Firebase Docs Reference Interface: DataMessagePayload admin.messaging.DataMessagePayload interface static Interface representing a data message payload. Data messages let developers send up to 4KB of custom key-value pairs. The keys and values must both be strings. Keys can be any custom string, except for the following reserved strings: "from" Anything starting with "google.". See Defining the message payload for code samples and detailed documentation. Except as otherwise noted, the content of this page is licensed under the Creative Commons Attribution 3.0 License, and code samples are licensed under the Apache 2.0 License. For details, see our Site Policies. Java is a registered trademark of Oracle and/or its affiliates. Last updated February 14, 2018.