2 phase locking

# to avoid double booking in the movie ticket

# user-1 and user-2 are booking the same seat simultaneously:
user-1:
begin transaction;
select * from seats where id = 14 for update; # key is for update, this locks the seat
update seats set isbooked=1, name = 'Hussein' where id = 14;
commit;

user-2:
begin transaction;
select * from seats where id = 14 for update; this command will be blocked because of 2 phase booking;
# once user-1 is done, this command will show the ticket is booked with name of Hussein

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exclusive locks conflicts with other exclusive lock or shared locks. 
So it appears that the other transaction is allowed to read because it is not obtaining any shared lock. 
If you want the other transaction to be blocked from reading try running it with FOR SHARE option