Implementing Pub/Sub Pattern

Publishers emit messages to topics; subscribers receive matching messages without direct coupling. WebSocket commonly carries the client side over a broker (Redis, NATS, Kafka).

Role	Action
Publisher	Send to topic
Broker	Fan-out
Subscriber	Receive matching
Topic	Routing key (e.g. `orders.*`)

2. Implementing Topic Subscription

ws.send(JSON.stringify({ "type": "sub", "topic": "prices.BTC" }));
ws.send(JSON.stringify({ "type": "unsub", "topic": "prices.BTC" }));

Message	Server Action
`sub`	Add to topic subscriber list
`unsub`	Remove from list
`list`	Return current subs

3. Managing Subscriber Lists

Structure	Use
`Map<topic, Set<WebSocket>>`	Direct fan-out
`Map<WebSocket, Set<topic>>`	Cleanup on disconnect
Trie	Wildcard matching
Redis SET / Pub-Sub	Multi-node

4. Publishing Messages to Topics

Example: Publish helper

function publish(topic, payload) {
  const subs = topics.get(topic);
  if (!subs) return;
  const msg = JSON.stringify({ "type":"msg", "topic":topic, "payload":payload });
  for (const c of subs) if (c.readyState === 1) c.send(msg);
}

5. Implementing Topic Wildcards

Convention	Match
`*` (single segment)	`orders.*` → `orders.new`
`#` (multi segment)	`orders.#` → `orders.eu.new`
`:param`	Named capture (custom)

6. Handling Private Channels

Type	Access
Public	Anyone may subscribe
Private (`private-` prefix)	Auth required
Presence	Auth + member list
Encrypted	End-to-end key per channel

7. Implementing Message Filtering

Example: Predicate filter

// per-subscriber filter
function publishFiltered(topic, payload) {
  for (const c of topics.get(topic) ?? []) {
    if (c.filter?.(payload) ?? true) c.send(JSON.stringify(payload));
  }
}

Filter	Use
Field predicate	`{ "sym":"BTC", "px>":50000 }`
Geo filter	Bounding box / radius
Permission filter	Hide fields user can't read

8. Managing Subscription State

State	Detail
Persist on reconnect	Resubscribe from client
Server snapshot	Send initial state on subscribe
Last seq	Resume from gap

9. Implementing Presence

Example: Presence events

{ "type": "presence.join", "topic": "room-42", "user": { "id": "u1", "name": "Ada" } }
{ "type": "presence.leave", "topic": "room-42", "user": { "id": "u1" } }
{ "type": "presence.list",  "topic": "room-42", "users": [ /* ... */ ] }

Event	Trigger
join	Subscriber added
leave	Subscriber removed/disconnected
update	Metadata change
sync	Periodic full state

10. Handling Subscription Limits

Limit	Detail
Max topics / connection	e.g. 100
Max subscribers / topic	e.g. 10k (fan-out cost)
Rate of sub/unsub	Token bucket
Wildcard depth	Cap to prevent explosions

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Implementing Pub/Sub Pattern

1. Understanding Publish-Subscribe Pattern

2. Implementing Topic Subscription

Example: Subscribe / unsubscribe

3. Managing Subscriber Lists

4. Publishing Messages to Topics

Example: Publish helper

5. Implementing Topic Wildcards

6. Handling Private Channels

7. Implementing Message Filtering

Example: Predicate filter

8. Managing Subscription State

9. Implementing Presence

Example: Presence events

10. Handling Subscription Limits

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