Devices coming online "en-masse"

I am asking this question out of my academic interest. I manage a very small fleet of devices and I have noticed a pattern that I would like to understand better.

I haven’t noticed this frequently but every now and then, all of my devices or at least a portion of them publish a spark/status event indicating that the device has just come online. This happens around the same time for all devices. I have attached a snapshot.

I find it weird because this happens to the entire fleet. It made me wonder if the devices renew their secure sessions every few days.

I have noticed that some of my devices have had their last handshake ten days ago and they stay connected to the cloud. I wonder what could trigger the devices to reconnect to the cloud.

The devices are located all over the country and they seem to come online within the same timeframe.

There are several reasons why a cellular device will reconnect to the cloud:

  • Powered on (cold boot or sleep without cellular standby)
  • Force session reset (from user firmware)
  • Periodic session reset (every 10 days)
  • Cloud initiated session reset

The last one is the one that can get devices in many locations offline then back online at the same time. There are a number of device-facing cloud servers. When we need to upgrade software, for example, we roll the devices over to different servers in groups. This will cause a reconnection for those devices when they start using a new server.

Another possible reason can also be the routers IP lease time. If all devices were initially connecting to the the same router at the same time they will all hit their lease time limit at the same time again.

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