function _M.check_schema(conf)
    local ok, err = core.schema.check(schema, conf)
    if not ok then
        return false, err
    end
    if conf.rules then
        for _, rule in ipairs(conf.rules) do
            if rule.match then
                for _, m in ipairs(rule.match) do
                    local ok, err = expr.new(m.vars)
                    if not ok then
                        return false, "failed to validate the 'vars' expression: " .. err
                    end
                end
            end
            if rule.weighted_upstreams then
                for _, wupstream in ipairs(rule.weighted_upstreams) do
                    local ups_id = wupstream.upstream_id
                    if ups_id then
                        local ups = upstream.get_by_id(ups_id)
                        core.log.error("upstream:", ups)
                        if not ups then
                            return false, "failed to fetch upstream info by "
                                        .. "upstream id: " .. ups_id
                        end
                    end
                end
            end
        end
    end
    return true
end