- Author: Cliff Brake
- Status: discussion
To date, SIOT has been deployed to systems with RTCs and solid network connections, so time is fairly stable, thus this has not been a big concern. However, we are looking to deploy to edge systems, some with cellular modem connections and some without a battery backed RTC, so they may boot without a valid time.
SIOT is very dependent on data having valid timestamps. If timestamps are not correct, the following probems may occur:
- old data may be preferred over newer data in the point CRDT merge algorithm
- data stored in time series databases may have the wrong time stamps
Additionally, there are edge systems that don't have a real-time clock and power up with an invalid time until a NTP process gets the current time.
We may need some systems to operate (run rules, etc) without a valid network connection (offline) and valid time.
- db (InfluxDB driver)
- sync (sends data upstream)
- store (not sure ???)
The db and sync clients should not process points (or perhaps buffer them until) until we are sure the system has a valid time. How does it get this information? Possibilities include:
- creating a broadcast or other special message subject that clients can optionally listen to. Perhaps the NTP client can send this message.
- synchronization may be a problem here if NTP client sends messages before a client has started.
- query for system state, and NTP sync status could be a field in this state.
- should this be part of the root device node?
- or a special hard-coded message?
- it would be useful to track system state as a standard point so it gets synchronized and stored in influxdb, therefore as part of the root node would be useful, or perhaps the NTP node.
System must function when offline without valid time. Again, for the point merge algorithm to work correctly, timestamps for new points coming into the store must be newer than what is currently stored. There are two possible scenarios:
- Problem: system powers up with old time, and points in DB have newer time.
- Solution: if we don't have a valid NTP time, then set system time to something later than the newest point timestamp in the store.
- Problem: NTP sets the time "back" and there are newer points in the DB.
- Solution: when we get a NTP time sync, verify it is not significantly earlier than the latest point timestamp in the system. If it is, update the point timestamps in the DB that are newer than the current time with the current time - 1yr. This ensures that settings upstream (which are likely newer than the edge device) will update the points in the edge device. This is not perfect, but if probably adequate for most systems.
We currently don't queue data when an edge device is offline. This is a different concern which we will address later.
The SIOT synchronization and point merge algorithm are designed to be simple and bandwidth efficient (works over Cat-M/NBIOT modems). There are design trade-offs. It is not a full-blown replicated, log-based database that will work correctly in every situation. It is designed so that changes can be made in multiple locations while disconnected and when a connection is resumed, that data is merged intelligently. Typically, configuration changes are made at the portal, and sensor data is generated at the edge, so this works well in practice. When in doubt, we prioritize changes made on the upstream (typically cloud instance), as that is the most user accessible system and is where most configuration changes will be made. Sensor data is updated periodically, so that will automatically get refreshed typically within 15m max. The system works best when we have a valid time at every location so we advise ensuring reliable network connections for every device, and at a minimum have a reliable battery backed RTC in every device.
It may make sense to write the latest point timestamp to the store meta table.
Will consider in future. Assume a valid network connection to NTP server for now.
It would be very useful to track events at edge devices where time is not correct and it requires a big jump to be corrected.
TODO: how can we determine this? From systemd-timedated logs?
This information could be used to diagnose when a RTC battery needs replaced, etc.
A final check that may be useful is to verify time between synchronized instances are relatively close. This is a final check to ensure the sync algorithm does not wreak havoc between systems, even if NTP is lying.
timedatectl statusproduces following output:
Local time: Thu 2023-06-01 18:22:23 EDT
Universal time: Thu 2023-06-01 22:22:23 UTC
RTC time: Thu 2023-06-01 22:22:23
Time zone: US/Eastern (EDT, -0400)
System clock synchronized: yes
NTP service: active
RTC in local TZ: no
There is a systemd-timedated D-Bus API.
what was decided.
what is the impact, both negative and positive.