PubsubMessage

A message data and its attributes. The message payload must not be empty; it must contain either a non-empty data field, or at least one attribute.

JSON representation 
{
  "data": string,
  "attributes": {
    string: string,
    ...
  },
  "messageId": string,
  "publishTime": string
}
Fields
data

string (bytes format)

The message payload. For JSON requests, the value of this field must be base64-encoded.

A base64-encoded string.
attributes

map (key: string, value: string)

Optional attributes for this message.

An object containing a list of "key": value pairs. Example: { "name": "wrench", "mass": "1.3kg", "count": "3" }.
messageId

string

ID of this message, assigned by the server when the message is published. Guaranteed to be unique within the topic. This value may be read by a subscriber that receives a PubsubMessage via a Pull call or a push delivery. It must not be populated by the publisher in a Publish call.
publishTime

string (Timestamp format)

The time at which the message was published, populated by the server when it receives the Publish call. It must not be populated by the publisher in a Publish call.

A timestamp in RFC3339 UTC "Zulu" format, with nanosecond resolution and up to nine fractional digits. Examples: "2014-10-02T15:01:23Z" and "2014-10-02T15:01:23.045123456Z".