waku.guide/diagrams/_protocol-interaction.md

```mermaid
sequenceDiagram
	participant A as A relay
	participant B as B relay(pubtopic1)
	participant C as C relay(pubtopic1)
	participant D as D relay(pubtopic1), store(pubtopic1), filter
	participant E as E relay, store
	participant F as F filter

	A ->> A: msg1=WakuMessage(contentTopic1, data) (1)
	F ->> D: FilterRequest(pubtopic1, contentTopic1) (2)
	D ->> D: Subscribe F to filter (2)
	A ->> B: Publish msg1 on pubtopic1 (3)
	B ->> D: relay msg1 on pubtopic1 (3)
	D ->> D: store: saves msg1 (4)
	D ->> C: relay msg1 on pubtopic1 (4)
	D ->> F: MessagePush(msg1) (5)
	E ->> E: E comes online (6)
	E ->> D: HistoryQuery(pubtopic1, contentTopic1) (6)
	D ->> E: HistoryResponse(msg1, ...) (6)
```

The Pub/Sub topic `pubtopic1` serves as a means of routing messages (the network employs a default Pub/Sub topic) and indicates that it is subscribed to messages on that topic for a relay. Node D serves as a `Store` and is responsible for persisting messages.

1. Node A creates a WakuMessage `msg1` with [Content Topic](/overview/concepts/content-topics) `contentTopic1`.
2. Node F requests to get messages filtered by Pub/Sub topic `pubtopic1` and Content Topic `contentTopic1`. Node D subscribes F to this filter and will forward messages that match that filter in the future.
3. Node A publishes `msg1` on `pubtopic1`. The message is sent from Node A to Node B and then forwarded to Node D.
4. Node D, upon receiving `msg1` both stores the message for future retrieval by other nodes and forwards it to Node C.
5. Node D also pushes `msg1` to Node F, informing it about the arrival of a new message.
6. At a later time, Node E comes online and requests messages matching `pubtopic1` and `contentTopic1` from Node D. Node D responds with `msg1` and potentially other messages that match the query.