configuration: "max_items" option: "max_items" is implemented using a text-single field, as it is done in the XEP-0060 example (there is no real formal description). When changing the node configuration, the max_items can't be set to a number lower than the total number of items in the node (the configuration will then be rejected), this is to avoid accidental deletion of items.

create table callbacks (
    service text not null,
    node text not null,
    uri text not null,
    PRIMARY KEY (service, node, uri)
);