Where is published message stored by mosquitto broker if offline

Question

I'm using the Eclipse paho to publish messages with the mosquitto broker and when both my publisher and subscriber are both connected, I'm successfully able to receive the message. And, even if I disconnect the subscriber, I still get the messages upon reconnecting it. That means there must be a storage somewhere on the broker level where these messages get stored while the subscriber is offline. I just really wanted to know how many such messages can be stored on broker level while subscriber is offline
I'm publishing messages through the broker in paho with QOS=2 through a session persistence using clean_session=false. So, will this be feasible for a real-time GPS tracker where there are chances of my subscriber being offline for hours together, also how much traffic can it take for a scalable application? And, is my session persistence still in place when offline?

Answer 1

Well, of course, they are stored! That is the main idea behind session persistence and if I'm correct, a 32-bit mosquitto broker uses a huge disk space of storage, like at least more than 1 GB for sure.
And, the file being written to disk in the mosquitto.conf file autosave* and persistence* options is 'mosquitto.db' and you can control where it is being written. But, I hope you don't have abnormally large lengths in your messages! It is a common problem though, avoid doing that! And, it will still depend on factors like how many publishers or subscribers you have or your message rates. Also, the session persistence is also maintained in the same .db file so that its state just resumes upon the broker starting again or the subscriber coming online.