I'm designing a chat application using AJAX, PHP, MySQL and am having a problem.
MySQL table is like this username[varchar11]-taken[enum-0,1](coloumns) generally a user who wants to talk to userX, firstly checks if taken value of userx is 0 or 1
SELECT * FROM usertable WHERE username=userX
If it is 0 the user updates taken as 1:
UPDATE usertable SET taken=1 WHERE username=userX
and starts to talk to userX.
But when there are two users who want to talk to userX and check the taken value at the same time unconsciously, they both get 0 response. As a result of it, both of their updates are taken as 1 afterwards and start to talk to userX at the same time. Effectively, 3 users will be talking each other. This chat site is supposed to allow a user to talk to maximum one user at the same time. I'm trying to design it like that.
I'd appreciate any suggestions. What I need is something which is going to stop other users from checking and updating taken value until one finishes to check and update taken value.
Related
I am new in forum, and need some help to do one functionality for my unity game. I am trying to save the progress of the player in one mysql database, like this:
userid level stars
29 1 2
29 2 1
45 1 3
50 1 2
50 2 3
50 3 1
29 3 3
so the script send the userid provided by the user registration in the begining of the game. and for each level he complete, the script send the number of the level, and the amount of stars collected in the level..
the problem and question is, how I configure this in the php script and mysql database to save the information only once? because if the player with the id 50 play the first level, will add a line with the information, but if the same player play the first level again and change the amount of stars, I dont want a new line, just update the stars amount.
I take a look in the INDEX, UNIQUE, PRIMARY, FULLTEXT, SPATIAL functions but dont figured out what is the correct combination and how to put in the php script, and take a look in other questions in the forum but nothing like this.
thanks for the help!
I recommend you use http://redis.io/ (in-memory data structure store, used as database, cache and message broker) to save progress in the games.
First you want an unique index on the combination (userid, level) and then you want to do an update if the combination exists and an insert otherwise.
For how to create the unique index please take a look at How do I specify unique constraint for multiple columns in MySQL?
For how to code the SQL query to do update/insert please take a look at SQL: If Exists Update Else Insert
The article above uses Microsoft SQL syntax. In PHP you can code this by issuing the query and then using mysql_affected_rows to see how many rows where affected. If 0 rows where affected then you issue the INSERT query from PHP.
in pseudo code you need to do something like this in SQL.
UPDATE $table set column=value WHERE id=$ID
Hi brayan actually the problems is that no one will write code for you, you have to do it yourself. I guess you are unaware with SQL i.e., you asked that
how I configure this in the php script and mysql database to save the
information only once? because if the player with the id 50 play the
first level, will add a line with the information, but if the same
player play the first level again and change the amount of stars, I
dont want a new line, just update the stars amount.
Anyhow You first required some basic understanding of SQL and PHP with Unity. I will recommend you this Guide Server_Side_Highscores of unityWiki it help you to make database and server logic intergartion with PHP.
Now for your Second important part of question.
You have to update user code after each level completion.Or you can simply ask to user about socre save.
Before inserting new record into the database you have to check that userId with level id alread exist or not. some thing like this
Select userid, level, stars
from youTableName
where userid = ?
and level = ?
if the above query return empty response then you simply need to add the record
INSERT INTO table_name (userid, level, stars)
VALUES (value1,value2,value3);
Otherwise you have to update that specific column.
Hi have a bunch of unique codes in a database which should only be used once.
Two users hit a script which assigns them at the same time and got the same codes!
The script is in Magento and the user can order multiple codes. The issue is if one customer orders 1000 codes the script grabs the top 1000 codes from the DB into an array and then runs through them setting them to "Used" and assigning them to an order. If a second user hits the same script at a similar time the script then grabs the top 1000 codes in the DB at that point in time which crosses over as the first script hasn't had a chance to finish assigning them.
This is unfortunate but has happened quite a few times!
My idea was to create a new table, once the user hits the script a row is made with "order_id" "code_type". Then in the same script a check is done so if a row is in this new table and the "code_type" matches that of which the user is ordering it will wait 60 seconds and check again until the previous codes are issued and the table is empty where it will then create a row and off it goes.
I am not sure if this is the best way or if two users hit at the same second again whether two rows will just be inserted and off we go with the same problem!
Any advice is much appreciated!
The correct answer depends on the database you use.
For example in MySQL with InnoDB the possible solution is a transaction with SELECT ... LOCK IN SHARE MODE.
Schematically it works this by firing following queries:
START TRANSACTION;
SELECT * FROM codes WHERE used = 0 LIMIT 1000 LOCK IN SHARE MODE;
// save ids
UPDATE codes SET used=1 WHERE id IN ( ...ids....);
COMMIT;
More information at http://dev.mysql.com/doc/refman/5.7/en/innodb-locking-reads.html
I am curious what path I should take to accomplish the following. I want multiple computers at one location to be able to view and make changes to data inside a mysql DB with a web browser. I dont have extensive knowledge in this area, but from what I do remember this was very difficult if not impossible.
Example: Lets say I have a record for John and I want 2 computers to be able to edit Johns record. Please note that the computers will not be editing the same portion of Johns record. Lets say one record is changing a status from need to be called to called and the other computer is changing the status of need to be ordered to ordered.
I want a solution that could natively handle this.
My current knowledge is building web interfaces with PHP and SQL. I would like to use these languages as I have some prior knowledge.
So my question: Is this possible? If, so exactly how would it work(flow of info)?
There are several ways that you can accomplish this. There's already some great PHP database editing software packages out there (phpMyAdmin).
To handle this in code though you can either use Transactions (depending on what flavor of SQL you're using this would be done differently)
One of the easier ways to ensure that you don't have people's data clashing with one another is just by adding additional where clauses to your statement.
Lets say you have a user record and you want to update the last name from Smith to Bill, and the user ID is 4.
Instead of writing
UPDATE users SET lastName='Bill' WHERE id='4'
You would add in:
UPDATE users SET lastName='Bill' WHERE id='4' AND lastName='Smith'
That way if someone else updates the last name field while you're working on it, your query will fail and you'll have to re-enter the data, thus faking a transaction
Use Transactions. Updating a single record at the exact same time isn't really supported, but applying one transaction followed immediately by another certainly is. This is native to MySQL.
START TRANSACTION;
SELECT #A:=SUM(salary) FROM table1 WHERE type=1;
UPDATE table2 SET summary=#A WHERE type=1;
COMMIT;
One other thing to do is the old desktop approach. Wich is almost mannualy control the flow of modifications. I will show:
Say that you have a client table with the fields id, firstname, lastname, age. In order to control multiple users updates you will add the version integer default 0 field to this table.
When you populate the object on the form to an user you will also store the actual version that the user has selected.
So lets assume that your client table is like this:
id firstname lastname age version
1 Tomas Luv 20 0
2 Lucas Duh 22 0
3 Christian Bah 30 0
When the user select the client with the id=1 the version of this row is, in this moment, 0. Then the user update the lastname of this client to Bob and submit it.
Here comes the magic:
Create a trigger (before update) that will check the current version of that registry with the version that the user previously selected, something like this (this is just pseudo code, as I'm doing it from my head):
create trigger check_client_version on client before update as
begin
if new.version != old.version then
throw some error saying that a modification already was done;
else
new.version = old.version + 1;
end if;
end;
On the application you check if the update has this error and inform to user that someone else made change on the registry he try to change.
So with the given example it would be like:
1 - The user A selected the row 1 and start editing it
2 - At the same time the user B selected the row 1 and save it before the user A
3 - The user A try to save his modifications and get the error from the application
On this context the user A has the version field pointed to 0 also is the user B but when the user B save the registry it now is 1 and when the user A try to save it it will fail because of the check trigger.
The problem with this approch is that you will have to have a before update trigger to every table in your model or at least the one you are concerned with.
I have a cron task running every x seconds on n servers. It will "SELECT FROM table WHERE time_scheduled<CURRENT_TIME" and then perform a lengthy task on this result set.
My problem is now: How do I avoid having two seperate servers perform the same task at the same time?
The idea is to update *time_scheduled* with a set interval after selecting it. But if two servers happen to run the query at the same time, that will be too late, no?
All ideas are welcome. It doesnt have to be a strict MySQL solution.
Thanks!
I am guessing you have a single MySQL instance, and connections from your n servers to run this processing job. You're implementing a job queue here.
The table you mention needs to use the InnoDB access method (or one of the other transaction-friendly access methods offered by Percona or MariaDB).
Do these items in your table need to be processed in batches? That is, are they somehow inter-related? Or is it possible for your server processes to handle them one-by-one? This is an important question, because you'll get better load balancing between your server processes if you can handle them individually or in small batches. Let's assume the small batches.
The idea is to prevent any server process from grabbing onto a row in your table if some other server process has that row. I've had to do this kind of thing a lot, and here is my suggestion; I know this works.
First, add an integer column to your table. Call it "working" or some such thing. Give it a default value of zero.
Second, assign a permanent id number to each server. The last part of the server's IP address (for example, if the server's IP address is 10.1.0.123, the id number is 123) is a good choice, because it's probably unique in your environment.
Then, when a server's grabbing work to do, use these two SQL queries.
UPDATE table
SET working = :this_server_id
WHERE working = 0
AND time_scheduled < CURRENT_TIME
ORDER BY time_scheduled
LIMIT 1
SELECT table_id, whatever, whatever
FROM table
WHERE working = :this_server_id
The first query will consistently grab a batch of rows to work on. If another server process comes in at the same time, it won't ever grab the same rows, because no process can grab rows unless working = 0. Notice that the LIMIT 1 will limit your batch size. You don't have to do this, but you can. I also threw in ORDER BY to process the rows first that have been waiting the longest. That's probably a useful way to do things.
The second query retrieves the information you need to do the work. Don't forget to retrieve the primary key values (I called them table_id) for the rows you're working on.
Then, your server process does whatever it needs to do.
When it's done, it needs to throw the row back into the queue for a later time. To do that, the server process needs to set the time_scheduled to whatever it needs to be, then to set working = 0. So, for example, you could run this query for each row you're processing.
UPDATE table
SET time_scheduled = CURRENT_TIME + INTERVAL 5 MINUTE,
working = 0
WHERE table_id = ?table_id_from_previous_query
That's it.
Except for one thing. In the real world these queuing systems get fouled up sometimes. Server processes crash. Etc. Etc. See Murphy's Law. You need a monitoring query. That's easy in this system.
This query will give a list of all jobs that are more than five minutes overdue, along with the server that's supposed to be working on them.
SELECT working, COUNT(*) stale_jobs
FROM table
WHERE time_scheduled < CURRENT_TIME - INTERVAL 5 MINUTE
GROUP BY WORKING
If this query comes up empty, all is well. If it comes up with lots of jobs with working set to zero, your servers aren't keeping up. If it comes up with jobs with working set to some server's id number, that server is taking a lunch break.
You can reset all the jobs assigned to the server that's gone to lunch with this query, if need be.
UPDATE table
SET working=0
WHERE working=?server_id_at_lunch
By the way, a compound index on (working, time_scheduled) will probably help this perform well.
In my requirements, every user on the website can see a score attached to other users. It gets calculated based of their profile parameters. My score to someone else will be one, but their score to me will be another one.
What I have done so far
Table in the MySQL database like so:
___UserID1___|___UserID2___|___Score___|___Last_Uopdated___
1 | 2 | 45 | 1235686744
2 | 1 | 24 | 1235645332
When a user views someones page, my score class is checking if the record for this pair exists in the database and if not, calculates it and records it. This works fine, because no one will look at absolutely every user page on the site.
Now I need to pull users and sort them based on score. SO I thought, I can create a cronjob, and run it every night, so it will update scores in the database and create them for every pair of user both ways.
Well, problem is I am planing a system for over 500,000 users and I am worried, it will bring my database down and create huge database. So for 500,000 we are talking about 250 billion records... :/
Does anyone know any other way of creating this feature? May be calculation on the fly... or any other way?
If I was in your situation I would create the calculation on the fly. I would generate the scores using your function and then store the values into the database then. That way whenever any user visits any page, the scores are updated. This is an incremental approach rather than trying to run the function on every single combination possible at once. Plus no more database disaster :)
If you have a page that ranks all the users by score, it would be much simpler if you use pagination and use the ORDER BY and OFFSET, LIMIT features of SQL queries instead of fetching all users at once.