# MQTT message payload
<a name="topicdata"></a>

The message payload that is sent in your MQTT messages isn't specified by Amazon IoT, unless it's for one of the [Reserved topics](reserved-topics.md). To accommodate your application's needs, we recommend you define the message payload for your topics within the constraints of the [Amazon IoT Core Service Quotas for Protocols](https://docs.aws.amazon.com/general/latest/gr/iot-core.html#iot-protocol-limits). 

Using a JSON format for your message payload enables the Amazon IoT rules engine to parse your messages and apply SQL queries to it. If your application doesn't require the rules engine to apply SQL queries to your message payloads, you can use any data format that your application requires. For information about limitations and reserved characters in a JSON document used in SQL queries, see [JSON extensions](iot-sql-json.md). 

For more information about designing your MQTT topics and their corresponding message payloads, see [Designing MQTT Topics for Amazon IoT Core](https://docs.amazonaws.cn/whitepapers/latest/designing-mqtt-topics-aws-iot-core/designing-mqtt-topics-aws-iot-core.html).

If a message size limit exceeds the service quotas, it will result in a `CLIENT_ERROR` with reason `PAYLOAD_LIMIT_EXCEEDED` and "Message payload exceeds size limit for message type." For more information about message size limit, see [Amazon IoT Core message broker limits and quotas](https://docs.amazonaws.cn//general/latest/gr/iot-core.html#message-broker-limits.html).