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i have an online hotel booking system which allows multiple users to make bookings almost simultaneously.

When a user requests for free rooms, the system performs a query to get the free rooms and then shows it in a some way ...

Now when user clicks on the "book" button, the free rooms are sent to the server to be booked, i am using a very similar query to this (this is simplified):

Insert into Bookings(room_id) select available_room_id from
dbo.GetAvailableRooms(@StartDate,@EndDate) where available_room_id = @ToBeBookedRoomID;

where " GetAvailableRooms " is a UDF.

what i am hoping to do by this query is to test for the latest time if room is available or not, but i am worried if this is exposed to race conditions ? i am thinking about the case that some user has booked that room in the time between the insert and select operations ... would "insert into .. select" guarantees no race conditions ?

Updated: This is the function sql query :

select AllDays.day_date,alloted_room_id as available_room_id,pl.room_number from HotelsContractsAllotments hl
inner join PlansRooms pl on pl.room_id = hl.alloted_room_id
inner join HotelsContracts hc on hc.contract_id = hl.allotment_contract_id
inner join Hotels h on h.hotel_id = hc.contract_hotel_id
inner join AllDays on day_date >= hc.contract_starting_date and day_date <= hc.contract_end_date
left join BookingsRooms br on day_date >= br.booking_starting_date and  day_date <= br.booking_end_date and br.booking_room_id = alloted_room_id
where br.booking_room_id is null and day_date >= @StartingDate and day_date <= @EndDate

Thanks.

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  • Without seeing the code from dbo.GetAvailableRooms, I would expect this to be exposed to race conditions.. Commented Nov 27, 2015 at 20:02
  • Sorry, i have just updated it, could you see it please ? @GordonLinoff Commented Nov 27, 2015 at 20:16
  • I would suggest using a transaction. Also see this: stackoverflow.com/questions/1071286/… Commented Nov 27, 2015 at 20:46

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Look at the locking on the table(s), and hints used. Based on the information provided I expect this to be open to race conditions (i.e. a lock is not maintained on the table). You can look at setting-up locks with SQL hits, or transactions. There are a few posts out there that discuss locking. An example of one that looks pertinent to your question is: SQL Server Table Lock during bulk insert

Without a continuous lock on the table(s) affected from the SELECT through the INSERT, yes you are open to a race condition.

NOTE - locking the table for extended periods will impact performance to varying degrees depending on the volume of traffic, and time locked (if a lot of users are trying to book at the same time, 1sec difference can become significant). Therefore, if you do lock the table, I recommend spending some time tuning your queries for performance. I wont say much on this as I do not have any metrics on your data, or know your indexes. Probably best to start by simply re-structuring your queries to hit the most restrictive table first. I assume this would be the table holding alloted_room_id (pass the ID so it can be used in the WHERE clause).

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It might be worthwhile to look at the models used for things like online shopping carts and how they handle allocations of 'reserved' resources that expire if the user does nothing.

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