What models of message transmission and delivery exist?

There are several main models of message transmission and delivery in messaging systems:

1. Push (sender-initiated transmission) — the sender actively sends the message to the receiver. Example: WebSocket, where the server sends data to the client without a request.

2. Pull (receiver-initiated transmission) — the receiver requests messages from the sender or intermediary. Example: polling a REST API for new data.

3. Message Queue — messages are placed in a queue, from which the receiver retrieves them as ready. Ensures asynchrony and reliability.

4. Publish/Subscribe — the sender publishes a message to a topic, and subscribers receive messages relevant to their subscription.

Message delivery models include:

• At most once — the message may be delivered 0 or 1 times, possible loss.
• At least once — the message is delivered at least once, duplicates may occur.
• Exactly once — the message is delivered exactly once, without loss or duplicates, the most complex model.

These models are chosen depending on requirements for reliability, performance, and implementation complexity.