MQTT and Retained Messages

[michaeljfalk]

Does Shairport-Sync ignore the

retain = "yes";

in the conf file? I have it set to retain yet MQTT never gets sent the Retain flag

this is my current file settings:

mqtt =
{
        log_verbosity = "debug";
        enabled = "yes"; 
        hostname = "127.0.0.1";
        port = 1883; 
        name = "Back Deck Speakers";
        topic = "shairport-sync/metadata"; 
        publish_raw = "no"; 
        publish_parsed = "yes"; 
        retain = "yes";
        publish_cover = "no"; 
}

[mikebrady]

Thanks for the post. There is no retain in the MQTT configuration set, so adding it will have no effect. What do you expect it to do?

[michaeljfalk]

I am writing a small python back-end/html front-end to some airplay speakers I was building using your Shairport-Sync and your Shairport-Sync only transmits song metadata when the song changes and not continually. MQTT lets you write to its stream using a -R flag and then it retains the info till it's removed. This way if/when the front-end accesses the data in MQTT it is supplied, even mid-stream, then the app could clear it manually. Currently the stream is empty so then the front-end just displays "Loading" till the song changes.
It would be a great little addition I think to add that functionality for when Shairport writes to MQTT as an option.

[mandresen]

Another practical use case, is making sure the active 0/1 information is available immediately in Home Assistant after a restart. As an example I use that information to decide whether to supply power to the relevant amplifier or not.

[mikebrady]

Thanks for the suggestion. I'm not sure how this works and what might be involved. Does it mean that Shairport Sync's interface would have to retain each value until it is replaced or updated?

[mandresen]

From an implementation standpoint I can't answer how difficult it is, sorry.

But more high-level, when a client publishes with the retain flag, the MQTT broker stores that message as the topic’s retained message and delivers it to current (and potential future) subscribers. The publishing client doesn’t need to manage or resend it. when you send a new retained publish (or an empty retained item) you replace or clear it.

As an example with mosquitto_pub, you add the '-r' flag to the CLI command to mark it as a retained message.

[lululombard]

FYI, I added MQTT retain support with #2142 and it has already been merged to development!

[mikebrady]

Haha, thanks. You beat me to it!