Create a subscription :

POST /_pubsub
{
    subscription: [
        {"db": "somedb"},
        {"db": "somedb2",}
    ]
}


update or create a subscription :

PUT /_pubsub
    {
        id: "someid",
        subscription: [
            {"db": "somedb", "since": "000"},
            {"db": "somedb2"}
        ]
    }
    
first line :
    { "ok": true; "id": "someid" }
    
Next lines :

    { "db": ...}


GET /_pubsub/someid
lines of changes

Each latest sequence of change for one pubsub id would be saved in a database so you can kept history when you go back on the feed ia GET.