Querying for a set time? - php

Question

Is there a way to query for a set time from a PHP script?
I want to write a PHP script that takes an id and then queries the MySQL database to see if there is a match. In the case where another user may have not yet uploaded their match, so I am aiming to query until I find a match or until 5 seconds have passed, which I will then return 0.
In pseudocode this is what I was thinking but it doesn't seem like a good method since I've read looping queries isn't good practice.
$id_in = 123;
time_c = time();
time_stop = time + 5; //seconds
while(time_c < time_stop){
time_c = time()
$result = mysql_query('SELECT * WHERE id=$id_in');
}

Answer 1

It sounds like your requirement is to poll some table until a row with a particular ID shows up. You'll need a query like this to do that:
SELECT some-column, another column FROM some-table WHERE id=$id_in
(Pro tip: don't use SELECT * in software.)
It seems that you want to poll for five seconds and then give up. So let's work through this.
One choice is to simply sleep(5), then poll the table using your query. The advantage of this is that it's very simple.
Another choice is what you have. This will make your php program hammer away at the table as fast as it can, over and over, until the poll succeeds or until your five seconds run out. The advantage of this approach is that your php program won't be asleep when the other program hits the table. In other words, it will pick up the change to the table with minimum latency. This choice, however, has an enormous disadvantage. By hammering away at the table as fast as you can, you'll tie up resources on the MySQL server. This is generally wasteful. It will prevent your application from scaling up efficiently (what if you have ten thousand users all doing this?) Specifically, it may slow down the other program trying to hit the table, so it can't get the update done in five seconds.
There's middle ground, however. Try doing a half-second wait
usleep(500000);
right before each time you poll the table. That won't waste MySQL resources as badly. Even if your php program is asleep when the other program hits the table, it won't be asleep for long.

Answer 2

There is no need to do simple polling and sleeping. I don't know your exact requirements, but in general your question asks for GET_LOCK() or PHP's Semaphore support.
Assuming your uploading process starts with
SELECT GET_LOCK("id-123", 0);
Your Query thread can then wait on that lock:
SELECT GET_LOCK("id-123", 5) as locked, entities.*
FROM entities WHERE id = 123;
You might eventually find that a TRIGGER is the thing what you were looking for.