡 No more events generated, subscription ends or waits for new events.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	Final
event	null	{id:101,...}	{id:102,...}	{id:103,...}	{id:104,...}	null
filterPass	null	true	false	true	false	null
clientReceived	[]	[{id:101,...}]	[{id:101,...}]	[{id:101,...},{id:103,...}]	[{id:101,...},{id:103,...}]	[{id:101,...},{id:103,...}]

Key Moments - 3 Insights

Why does the client not receive events where recipientId does not match userId?

Because the filter condition fails (see execution_table rows 2 and 4), so those events are ignored and not sent to the client.

What happens if no events match the filter?

The client receives no events (empty list), as shown in execution_table rows where filterPass is false and action is 'Ignore event'.

Does the server send all events to the client before filtering?

No, the server applies the filter first and only sends events that pass the filter, as shown in the 'Action' column.

Visual Quiz - 3 Questions

Look at the execution_table, what is the 'filterPass' value at Step 3?

A true

B false

C null

D undefined

💡 Hint

Check the 'Filter Condition' and 'Pass Filter?' columns at Step 3 in the execution_table.

At which step does the client receive the event with id 103?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the 'Client Receives' column for event id 103 in the execution_table.

If the filter condition changed to recipientId == 'U2', which step would send an event to the client?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Refer to the 'Event Generated' and 'Filter Condition' columns to see which event has recipientId 'U2'.

Concept Snapshot

Subscription filtering lets clients receive only events matching criteria.
Client sends filter with subscription.
Server checks each event against filter.
Only matching events are sent to client.
Non-matching events are ignored.
This saves bandwidth and processing.

Full Transcript

Subscription filtering in GraphQL means the client asks to get only certain events, like messages for a specific user. The server gets this request and checks every new event. If the event matches the filter, it sends it to the client. If not, it ignores it. This way, the client only gets relevant updates. For example, if the client wants messages where recipientId equals their userId, only those messages are sent. Events with other recipientIds are skipped. This filtering happens on the server before sending data, saving resources and making updates efficient.