键表达式会在如下结构的消息上下文中进行求值：

JSON 示例

{
  "message": {
    "qos": 0,
    "topic": "some/topic",
    "payload": "some-payload",
    "headers": {
      "client_attrs": {},
      "proto_ver": 5,
      "properties": {
        "User-Property": {
          "user-prop": "some-value"
        }
      },
      "peerhost": "127.0.0.1",
      "username": "undefined",
      "protocol": "mqtt",
      "peername": "127.0.0.1:49352"
    },
    "from": "clientid",
    "timestamp": 1759238376252,
    "id": "..non utf8 bytes...",
    "flags": {
      "retain": false,
      "dup": false
    },
    "extra": {}
  }
}

Erlang Term 示例

#{message =>
      #{extra => #{},
        flags => #{dup => false, retain => false},
        id => <<0,6,64,4,154,125,229,77,244,69,0,0,28,21,0,2>>,
        timestamp => 1759238376252, from => <<"clientid">>,
        headers =>
            #{peername => <<"127.0.0.1:49352">>, protocol => mqtt,
              username => undefined, peerhost => <<"127.0.0.1">>,
              properties =>
                  #{'User-Property' => #{<<"user-prop">> => <<"some-value">>}},
              proto_ver => 5, client_attrs => #{}
            },
        payload => <<"some-payload">>, topic => <<"some/topic">>,
        qos => 0
      }
    }