SMF Topics

Solace Message Format (SMF) is Solace PubSub+’s underlying messaging protocol.

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 subscribe topics:
      • / is a level separator.
      • <NULL> is an invalid character.
      • * is a single-level wildcard.
      • > 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, refer to System-Level Subscription Exception Configuration.
    • For publish topics:
      • / is a level separator.
      • <NULL> is an invalid character.
      • * is treated as a literal character. Avoid using this character when creating topics.
      • > is treated as a literal character. Avoid using this character when creating topics.
      • A leading ! is treated as a literal character. Avoid using 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/), Solace PubSub+ Version 7.1.1 or greater also allow empty topic levels in SMF topics. However, there are some limitations to using empty topic levels with SMF topics:

  • Client applications using Solace messaging APIs can publish to topics with empty levels, but they can't use topic subscriptions with empty levels.
  • In ACL profiles, SMF topic exceptions may not have empty topic levels (for publishing or subscribing).