I implemented real time sync following Realm's tasks demo app.
There a dummy container is used, to hold a List with the models.
The demo app doesn't seem to support offline usage.
I wondered what happens when, given this setup, I start the app on an online as well as an offline device and then go online with the offline device.
My initial expectation was that I'd end with 2 containers (which would be an invalid state), but when I tested surprisingly there was only 1 container at the end.
But sometimes I get 2 containers and haven't been able to identify what causes this.
The question then is, how does this exactly work? I assume the reason that the container is normally not duplicated when I sync the offline device for the first time is that it's handled as the same object, maybe because it doesn't have a primary key or something? But then why is it sometimes duplicated? And what would be the best practice here? Do I maybe have to use a primary key or check after connecting if there's duplication and if yes do a manual merge of the containers?
At the moment, Realm Tasks merely checks if the default Realm is empty before it tries to add a new base list container object. If the synchronization process hasn't completed by the time this check occurs, it's reasonable that a second container would be created. When testing the app on a local network, this usually isn't a problem since the download speeds are so fast, but we definitely should test this a bit more thoroughly.
Adding a primary key will definitely help since it means that if a second list is created locally, it will get merged with the version that comes down from the server.
We've recently been focusing on the 'on-boarding' process when a second device connects to a user's Realm Mobile Platform account via the new progress notification system. A more logical approach would be to wait for the synchronization to complete the initial download after logging in, and then checking for the presence of the objects. Once the documentation is complete, we'll most likely be revamping how Realm Tasks handles this.
The demo app (as well as the Realm Mobile Platform) does support offline, but only after the user has logged in for the first time (which is when these container objects are initially generated). After that time, the apps can be used offline, and any changes done in that interim are synchronized the next time it comes online.
We're planning on building 'anonymous user' feature where a user can start using the app straight away (even offline) and then any changes they made before they log in (due to them being offline) are then transferred to the user account after they do so.