We have an android(or iphone) client we are developing. The client allows the android user to send entries to a server which we also develop. If the client does not have data services (GPRS) at the moment the user sends the entry to the server, the client also supports saving the entry to an offline database and sending it later to the server.
One important aspect of the whole process is accuracy of the timestamps on which the user sent the entry to the server (whether the entry is made in real time or sent by the client from the offline database)
When available on the client, we get a GPS location and are able to use the GPS timestamp to send that to the server (or save the GPS timestamp on the offline DB and send it later to the server). However if the user has turned off the GPS (and all other location services), the device will not have a GPS fix and therefore the server can not determine accurately when an entry was made.
We can not use the local device clock as the user may change the clock to make entries on different times than they actually occurred (these entries are part of the users salary so he might have an interest to “fix” them).
So basically I am searching for a way to determine as best I can the time some entry was made when I can not trust the internal clock of the mobile. The algorithm should support both entries sent in real time or entries sent from an offline DB. the algorithm should also support cases where the user changes the time of the mobile, turns the mobile on/off, turns the GPS on/off while the application is running on the mobile etc…
Few ideas that I thought of:
- Although I can not trust the mobile’s time, it can still perform as a stop watch:
Have a class that will loop until the application exists, the loop will sleep 1 second and increase an internal clock variable by 1 second. On every GPS location my code gets we update the internal clock variable. This way I have an absolute clock that came from outside the device (from the GPS) and when the client sends an entry to the server, we can use the internal clock as an absolute time.
PROS: the user can not modify this clock as it is only updated when we get a location from the GPS
CONS: the application needs at least one GPS fix before the user can make any reliable entries
- I can take advantage of the fact that the server has an accurate clock which is correct. If the client would send to the server info that the age of the entry is 10 minutes, the server could use its internal time and know the exact time the entry was made on.
The biggest problem is how to know the entry age? I thought about saving the entries to the offline DB with an age of 0, then every 1 second increase the age of the entry in the DB. The problem is that if the app is closed and/or the device is off this will now happen
This is where I am currently stuck. Any ideas on how to solve this are more than welcome
Thanks
i think i am going to combine Jules and Joel’s answers into one solution which will provide for my needs the best solution:
since the user might change the clock when the mobile doed not have GPRS, just detecting the time change event will not help us as we can not validate at that moment the new time is correct.
As Joel recommended i will pull the time from my server when my application is started (at that point i still must have communications with the server or else my application will not start). The time pulled from the server along with the current device upTime will be saved.
when the user wants to make an entry i will calculate the current time using (Server Base Time + Current UpTime – Base UpTime). this way i will have an independent source of time regardless of the current clock of the device
this will defenitly work on android
on iPhone we will try to use something out of http://www.cocoadev.com/index.pl?FindingUptime to get the upTime
Jules & Joel, thanks for your answers!