Sequence diagrams describing possible data synchronization methods. You can visualize these using http://www.websequencediagrams.com

sync_phase1_part1.diagram

# Syncronization Sequence (Phase 1)

User->App: Open & Login
App->Server: GET ToDos
Server->App: 200 OK\n{title:eat breakfast, updated:02:00}
Web->Server: Update ToDo #1 via web at 02:30\n{title:eat lunch}
User->App: Update ToDo #1 in app at 03:00
App->Server: PUT update ToDo #1\n{title:eat dinner}
note over Server
The web changes are overwritten
end note
Server->App: 200 OK\n{title:eat dinner updated:03:00}

sync_phase2_part1.diagram

# Syncronization Sequence (Phase 2)

User->App: Open & Login
App->Server: GET ToDos
Server->App: 200 OK\n{title:eat breakfast, updated:02:00}
Web->Server: Update ToDo #1 via web at 02:30\n{title:eat lunch}
User->App: Update ToDo #1 in app at 03:00
note over App
Send the updated value last received from the Server, *not* the current time
end note
App->Server: PUT update ToDo #1\n{title:eat dinner, updated:02:00}
Server->App: Reject PUT with 40X status\n{title:eat lunch, updated:02:30}
App->User: Ask user about overwriting changes\n(use "eat lunch" or "eat dinner"?)

sync_phase2_part2.diagram

# Syncronization Sequence (Phase 2) continued...
# When a change is rejected

# This first line is only here to order the columns correctly
User-->>App:

Server->App: Reject PUT with 40X status\n{title:eat lunch, updated:02:30}
App->User: Ask user about overwriting changes\n(use "eat lunch" or "eat dinner"?)

alt User chooses the Server version
User->App: Use "eat lunch" at 3:30
note over App: Update the local updated value to 02:30 and discard the local "eat dinner" title

else User chooses the App version
User->App: Use "eat dinner" at 3:30
note over App
Send the updated value last received from the Server, *not* the current time
end note
App->Server: PUT update ToDo #1\n{title:eat dinner, updated:02:30}
Server->App: 200 OK\n{title:eat dinner, updated:03:30}