Webapp - should a constraint be checked at database or application, or both - sql

Question

Simplified context:
let say I have 2 table in my database: Room(id, maxContract) and Contract(id, roomid, status)
let say I have a room 17 which allow max 2 clients, now I would search the Contract table of row roomid = 17 and status = active, if the more more than max (in this case 2) rows, I would prevent further INSERT until a contract expire.
Question:
Now I see 2 ways of doing this, first is in the database itself, maybe on a TRIGGER, and the second is doing this in my webapp DAO: query the Contract table to get the count, if-else to check the logic and only run the insert if true
But I am just a newbie, I don't know what is the best (or common) approach, which way I should do it? If this was my personal app, I would do both for max security, but designing a web I had to also take performance into consideration.
In case of frontend - backend, I know that validation is mandatory at backend and optional at the frontend, but between backend-database I don't know exactly
(In case this is opinion-based and there is no best-practice, I would like to know the pros and cons of both implementation)
EDIT:
to be more exact: user click JOIN ROOM => call an insertToRoom() method
+solution 1:
insertToRoom(){
if (roomIsAvailable()){
execute INSERT query;
}
else alert: "room is full";
}
roomIsAvailable() is a method to query and count how many contracts are bound to the room
+solution 2:
insertToRoom(){
execute INSERT query;
}
database:
CREATE TRIGGER before INSERT
if (some code to count the rooms)
ROLLBACK TRANSACTION;
in this case, if an unavailable room is join, database will return error, which in turn cause the execute INSERT query in the application to return false.
Either way, the falsy data is not inserted end the end user will get an error alert

Answer 1

I expect that more than one user can work with your application.
If you will get the current status from DB to the application, evaluate the condition there and then you will run the insert into DB table, then the condition can be violated. Let's imagine this sequence of steps:
User A read the current status from DB.
User B read the current status from DB.
User A evaluate the condition with result "there is a space for one client".
User B evaluate the condition with result "there is a space for one client".
User A update DB -> the room is full.
User B update DB -> in the room is more than allowed number of clients.
So that is not an option.
You can use triggers (as you mentioned), if your DB has such possibility.
You can also create one SQL statement which will check conditions and update the record in DB in one step (atomically). It depends on your DB engine, whether it is possible.
Update 2022-05-27
I was asked to explain in detail the atomic insert solution.
I'm not MySQL guru and I'm pretty sure, that there is more elegant way, how to do it, but something like this works too:
Let's create required DB objects first:
create table Room(
id INT,
maxContract INT);
create table Contract(
id INT AUTO_INCREMENT,
roomid INT,
status VARCHAR(30),
PRIMARY KEY(id)
);
create view UsedSpace as
select r.id as roomid, count(c.id) as used
from Room r
left outer join Contract c on c.roomid = r.id and status='active'
group by r.id;
Then we can use this statement to insert new row to Contract table:
insert into Contract(roomid, status)
select r.id, 'active'
from Room r
inner join UsedSpace us on r.id = us.roomid and r.maxContract > us.used
where id = 17;
When there is too much active contracts, then new row is not inserted.
You can check if the row was inserted or not via
select row_count();
Here is a fiddle to show results quickly.