As far as I know, Whatsapp relied on Mnesia only as database (for non media content). The Mnesia tables had to be fragmented to hold all data. Whatsapp contributed to some optimizations and improvements of the Erlang code base related to handling the Mnesia database.
Whatsapp does not store any messages on their servers, thought. Accordingly, the database was for user credentials, the roster, group chat meta infos, non delivered messages and presence info only.
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