sync.md

Product quantity synchronization strategy

Action	Mobile A (global quantity) [local delta]	Mobile B (global quantity) [local delta]	Backend [starting quantity, delta A, delta B, real quantity]	Real state
Initial state	(27) [0]	(27) [0]	[27, 0, 0, 27]	27
Mobile A adds 5 items locally	(27) [-5]	(27) [0]	[27, 0, 0, 27]	22
Mobile A sync	(27) [-5]	(27) [0]	[27, -5, 0, 22]	22 (+)
Mobile B adds 3 items locally	(27) [-5]	(27) [+3]	[27, -5, 0, 22]	25
Mobile B sync	(27) [-5]	(22) [+3]	[27, -5, +3, 25]	25 (+)
Mobile A removes 14 items locally	(27) [-19]	(22) [+3]	[27, -5, +3, 25]	11
Mobile A sync	(30) [-19]	(22) [+3]	[27, -19, +3, 11]	11 (+)
Mobile B sync	(30) [-19]	(8) [+3]	[27, -19, +3, 11]	11 (+)
Mobile A adds 7 items locally	(30) [-12]	(8) [+3]	[27, -19, +3, 11]	18
Mobile A sync (failed to contact backend)	(30) [-12]	(8) [+3]	[27, -19, +3, 11]	18 (-)
Mobile B adds 4 items and sync	(30) [-12]	(8) [+7]	[27, -19, +7, 15]	22 (-)
Mobile A sync	(34) [-12]	(8) [+7]	[27, -12, +7, 22]	22 (+)
Mobile B adds 9 items	(34) [-12]	(8) [+16]	[27, -12, +7, 22]	31
Mobile B sync (backend contact success, return from backend lost)	(34) [-12]	(8) [+16]	[27, -12, +16, 31]	31 (+)
Mobile A sync	(43) [-12]	(8) [+16]	[27, -12, +16, 31]	31 (+)
Mobile B sync	(43) [-12]	(15) [+16]	[27, -12, +16, 31]	31 (+)

Legend

global quantity = sum(mobile_deltas) - local_delta for given device starting quantity = const value (+) -> backend real quantity equals real state after sync (-) -> backend real quantity differs from real state after sync