Offline db out of sync

[mpfau]

On my machine, the offline DB got out of sync. The same mail is displayed in the Trash and Inbox folders. I can't delete this mail from the Inbox anymore as it is always fetched from the Offline db.

We assume that this happened after a websocket event for an EntityUpdate (move mail to trash) was missed.

Discussion during Refinement on Oct 18:
We had missing ws updates for entity events because of the server migration. This caused some undeletable emails. It seems that there were such emails before but maybe for another reason.

• Do we currently have a strategy to detect missed events?
• Theoretically can happen if some WS message got lost.
• previously, we loaded 1000 events before the last processed one
• proposal: give updates a proper sequence number so the client can detect gaps
  • how to do that? will it scale to multiple servers?
  • can ws message get lost? https://stackoverflow.com/a/32475028/1923879
    • guarantees order: if we miss one, we won't receive another until the missed one is retransmitted
  • if we had sequence number: we could detect missing events in websocket session. Instead of processing the event we would re-initialize since the last processed one instead. For this we don't need to record/persist sequence numbers, it is enough to have them consistent in the WS session.
• let the client download all events since the last one it recorded (only needs timestamp)
• we could build mitigation: when we try to modify/delete email and get 404 we just nuke it from DB
  • this would be the first use case to move away from read-only cache, so may be worth more design effort

Further Actions

• We must investigate what happens with unreliable WS connections: can the server send messages that the client never receives without the server noticing
• is there an error thrown that we don't handle properly
• Check event optimization in the client
• Ask if this may be only related to the infra migration (did this happen before about Oct 10)
• what is with missed CREATE events. We may have mail on the server that's just not shown because the cache thinks it has everything.

[alstr]

This is a major problem that is making it very difficult to continue to use Tutanota. This problem has cropped up several times now over the last couple of months and today I have more emails in my iOS inbox showing as "cannot be displayed", nor can they be deleted, and whenever I open the app and the emails reappear it messes up the new email count on the macOS app. Removing apps and local files is not a practical day to day solution when it occurs at this frequency. At the moment I am finding offline mode to be more of a problem than a solution. Please fix this as a priority over adding new functionality.

[charlag]

thanks for the info. We are still trying to find out in which cases it happens.

[alstr]

In my case it's pretty frequent. I've given up trying to maintain the drafts folder.

[charlag]

I experience the same problem with the undeletable mail but we are not sure how to reproduce it yet