Shared Subscription Syntax for MQTT

General Discussions
1

Hello Solace Community,
I have a question regarding the usage of shared subscriptions with an MQTT client.
I found two different prefixes in the documentation and am curious to learn which is used in which case.

I found in the documentation the following hint (see:

) :

Syntax of Shared Subscriptions

A shared subscription has the form:

#share/<ShareName>/<topicFilter>

Where:

ShareName is the identifier associated with the shared subscription. It may not contain * or >.

topicFilter is the topic filter.

But inside the MQTT operational behaviour I find this (see on 4.8.2

):

A Shared Subscription’s Topic Filter MUST start with $share/ and MUST contain a ShareName that is at least one character long[MQTT-4.8.2-1]. The ShareName MUST NOT contain the characters “/”, “+” or “#”, but MUST be followed by a “/” character. This “/” character MUST be followed by a Topic Filter[MQTT-4.8.2-2] as described in section 4.7 .

For me it only worked with $share in the beginning. But I am now unsure if I missunderstood something. Is the first documentation I quoted and linked not relevant for me ? I was under the assumption that I am using Direct Messaging with QoS 0 and MQTT. So the documentation above should also have worked in my opinion.

So is $share or #share correct ? Or maybe both ? :slight_smile:

Greetings,

Jan-Filip

2

Hey @jfg1306 - so #share/<ShareName>/<topicFilter> notation is when you are using solace SMF. However when you use MQTT protocol and want to take advantage of the shared subscriptions then you will use the $share/<ShareName>/<topicFilter>

So technically, both are correct but depending on which protocol you are using :slight_smile: Hopefully this helps!

3

Hi Jan-Filip

If you are working with the Solace SMF based APIs like JCSMP, Java, Python etc. shared subscription has the form: #share/<ShareName>/<topicFilter>

For MQTT client, $share/{ShareName}/{filter} is the correct form as mentioned in section 4.8.2

and it is as per the MQTT v5 specs.

4

Hi @Tamimi and @MayurP ,

thank you both the answer. Now I got it, so both are valid but are used based on the used messaging API.

Greetings,

Jan-Filip.