Question: $SYS topic lists

[PranishShresth]

Hello,

I want to understand SYS topics that flashmq currently supports. I am currently in process of migrating mosquitto to flashmq and some of the SYS topics that we used before are not working. For example: load/messages/received/1min is not supported. I can see there is total and persecond though. Just want to understand what can be used or not.

I can see number of SYS topics defined in

FlashMQ/threaddata.cpp

Line 85 in 4b8c15c

* @brief ThreadData::queuePublishStatsOnDollarTopic makes this thread publish the $SYS topics.

Also regarding sub count, i saw that you mentioned that is only updated or corrected every 30 min.

[halfgaar]

It sounds to me you got it all right :)

The counters are specified as 'total' and 'per second'. The per second is mostly just for convenience, because it's a snapshot of the specific second you're looking at it. More useful is the total. If you write that to a time series database a regular intervals, you can perform a mathematical derivative on it, and calculate any X per Y (so per second, minute, hour, etc). One tool stack to do so is Telegraf + influxdb + Grafana. Telegraf can connect to the MQTT server and receive the SYS topics and write them to influxdb. In Grafana you can easily point-click a bunch of graphs together. One tip about Telegraf: it doesn't gracefully reconnect on lost mqtt connections, so you have to monitor that and restart if it breaks.

ACL checks are also a useful metric. It may show if you have an inefficient subscribtion pattern. If you have way more ACL read checks than incoming messages, you may want to optimize your clients. That is, if there are performance problems.

The drift topics are useful for monitoring performance. It counts its internal delay in keeping up on events. If that goes up too much, you know the server (or one thread) is overloaded.

The subscription count is indeed corrected once per 30 minutes or so. Only if you have a high subscribe frequency will the correction be large.

[PranishShresth]

Thank you for your breakdown. Appreciate your prompt response.

What interval is the /total topic published ?

[halfgaar]

The SYS topics are published once per 10 seconds. But, they are retained messages, so there will always be 'current' value, which gets updated once per 10s.

[PranishShresth]

Thank you! Adding thread drift to our metrics too :)