My database is SQL Server 2005/8. In a booking system we have a limit of 24 bookings on an event. This code in a stored procedure checks:
- that the current user (#UserId) is not already booked on the event (#EventsID)
- that the current event has a current booking list of under 24
- inserts a new booking.
BEGIN TRANSACTION
IF (((select count (*) from dbo.aspnet_UsersEvents with (updlock)
where UserId = #UserId and EventsId = #EventsId) = 0)
AND ((SELECT Count(*) FROM dbo.aspnet_UsersEvents with (updlock)
WHERE EventsId = #EventsId) < 24))
BEGIN
insert into dbo.aspnet_UsersEvents (UserId, EventsId)
Values (#UserId, #EventsId)
END
COMMIT
The problem is that it is not safe. Two users might perform the test simultaneously and conclude they can both book. Both insert a row and we end up with 25 bookings.
Simply enclosing it in a transaction does not work. I tried adding WITH (UPDLOCK) to the selects in the hope that one would take update locks and keep the other out. That does not work.
Three options:
SET TRANSACTION ISOLATION LEVEL REPEATABLE READ
Change the lock hint to WITH (UPDLOCK, HOLDLOCK)
Add a unique constraint to dbo.aspnet_UsersEvents and a TRY/CATCH around the insert.
You can use the following script to confirm that the lock is taken and immediately released if you omit HOLDLOCK. You will also see that the lock is not released (no 'releasing lock reference on KEY' output) when HOLDLOCK is used.
(Gist script)
Just do it in one statement, at READ COMMITTED or higher.
INSERT dbo.aspnet_UsersEvents
(UserId,EventsId)
OUTPUT inserted.UserEventsId -- Or whatever, just getting back one row identifies the insert was successful
SELECT #UserId
, #EventsId
WHERE ( SELECT COUNT (*)
FROM dbo.aspnet_UsersEvents
WHERE UserId = #UserId
AND EventsId = #EventsId ) = 0
AND ( SELECT COUNT(*)
FROM dbo.aspnet_UsersEvents
WHERE EventsId = #EventsId ) < 24
Side note: your SELECT COUNT(*) for duplicate checking seems excessive, personally I'd use NOT EXISTS(SELECT NULL FROM ... WHERE UserID = ..., EventsID = ....
Related
I have a console application which will be running in 3 pc. I want console application 2 & 3 will not be able to access those records which have been selected by console application1.
I found a sample code that shown some way to access table as a queue. in the below SQL working 1 data at a time but my requirement is to work with 25 data. So guide me what to change in below SQL?
I rarely work with sql server. i found code which close to my requirement but what i need to change there to get it work not very sure. so looking for suggestion.
Please see this code
DECLARE #NextID INTEGER
BEGIN TRANSACTION
-- Find the next queued item that is waiting to be processed
SELECT TOP 1 #NextID = ID
FROM MyQueueTable WITH (UPDLOCK, READPAST)
WHERE StateField = 0
ORDER BY ID ASC
-- if we've found one, mark it as being processed
IF #NextId IS NOT NULL
UPDATE MyQueueTable SET Status = 1 WHERE ID = #NextId
COMMIT TRANSACTION
-- If we've got an item from the queue, return to whatever is going to process it
IF #NextId IS NOT NULL
SELECT * FROM MyQueueTable WHERE ID = #NextID
I need to process at a time 25 records. so what I need to change in above code?
Why last select is required?
IF #NextId IS NOT NULL
SELECT * FROM MyQueueTable WHERE ID = #NextID
My first select will return data then last select is required at all? please tell me purpose of last select.
You can use UPDATE with an OUTPUT clause to output the data into a temp table in one statement. This does not require a transaction.
DROP TABLE IF EXISTS #process;
CREATE TABLE #process (ID INT);
-- Find the next queued item that is waiting to be processed
UPDATE mqt
SET Status = 1
OUTPUT inserted.ID
INTO #process (ID)
FROM (
SELECT TOP (25) *
FROM MyQueueTable WITH (READPAST)
WHERE StateField = 0
ORDER BY ID ASC
) AS mqt;
Rather than using a scalar variable, you could create a temp table that you INSERT the top 25 records into.
-- Find the next queued item that is waiting to be processed
DROP TABLE IF EXISTS #process;
CREATE TABLE #process (ID INT);
INSERT INTO #process
SELECT TOP (25) ID
FROM MyQueueTable WITH (UPDLOCK, READPAST)
WHERE StateField = 0
ORDER BY ID ASC;
-- if we've found one, mark it as being processed
IF EXISTS (SELECT TOP (1) ID FROM #process)
BEGIN
UPDATE MyQueueTable
SET Status = 1
FROM MyQueueTable AS mqt
JOIN #process AS p
ON mqt.ID = p.ID;
END;
And a couple of tips on coding practice, if I may:
be sure to end your statements with semicolons (;) to separate them and improve readability (also it will one day be a requirement)
avoid using SELECT * in deployed code. Instead, specify the columns you want to SELECT
I created a stored procedure to implement rate limiting on my API, this is called about 5-10k times a second and each day I'm noticing dupes in the counter table.
It looks up the API key being passed in and then checks the counter table with the ID and date combination using an "UPSERT" and if it finds a result it does an UPDATE [count]+1 and if not it will INSERT a new row.
There is no primary key in the counter table.
Here is the stored procedure:
USE [omdb]
GO
/****** Object: StoredProcedure [dbo].[CheckKey] Script Date: 6/17/2017 10:39:37 PM ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[CheckKey] (
#apikey AS VARCHAR(10)
)
AS
BEGIN
SET NOCOUNT ON;
DECLARE #userID as int
DECLARE #limit as int
DECLARE #curCount as int
DECLARE #curDate as Date = GETDATE()
SELECT #userID = id, #limit = limit FROM [users] WHERE apiKey = #apikey
IF #userID IS NULL
BEGIN
--Key not found
SELECT 'False' as [Response], 'Invalid API key!' as [Reason]
END
ELSE
BEGIN
--Key found
BEGIN TRANSACTION Upsert
MERGE [counter] AS t
USING (SELECT #userID AS ID) AS s
ON t.[ID] = s.[ID] AND t.[date] = #curDate
WHEN MATCHED THEN UPDATE SET t.[count] = t.[count]+1
WHEN NOT MATCHED THEN INSERT ([ID], [date], [count]) VALUES (#userID, #curDate, 1);
COMMIT TRANSACTION Upsert
SELECT #curCount = [count] FROM [counter] WHERE ID = #userID AND [date] = #curDate
IF #limit IS NOT NULL AND #curCount > #limit
BEGIN
SELECT 'False' as [Response], 'Request limit reached!' as [Reason]
END
ELSE
BEGIN
SELECT 'True' as [Response], NULL as [Reason]
END
END
END
I also think some locks are happening after introducing this SP.
The dupes aren't breaking anything, but I'm curious if it's something fundamentally wrong with my code or if I should setup a constraint in the table to prevent this. Thanks
Update 6/23/17: I dropped the MERGE statement and tried using ##ROWCOUNT but it also caused dupes
BEGIN TRANSACTION Upsert
UPDATE [counter] SET [count] = [count]+1 WHERE [ID] = #userID AND [date] = #curDate
IF ##ROWCOUNT = 0 AND ##ERROR = 0
INSERT INTO [counter] ([ID], [date], [count]) VALUES (#userID, #curDate, 1)
COMMIT TRANSACTION Upsert
A HOLDLOCK hint on the update statement will avoid the race condition. To prevent deadlocks, I suggest a clustered composite primary key (or unique index) on ID and date.
The example below incorporates these changes and uses the SET <variable> = <column> = <expression> form of the SET clause to avoid the need for the subsequent SELECT of the final counter value and thereby improve performance.
ALTER PROCEDURE [dbo].[CheckKey]
#apikey AS VARCHAR(10)
AS
SET NOCOUNT ON;
--SET XACT_ABORT ON is a best practice for procs with explcit transactions
SET XACT_ABORT ON;
DECLARE
#userID as int
, #limit as int
, #curCount as int
, #curDate as Date = GETDATE();
BEGIN TRY;
SELECT
#userID = id
, #limit = limit
FROM [users]
WHERE apiKey = #apikey;
IF #userID IS NULL
BEGIN
--Key not found
SELECT 'False' as [Response], 'Invalid API key!' as [Reason];
END
ELSE
BEGIN
--Key found
BEGIN TRANSACTION Upsert;
UPDATE [counter] WITH(HOLDLOCK)
SET #curCount = [count] = [count] + 1
WHERE
[ID] = #userID
AND [date] = #curDate;
IF ##ROWCOUNT = 0
BEGIN
INSERT INTO [counter] ([ID], [date], [count])
VALUES (#userID, #curDate, 1);
END;
IF #limit IS NOT NULL AND #curCount > #limit
BEGIN
SELECT 'False' as [Response], 'Request limit reached!' as [Reason]
END
ELSE
BEGIN
SELECT 'True' as [Response], NULL as [Reason]
END;
COMMIT TRANSACTION Upsert;
END;
END TRY
BEGIN CATCH
IF ##TRANCOUNT > 0 ROLLBACK;
THROW;
END CATCH;
GO
Probably not the answer you're looking for but for a rate-limiting counter I would use a cache like Redis in a middleware before hitting the API. Performance-wise it's pretty great since Redis would have no problem with the load and your DB won’t be impacted.
And if you want to keep a history of hits per api key per day in SQL, run a daily task to import yesterday's counts from Redis to SQL.
The data-set would be small enough to get a Redis instance that would cost literally nothing (or close).
It will be the merge statement getting into a race condition with itself, i.e. your API is getting called by the same client and both times the merge statement finds no row so inserts one. Merge isn't an atomic operation, even though it's reasonable to assume it is. For example see this bug report for SQL 2008, about merge causing deadlocks, the SQL server team said this is by design.
From your post I think the immediate issue is that your clients will be potentially getting​ a small number of free hits on your API. For example if two requests come in and see no row you'll start with two rows with a count of 1 when you'd actually want one row with a count of 2 and the client could end up getting 1 free API hit that day. If three requests crossed over you'd get three rows with a count of 1 and they could get 2 free API hits, etc.
Edit
So as your link suggests you've got two categories of options you could explore, firstly just try and get this working in SQL server, secondly other architectural solutions.
For the SQL option I would do away with the merge and consider pre-populating your clients ahead of time, either nightly or less often for several days at a time, this will leave you a single update instead of the merge/update and insert. Then you can confirm both your update and your select are fully optimised, ie have the necessary index and that they aren't causing scans. Next you could look at tweaking locking so you're only locking at the row level, see this for some more info. For the select you could also look at using NOLOCK which means you could get slightly incorrect data but this shouldn't matter in your case, you'll be using a WHERE which targets a single row always as well.
For the non-SQL options, as your link says you could look at queuing things up, obviously these would be the updates/inserts so your selects would be seeing old data. This may or may not be acceptable depending on how far apart they are although you could have this as an "eventually consistent" solution if you wanted to be strict and charge extra or take off API hits the next day or something. You could also look at caching options to store the counts, this would get more complex if your app is distributed but there are caching solutions for this. If you went with caching you could choose to not persist anything but then you'd potentially give away a load of free hits if your site went down, but you'd probably have bigger issues to worry about then anyway!
At a high level, have you considered pursuing the following scenario?
Restructuring: Set the primary key on on your table to be a composite of (ID, date). Possibly even better, just use the API Key itself instead of the arbitrary ID you're assigning it.
Query A: Do SQL Server's equivalent of "INSERT IGNORE" (it seems there are semantic equivalents for SQL Server based on a Google search) with the values (ID, TODAY(), 1). You'll also want to specify a WHERE clause that checks the ID actually exists in your API/limits table).
Query B: Update the row with (ID, TODAY()) as its primary key, setting count := count + 1, and in the very same query, do an inner join with your limits table, so that in the where clause you can specify that you'll only update the count if the count < limit.
If the majority of your requests are valid API requests or rate-limited requests, I would perform queries in the following order on every request:
Run Query B.
If 0 rows updated:
Run query A.
If 0 rows updated:
Run query B.
If 0 rows updated, reject because of rate limit.
If 1 rows updated, continue.
If 1 rows updated:
continue.
If 1 row updated:
continue.
If the majority of your requests are invalid API requests, I'd do the following:
Run query A.
If 0 rows updated:
Run query B.
If 0 rows updated, reject because of rate limit.
If 1 rows updated, continue.
If 1 rows updated:
continue.
I'm implementing a queue in SQL Server (please no discussions about this) and am running into a race condition issue. The T-SQL of interest is the following:
set transaction isolation level serializable
begin tran
declare #RecordId int
declare #CurrentTS datetime2
set #CurrentTS=CURRENT_TIMESTAMP
select top 1 #RecordId=Id from QueuedImportJobs with (updlock) where Status=#Status and (LeaseTimeout is null or #CurrentTS>LeaseTimeout) order by Id asc
if ##ROWCOUNT> 0
begin
update QueuedImportJobs set LeaseTimeout = DATEADD(mi,5,#CurrentTS), LeaseTicket=newid() where Id=#RecordId
select * from QueuedImportJobs where Id = #RecordId
end
commit tran
RecordId is the PK and there is also an index on Status,LeaseTimeout.
What I'm basically doing is select a record of which the lease happens to be expired, while simultaneously updating the lease time with 5 minutes and setting a new lease ticket.
So the problem is that I'm getting deadlocks when I run this code in parallel using a couple of threads. I've debugged it up to the point where I found out that the update statement sometimes gets executing twice for the same record. Now, I was under the impression that the with (updlock) should prevent this (it also happens with xlock btw, not with tablockx). So it actually look like there is a RangeS-U and a RangeX-X lock on the same range of records, which ought to be impossible.
So what am I missing? I'm thinking it might have something to do with the top 1 clause or that SQL Server does not know that where Id=#RecordId is actually in the locked range?
Deadlock graph:
Table schema (simplified):
It looks like the locks are on different HOBT's. Are there multiple indexes on the table?
If so, the select with (updlock) might only take an update lock on one index.
Why not just:
DECLARE #t TABLE(Id INT);
UPDATE TOP (1) dbo.QueuedImportJobs
SET LeaseTimeout = DATEADD(MINUTE, 5, CURRENT_TIMESTAMP)
OUTPUT inserted.Id INTO #t
WHERE Status = #Status
AND COALESCE(LeaseTimeout, '19000101') < CURRENT_TIMESTAMP;
SELECT <cols> FROM dbo.QueuedImportJobs
WHERE Id IN (SELECT Id FROM #t);
As an aside you might want to have ORDER BY to ensure the selected row is the first one on the queue according to the desired index order. If the index on Id is clustered, this is probably how it will work anyway, but there is no guarantee unless you say so. This will require a slight re-structuring of the query, since you can't apply ORDER BY (or an index hint) directly on an UPDATE, e.g.:
WITH x AS
(
SELECT TOP (1) Id, LeaseTimeout
FROM dbo.QueuedImportJobs
WHERE Status = #Status
AND COALESCE(LeaseTimeout, '19000101') < CURRENT_TIMESTAMP
ORDER BY Id
)
UPDATE x
SET LeaseTimeout = DATEADD(MINUTE, 5, CURRENT_TIMESTAMP)
OUTPUT inserted.id INTO #t;
First off, I want to start by saying I am not an SQL programmer (I'm a C++/Delphi guy), so some of my questions might be really obvious. So pardon my ignorance :o)
I've been charged with writing a script that will update certain tables in a database based on the contents of a CSV file. I have it working it would seem, but I am worried about atomicity for one of the steps:
One of the tables contains only one field - an int which must be incremented each time, but from what I can see is not defined as an identity for some reason. I must create a new row in this table, and insert that row's value into another newly-created row in another table.
This is how I did it (as part of a larger script):
DECLARE #uniqueID INT,
#counter INT,
#maxCount INT
SELECT #maxCount = COUNT(*) FROM tempTable
SET #counter = 1
WHILE (#counter <= #maxCount)
BEGIN
SELECT #uniqueID = MAX(id) FROM uniqueIDTable <----Line 1
INSERT INTO uniqueIDTableVALUES (#uniqueID + 1) <----Line 2
SELECT #uniqueID = #uniqueID + 1
UPDATE TOP(1) tempTable
SET userID = #uniqueID
WHERE userID IS NULL
SET #counter = #counter + 1
END
GO
First of all, am I correct using a "WHILE" construct? I couldn't find a way to achieve this with a simple UPDATE statement.
Second of all, how can I be sure that no other operation will be carried out on the database between Lines 1 and 2 that would insert a value into the uniqueIDTable before I do? Is there a way to "synchronize" operations in SQL Server Express?
Also, keep in mind that I have no control over the database design.
Thanks a lot!
You can do the whole 9 yards in one single statement:
WITH cteUsers AS (
SELECT t.*
, ROW_NUMBER() OVER (ORDER BY userID) as rn
, COALESCE(m.id,0) as max_id
FROM tempTable t WITH(UPDLOCK)
JOIN (
SELECT MAX(id) as id
FROM uniqueIDTable WITH (UPDLOCK)
) as m ON 1=1
WHERE userID IS NULL)
UPDATE cteUsers
SET userID = rn + max_id
OUTPUT INSERTED.userID
INTO uniqueIDTable (id);
You get the MAX(id), lock the uniqueIDTable, compute sequential userIDs for users with NULL userID by using ROW_NUMBER(), update the tempTable and insert the new ids into uniqueIDTable. All in one operation.
For performance you need and index on uniqueIDTable(id) and index on tempTable(userID).
SQL is all about set oriented operations, WHILE loops are the code smell of SQL.
You need a transaction to ensure atomicity and you need to move the select and insert into one statement or do the select with an updlock to prevent two people from running the select at the same time, getting the same value and then trying to insert the same value into the table.
Basically
DECLARE #MaxValTable TABLE (MaxID int)
BEGIN TRANSACTION
BEGIN TRY
INSERT INTO uniqueIDTable VALUES (id)
OUTPUT inserted.id INTO #MaxValTable
SELECT MAX(id) + 1 FROM uniqueIDTable
UPDATE TOP(1) tempTable
SET userID = (SELECT MAXid FROM #MaxValTable)
WHERE userID IS NULL
COMMIT TRANSACTION
END TRY
BEGIN CATCH
ROLLBACK TRANSACTION
RAISERROR 'Error occurred updating tempTable' -- more detail here is good
END CATCH
That said, using an identity would make things far simpler. This is a potential concurrency problem. Is there any way you can change the column to be identity?
Edit: Ensuring that only one connection at a time will be able to insert into the uniqueIDtable. Not going to scale well though.
Edit: Table variable's better than exclusive table lock. If need be, this can be used when inserting users as well.
I've got a stored procedure, which selects 1 record back. the stored procedure could be called from several different applications on different PCs. The idea is that the stored procedure brings back the next record that needs to be processed, and if two applications call the stored proc at the same time, the same record should not be brought back. My query is below, I'm trying to write the query as efficiently as possible (sql 2008). Can it get done more efficiently than this?
CREATE PROCEDURE GetNextUnprocessedRecord
AS
BEGIN
SET NOCOUNT ON;
--ID of record we want to select back
DECLARE #iID BIGINT
-- Find the next processable record, and mark it as dispatched
-- Must be done in a transaction to ensure no other query can get
-- this record between the read and update
BEGIN TRAN
SELECT TOP 1
#iID = [ID]
FROM
--Don't read locked records, only lock the specific record
[MyRecords] WITH (READPAST, ROWLOCK)
WHERE
[Dispatched] is null
ORDER BY
[Received]
--Mark record as picked up for processing
UPDATE
[MyRecords]
SET
[Dispatched] = GETDATE()
WHERE
[ID] = #iID
COMMIT TRAN
--Select back the specific record
SELECT
[ID],
[Data]
FROM
[MyRecords] WITH (NOLOCK, READPAST)
WHERE
[ID] = #iID
END
Using the READPAST locking hint is correct and your SQL looks OK.
I'd add use XLOCK though which is also HOLDLOCK/SERIALIZABLE
...
[MyRecords] WITH (READPAST, ROWLOCK, XLOCK)
...
This means you get the ID, and exclusively lock that row while you carry on and update it.
Edit: add an index on Dispatched and Received columns to make it quicker. If [ID] (I assume it's the PK) is not clustered, INCLUDE [ID]. And filter the index too because it's SQL 2008
You could also use this construct which does it all in one go without XLOCK or HOLDLOCK
UPDATE
MyRecords
SET
--record the row ID
#id = [ID],
--flag doing stuff
[Dispatched] = GETDATE()
WHERE
[ID] = (SELECT TOP 1 [ID] FROM MyRecords WITH (ROWLOCK, READPAST) WHERE Dispatched IS NULL ORDER BY Received)
UPDATE, assign, set in one
You can assign each picker process a unique id, and add columns pickerproc and pickstate to your records. Then
UPDATE MyRecords
SET pickerproc = myproc,
pickstate = 'I' -- for 'I'n process
WHERE Id = (SELECT MAX(Id) FROM MyRecords WHERE pickstate = 'A') -- 'A'vailable
That gets you your record in one atomic step, and you can do the rest of your processing at your leisure. Then you can set pickstate to 'C'omplete', 'E'rror, or whatever when it's resolved.
I think Mitch is referring to another good technique where you create a message-queue table and insert the Ids there. There are several SO threads - search for 'message queue table'.
You can keep MyRecords on a "MEMORY" table for faster processing.