Storing all genserver pid in ets table in each node

see i am working for someone so i can’t say for whom i am working.

but i will explain what i am trying to do, see this application have group which contain something like 2 lakh people and and we are thinking to increase it. we are using mnesia to keep user session pid with user detail in mnesia to broadcast to all online users. but if we increase group size, then we know mnesia will not work, write and sync in mnesia is wrost when size grow so we are thinking of using ets table because it is fast.

the main problem is sync