Is there any possibility to run update at some specific point of time, but in different/parallel session? In the provided example I want some specific update to be run at the time when I run WAITFOR. Currently I have this WAITFOR block to have some time to switch to another SSMS (or other tool) window/tab and run update while it's waiting for 10 secs. Logically the only thing is needed to be done, is that transaction started at this point of time.
EXEC dbo.p_sync_from_accounts_ext_test #enable_snapshot_isolation = 1
, #run_update_flag = NULL
, #run_wait_for_10 = NULL
, #acc = #acc;
WAITFOR DELAY '00:00:10'; -- Execute update in parallel transaction
-- table update should be performed in that parallel transaction
EXEC dbo.p_finish_sync_attributes;
Yes, you can do it.
Method 1: loop-back linked server (linked server that points to your current server) that does not have DTC enabled. Call your SP from your linked server.
Methd 2: create an SQL Server Job and start the job programmatically.
Note that in the first case your update statement must be included in an SP. In the second case it is advisable but not necessary.
Related
For testing, I am trying to simulate a condition in which a query from our web application to our SQL Server backend would timeout. The web application is configured so this happens if the query runs longer than 30 seconds. I felt the easiest way to do this would be to take and hold an exclusive lock on the the table that the web application wants to query. As I understand it, an exclusive lock should prevent any additional locks (even the shared locks taken by a SELECT statement).
I used the following methodology:
CREATE A LONG-HELD LOCK
Open a first query window in SSMS and run
BEGIN TRAN;
SELECT * FROM MyTable WITH (TABLOCKX);
WAITFOR DELAY '00:02:00';
ROLLBACK;
(see https://stackoverflow.com/a/25274225/2824445 )
CONFIRM THE LOCK
I can EXEC sp_lock and see results with ObjId matching MyTable, Type of TAB, Mode of X
TRY TO GET BLOCKED BY THE LOCK
Open a second query window in SSMS and run SELECT * FROM MyTable
I would expect this to sit and wait, not returning any results until after the lock is released by the first query. Instead, the second query returns with full results immediately.
STUFF I TRIED
In the second query window, if I SET TRANSACTION ISOLATION LEVEL SERIALIZABLE, then the second query waits until the first completes as expected. However, the point is to simulate a timeout in our web application, and I do not have any easy way to alter the transaction isolation level of the web application's connections away from the default of READ COMMITTED.
In the first window, I tried modifying the table's values inside the transaction. In this case, when the second query returns immediately, the values it shows are the unmodified values.
Figured it out. We had READ_COMMITTED_SNAPSHOT turned on, which is how the second query was able to return the previous, unmodified values in part 2 of "Stuff I tried". I was able to determine this with SELECT is_read_committed_snapshot_on FROM sys.databases WHERE name = 'MyDatabase'. Once it was turned off with ALTER DATABASE MyDatabase SET READ_COMMITTED_SNAPSHOT OFF, I began to see the expected behavior in which the second query would wait for the first to complete.
We have an ETL pipeline that runs for each CSV uploaded into an storage account (Azure). It runs some transformations on the CSV and writes the outputs to another location, also as CSV, and calls a stored procedure on the database (SQL Azure) which ingests (BULK INSERT) this resulting CSV into a staging table.
This pipeline can have concurrent executions as multiple resources can be uploading files to the storage. Hence, the staging table is getting data inserted pretty often.
Then, we have an scheduled SQL job (Elastic Job) that triggers an SP that moves the data from the staging table into the final table.
At this point, we would want to truncate/empty the staging table so that we do not re-insert them in the next execution of the job.
Problem is, we cannot be sure that between the load from the staging table to the final table and the truncate command, there has not been any new data written into the staging table that could be truncated without first being inserted in to the final table.
Is there a way to lock the staging table while we're copying the data into the final table so that the SP (called from the ETL pipeline) trying to write to it will just wait until the lock is release? Is this achievable by using transactions or maybe some manual lock commands?
If not, what's the best approach to handle this?
I would propose solution with two identical staging tables. Lets name them StageLoading and StageProcessing.
Load process would have following steps:
1. At the beginning both tables are empty.
2. We load some data into StageLoading table (I assume each load is a transaction).
3. When Elastic job starts it will do:
- ALTER TABLE SWITCH to move all data from StageLoading to StageProcessing. It will make StageLoading empty and ready for next loads. It is a metadata operation, so takes miliseconds and it is fully blocking, so will be done between loads.
- load the data from StageProcessing to final tables.
- truncate table StageProcessing.
4. Now we are ready for next Elastic job.
If we try to do SWITCH when StageProcessing is not empty, ALTER will fail and it will mean that last load process failed.
I like the sp_getapplock and use this method myself in few places for its flexibility and that you have full control over the locking logic and wait times.
The only problem that I see is that in your case concurrent processes are not all equal.
You have SP1 that moves data from the staging table into the main table. Your system never tries to run several instances of this SP.
Another SP2 that inserts data into the staging table can be run several times simultaneously and it is fine to do it.
It is easy to implement the locking that would prevent any concurrent run of any combination of SP1 or SP2. In other words, it is easy if the locking logic is the same for SP1 and SP2 and they are treated equal. But, then you can't have several instances of SP2 running simultaneously.
It is not obvious how to implement the locking that would prevent concurrent run of SP1 and SP2, while allowing several instances of SP2 to run simultaneously.
There is another approach that doesn't attempt to prevent concurrent run of SPs, but embraces and expects that simultaneous runs are possible.
One way to do it is to add an IDENTITY column to the staging table. Or an automatically populated datetime if you can guarantee that it is unique and never decreases, which can be tricky. Or rowversion column.
The logic inside SP2 that inserts data into the staging table doesn't change.
The logic inside SP1 that moves data from the staging table into the main table needs to use these identity values.
At first read the current maximum value of identity from the staging table and remember it in a variable, say, #MaxID. All subsequent SELECTs, UPDATEs and DELETEs from the staging table in that SP1 should include a filter WHERE ID <= #MaxID.
This would ensure that if there happen to be a new row added to the staging table while SP1 is running, that row would not be processed and would remain in the staging table until the next run of SP1.
The drawback of this approach is that you can't use TRUNCATE, you need to use DELETE with WHERE ID <= #MaxID.
If you are OK with several instances of SP2 waiting for each other (and SP1), then you can use sp_getapplock similar to the following. I have this code in my stored procedure. You should put this logic into both SP1 and SP2.
I'm not calling sp_releaseapplock explicitly here, because the lock owner is set to Transaction and engine will release the lock automatically when transaction ends.
You don't have to put retry logic in the stored procedure, it can be within external code that runs these stored procedures. In any case, your code should be ready to retry.
CREATE PROCEDURE SP2 -- or SP1
AS
BEGIN
SET NOCOUNT ON;
SET XACT_ABORT ON;
BEGIN TRANSACTION;
BEGIN TRY
-- Maximum number of retries
DECLARE #VarCount int = 10;
WHILE (#VarCount > 0)
BEGIN
SET #VarCount = #VarCount - 1;
DECLARE #VarLockResult int;
EXEC #VarLockResult = sp_getapplock
#Resource = 'StagingTable_app_lock',
-- this resource name should be the same in SP1 and SP2
#LockMode = 'Exclusive',
#LockOwner = 'Transaction',
#LockTimeout = 60000,
-- I'd set this timeout to be about twice the time
-- you expect SP to run normally
#DbPrincipal = 'public';
IF #VarLockResult >= 0
BEGIN
-- Acquired the lock
-- for SP2
-- INSERT INTO StagingTable ...
-- for SP1
-- SELECT FROM StagingTable ...
-- TRUNCATE StagingTable ...
-- don't retry any more
BREAK;
END ELSE BEGIN
-- wait for 5 seconds and retry
WAITFOR DELAY '00:00:05';
END;
END;
COMMIT TRANSACTION;
END TRY
BEGIN CATCH
ROLLBACK TRANSACTION;
-- log error
END CATCH;
END
This code guarantees that only one procedure is working with the staging table at any given moment. There is no concurrency. All other instances will wait.
Obviously, if you try to access the staging table not through these SP1 or SP2 (which try to acquire the lock first), then such access will not be blocked.
Is there a way to lock the staging table while we're copying the data into the final table so that the SP (called from the ETL pipeline) trying to write to it will just wait until the lock is release? Is this achievable by using transactions or maybe some manual lock commands?
It looks you are searching for a mechanism that is wider than a transaction level. SQL Server/Azure SQL DB has one and it is called application lock:
sp_getapplock
Places a lock on an application resource.
Locks placed on a resource are associated with either the current transaction or the current session. Locks associated with the current transaction are released when the transaction commits or rolls back.Locks associated with the session are released when the session is logged out. When the server shuts down for any reason, all locks are released.
Locks can be explicitly released with sp_releaseapplock. When an application calls sp_getapplock multiple times for the same lock resource, sp_releaseapplock must be called the same number of times to release the lock. When a lock is opened with the Transaction lock owner, that lock is released when the transaction is committed or rolled back.
It basically means that your ETL Tool should open single session to DB, acquire the lock and release when finished. Other sessions before trying to do anything should try to acquire the lock(they cannot because it already taken), wait until when it released and continue to work.
Assuming you have a single outbound job
Add an OutboundProcessing BIT DEFAULT 0 to the table
In the job, SET OutboundProcessing = 1 WHERE OutboundProcessing = 0 (claim the rows)
For the ETL, incorporate WHERE OutboundProcessing = 1 in the query that sources the data (transfer the rows)
After the ETL, DELETE FROM TABLE WHERE OutboundProcessing = 1 (remove the rows you transferred)
If the ETL fails, SET OutboundProcessing = 0 WHERE OutboundProcessing = 1
I always prefer to "ID" each file I receive. If you can do this, you can associate the records from a given file throughout your load process. You haven't called out a need for this, but jus sayin.
However, with each file having an identity (just a int/bigint identity value should do) you can then dynamically create as many load tables as you like from a "template" load table.
When a file arrives, create a new load table named with the ID of the file.
Process your data from load to final table.
drop the load table for the file being processed.
This is somewhat similar to the other solution about using 2 tables (load and stage) but even in that solution you are still limited to having 2 files "loaded" (your still only applying one file to the final table though?)
Last, it is not clear if your "Elastic Job" is detached from the actual "load" pipeline/processing or if it is included. Being a job, I assume it is not included, if a job, you can only run a single instance at time? So its not clear why it's important to load multiple files at once if you can only move one from load to final at a time. Why the rush to get files into load?
I have an isolated Azure SQL test database that has no active connections except my development machine through SSMS and a development web application instance. I am the only one using this database.
I am running some tests on a table of ~1M records where we need to do a large UPDATE to data in nearly all of the ~1M records.
DECLARE #BatchSize INT = 1000
WHILE #BatchSize > 0
BEGIN
UPDATE TOP (#BatchSize)
[MyTable]
SET
[Data] = [Data] + ' a change'
WHERE
[Data] IS NOT NULL
SET #BatchSize = ##ROWCOUNT
RAISERROR('Updated %d records', 0, 1, #BatchSize) WITH NOWAIT
END
This query works fine, and I can see my data being updated 1000 records at a time every few seconds.
Performing additional INSERT/UPDATE/DELETE commands on MyTable seem to be somewhat affected by this batch query running, but these operations do execute within a few seconds when ran. I assume this is because locks are being taken on MyTable and my other commands will execute in between the batch query's locks/looping iterations.
This behavior is all expected.
However, every so often while the batch query is running I notice that additional INSERT/UPDATE/DELETE commands on MyTable will no longer execute. They always time out/never finish. I assume some type of lock has occurred on MyTable, but it seems that the lock is never being released. Further, even if I cancel the long-running update batch query I can still no longer run any INSERT/UPDATE/DELETE commands on MyTable. Even after 10-15 minutes of the database sitting stale with nothing happening on it anymore I cannot execute write commands on MyTable. The only way I have found to "free up" the database from whatever is occurring is to scale it up and down to a new pricing tier. I assume that this pricing tier change is recycling/rebooting the instance or something.
I have reproduced this behavior multiple times during my testing today.
What is going on here?
Scaling up/down the tier rollback all open transactions and disconnect server logins.
About what you are seeing it seems is lock escalation. Try to serialize access to the database using sp_getapplock. You can also try lock hints.
We have a clean-up job, which calls a stored procedure, which in turn deletes one day's worth of records for a log table. This job runs every five minutes and usually completes in less than 10 seconds. Sometimes, it take much longer, as long as 15 minutes. During such instances, the log table gets locked and subsequent transactions timeout, till the job completes.
In order to address this, we came up with this solution -
1) Remove the scheduling of the existing job
2) Create a new job, to call the original job
3) Schedule the new job to run every 5 minutes
4) See below code of the new job
DECLARE #JobToRun NVARCHAR(128) = N'OM_EDU_Purge logs'
EXEC msdb.dbo.sp_start_job #JobToRun
WAITFOR DELAY '00:00:20'
IF EXISTS(SELECT 1
FROM msdb.dbo.sysjobs J
JOIN msdb.dbo.sysjobactivity A
ON A.job_id=J.job_id
WHERE J.name=#JobToRun
AND A.run_requested_date IS NOT NULL
AND A.stop_execution_date IS NULL
)
BEGIN -- job is running or finishing (not idle)
EXEC msdb.dbo.sp_stop_job #job_name=#JobToRun
-- could log info, raise error, send email etc here
END
This seems to work fine and stops the job if it is still running after 20 seconds. However, since the job calls a stored procedure, here is my question:
When the job is stopped, will it also terminate the stored procedure that is executing?
I think you query gets stuck because the log table being updated or creating records concurrently with you delete statement. So you might try to lock the table while delete statement execution. update your procedure inside query like this exp: delete from logs with(tablock)
Here, a stored proc is just calling another nested stored proc. So no, stored proc won't be stopped. The control will return to the calling stored proc. You should have sufficient error-handling in the proc to take care of scenarios where the called sproc errors out.
The stored procedure contains 2 dml statements,ie 2 update queries.
There is a need of executing the 2nd statement only after the complete exection of first query.
CREATE PROCEDURE usp_pn
AS
BEGIN
Update [db1].dbo.[table1]
SET
[date1] = DateDiff(MI,[Ar_DateTime],[Departure_DateTime])
Update [db1].dbo.[table1]
SET
[InMinutes] = [date1]+some_calculation
END
GO
here i want to ensure the 2nd update has to run only after the first update completed
So can i write the stored procedure as shown above or is there any modification required??#
Please sugest
I just want to know the execution details:that is there are more than one dml statements in an sp then are these 2 going to run parallely or run one after another..
It might be a basic question but just want to know some thoughts...
Thanks
The statements always run sequentially.
Individual statements can be parallelised but SQL Server will never run different statements in the same batch in parallel.
A great Remus Rusanu article discussing this (and more) is Understanding how SQL Server executes a query
This should settle the often asked question whether statements in a
SQL batch (=> request => task => worker) can execute in parallel: no,
as they are executed on a single thread (=> worker) then each
statement must complete before the next one starts.
For the specific example in your question though (perhaps this is an over simplified example?) I would use a single statement
UPDATE [db1].dbo.[table1]
SET [date1] = DateDiff(MI, [Ar_DateTime], [Departure_DateTime]),
[InMinutes] = DateDiff(MI, [Ar_DateTime], [Departure_DateTime]) + some_calculation