Periodically update boards' local time and synchronize flushes

[marcomicera]

Could be done:

• Periodically, by another task
• Every time the boards gets an IP address from the AP

Second one looks easier.

[suidpit]

Agreed, second one looks cool and we stay super in sync. Maybe we can establish a counter, like every 5 flushes. I don't think the performance would improve any noticeably, but still I think syncining at every flush is not needed.

[marcomicera]

So the only way to send an SNTP request via a method exposed by sntp.h is through void sntp_init(void), which is defined here and this is its definition. At this line sntp_request gets called only if (sntp_pcb == NULL).

sntp_pcb can only be set to NULL inside the sntp_stop function, which would make me think that I'd need to stop and start the SNTP module every time I want to synchronize the board's current date/time.

However, this makes me think that the board is already sending SNTP requests periodically in the background, and the board is already set in this mode:

followifier/board/followifier/main/components/sync.c

Lines 24 to 29 in f44a3f2

	/* Choices:
	*
	* - SNTP_OPMODE_POLL: polling mode using unicast.
	* - SNTP_OPMODE_LISTENONLY
	*/
	sntp_setoperatingmode(SNTP_OPMODE_POLL);

@madt1m what do you think?

[suidpit]

Let's sniff and see if NTP requests are sent periodically. I will also check the source code of the sntp module.

[marcomicera]

Nope, NTP requests are not sent periodically, I was wrong. I'll keep diving in.

[simonasaitta]

What about have it set up by the server together with the synchronization? I have some idea of how it could be done. I can work on it tonight

[suidpit]

After a brief discussion, we've come up with some ideas to mantain timestamps synced and use that information to sync flushes on the same timeslot for every board.
Basically, once the boards are synced, it will be enough for the board to wait until the threshold of the minute (hh:mm:00) and then trigger the flush. This should ensure somewhat synced flushes among all the system.

[simonasaitta]

Synchronization among boards should have been fixed here 23c7c08. Only synchronization with server is left

[marcomicera]

Needed for long-term statistics.

[suidpit]

Cherry-picked in 177eb20. After a few tests, we can close this.

[marcomicera]

I'll test this one out since I'm the only one that has more than one board. If I'm wrong, please let me know...