Backends

    1. PUB publishes a message;
    2. Backend records this message in DB;
    3. SUB subscribes to a topic;
    4. Backend retrieves the messages of this topic from DB;
    5. Messages are sent to SUB;
    1. PUB publishes a message;
    2. Backend records the message in DB;
    3. SUB1 and SUB2 subscribe to a topic;
    4. Backend retrieves the messages of this topic;
    5. Messages are sent to SUB1 and SUB2;