Publishing messages to topics

A publisher application creates and sends messages to a topic. Metro offers at-least-once message delivery and best-effort ordering to the existing subscribers, as explained in the Subscriber Overview.

The general flow for a publisher application is:

• Create a message containing your data.
• Send a request to the Metro Server to publish the message to the desired topic.

Message format

A message consists of fields with the message data and metadata. Specify at least one of the following in the message:

• The message data
• An ordering key
• Attributes with additional metadata

If you’re using the REST API, the message data must be base64-encoded.

The Metro service adds the following fields to the message:

• A message ID unique to the topic
• A timestamp for when the Metro service receives the message

Publishing messages

You can publish messages with the Metro API.

After you publish a message, the Metro service returns the message ID to the publisher.

Using attributes

You can embed custom attributes as metadata in Metro messages. Attributes can be text strings or byte strings. The message schema can be represented as follows:

{
  "data": "<string>",
  "attributes": {
    "<key1 string>": "<value1 string>",
    "<key2 string>": "<value2 string>"
  },
  "messageId": "<string>",
  "publishTime": "<string>",
  "orderingKey": "<string>"
}

The PubsubMessage JSON schema is published as part of the REST and RPC documentation.