Solace Message Format Topics

Solace Message Format (SMF) is the underlying messaging protocol for PubSub+.

SMF topics have the following properties:

• Consist of NULL-terminated UTF-8 strings composed of one or more levels in the format: a/b/c.
• The / character is used as a separator between levels - which are also referred to as elements - of the string.
• A maximum of 128 topic levels are supported.
• The maximum length of a string, excluding the NULL terminator, is 250 bytes.
• Characters in topic strings are case-sensitive. For example, the topic subscription animals/domestic/cats is not equal to Animals/Domestic/Cats.
• All UTF-8 characters are supported. However, the following characters have special meaning:
• For topic subscriptions:
• / is a level separator.
• <NULL> is an invalid character.
• * is a single-level wildcard. For more information, see Wildcard Characters in Topic Subscriptions.
• > can be used as a wildcard, and is a multi-level wildcard when used at the last level of a topic subscription.
• A leading !, as for example in !a/b/c, in queue topic subscriptions indicates a subscription exception. For more information, see System-Level Subscription Exception Configuration.
• For publishing topics:
• / is a level separator.
• <NULL> is an invalid character.
• * is treated as a literal character. Do not use this character when creating topics.
• > is treated as a literal character. Do not use this character when creating topics.
• A leading ! is treated as a literal character. Do not use this character when creating topics.

MQTT Topic Compatibility

To allow compatibility with MQTT topics, which permit empty topic levels (for example, /a/b, a//b, or a/b/), PubSub+ also allows empty topic levels in SMF topics. However, using empty topic levels in SMF topics has some limitations:

• Client applications using the PubSub+ Messaging APIs for C, .NET, Go, Java RTO, and Python:
  • can publish to topics with empty levels
  • cannot use topic subscriptions with empty levels
• Client applications using the PubSub+ Messaging APIs for Java, JCSMP, JMS, JavaScript, and Node.js:
  • cannot publish to topics with empty levels
  • cannot use topic subscriptions with empty levels
• In ACL profiles, SMF topic exceptions cannot have empty topic levels for publishing or subscribing.

The event broker automatically creates a topic subscription beginning with the prefix #P2P/ for each client, which allows messages to be sent directly to that client (for example, in request/reply scenarios). However, the leading "#" in #P2P is not valid MQTT syntax. Therefore, to make use of these special Solace subscriptions, MQTT clients can use the string _P2P instead of #P2P. The event broker considers these two strings to be equivalent.

For more information, see Using MQTT.