Question

If a user inserts rows into a table, i would like SQL Server to perform some additional processing – but not in the context of the user’s transaction.

e.g. The user gives read access to a folder:

UPDATE Folders SET ReadAccess = 1
WHERE FolderID = 7

As far as the user is concerned i want that to be the end of the atomic operation. In reality i have to now go find all child files and folders and give them ReadAccess.

EXECUTE SynchronizePermissions

This is a potentially lengthy operation (over 2s). i want this lengthy operation to happen “later“. It can happen 0 seconds later, and before the carbon-unit has a chance to think about it the asynchronous update is done.

How can i run this required operation asychronously when it’s required (i.e. triggered)?

The ideal would be:

CREATE TRIGGER dbo.Folders FOR INSERT, UPDATE, DELETE AS
   EXECUTEASYNCHRONOUS SynchronizePermissions

or

CREATE TRIGGER dbo.Folders FOR INSERT, UPDATE, DELETE AS
   EXECUTE SynchronizePermissions WITH(ASYNCHRONOUS)

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Right now this happens as a trigger:

CREATE TRIGGER dbo.Folders FOR INSERT, UPDATE, DELETE AS
   EXECUTE SynchronizePermissions

and the user is forced to wait the 3 seconds every time they make a change to the Folders table.

i’ve thought about creating a Scheduled Task on the user, that runs every minute, and check for an PermissionsNeedSynchronizing flag:

CREATE TRIGGER dbo.Folders FOR INSERT, UPDATE, DELETE AS
   UPDATE SystemState SET PermissionsNeedsSynchronizing = 1

The scheduled task binary can check for this flag, run if the flag is on:

DECLARE @FlagValue int
SET @FlagValue = 0;

UPDATE SystemState SET @FlagValue = PermissionsNeedsSynchronizing+1
WHERE PermissionsNeedsSynchronizing = 1

IF @FlagValue = 2
BEGIN
   EXECUTE SynchronizePermissions

   UPDATE SystemState SET PermissionsNeedsSynchronizing = 0
   WHERE PermissionsNeedsSynchronizing = 2
END

The problem with a scheduled task is:
– the fastest it can run is every 60 seconds
– it’s suffers from being a polling solution
– it requires an executable

What i’d prefer is a way that SQL Server could trigger the scheduled task:

CREATE TRIGGER dbo.Folders FOR INSERT, UPDATE, DELETE AS
   EXECUTE SynchronizePermissionsAsychronous


CREATE PROCEDURE dbo.SynchronizePermissionsAsychronous AS

   EXECUTE sp_ms_StartWindowsScheduledTask @taskName="SynchronousPermissions"

The problem with this is:
– there is no sp_ms_StartWinodowsScheduledTask system stored procedure

So i’m looking for ideas for better solutions.

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Update: The previous example is a problem, that has has no good solution, for five years now. A problem from 3 years ago, that has no good solution is a table that i need to update a meta-data column after an insert/update. The metadata takes too long to calculate in online transaction processing, but i am ok with it appearing 3 or 5 seconds later:

CREATE TRIGGER dbo.UpdateFundsTransferValues FOR INSERT, UPDATE AS

UPDATE FundsTransfers
SET TotalOrderValue = (SELECT ....[snip]....),
    TotalDropValue = (SELECT ....,[snip]....)
WHERE FundsTransfers.FundsTransferID IN (
    SELECT i.FundsTransferID
    FROM INSERTED i
)

And the problem that i’m having today is a way to asychronously update some metadata after a row has been transitionally inserted or modified:

CREATE TRIGGER dbo.UpdateCDRValue FOR INSERT, UPDATE AS

UPDATE LCDs
SET CDRValue = (SELECT ....[snip]....)
WHERE LCDs.LCDGUID IN (
    SELECT i.LCDGUID
    FROM INSERTED i
)

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Update 2: i’ve thought about creating a native, or managed, dll and using it as an extended stored procedure. The problem with that is:

• you can’t script a binary
• i’m now allowed to do it

Answer 1

Use a queue table, and have a different background process pick things up off the queue and process them. The trigger itself is by definition a part of the user’s transaction – this is precisely why they are often discouraged (or at least people are warned to not use expensive techniques inside triggers).