SSIS Intermittent variable error: The system canno

问题:

Our SSIS pacakges a structured as one Control package and many child packages (about 30) that are invoked from the control package. The child packages are invoked with Execute Package Task. There is one Execute Package Task per child package. Each Execute Package Task uses File Connection Manager to specify path to the child package dtsx file. There is one File Connection Manager per child package. Each File Connection Manager has an expression defined for ConnectionString property. This expression looks like this:

@[Template::FolderPackages]+"MyPackage.dtsx" 

The file name is different for each package. The variable (FolderPackages) is specified in the SSIS package configuration file.

The error that is generated during run time is

Error 0x80070002 while loading package file "MyPackage.dtsx"

The system cannot find the file specified." The package that fails is different from run to run and sometimes no packages fail at all. This is when run on exactly the same environment/data etc.

I ran FileMon during this error and found out that when the error happens SSIS tries to read the dtsx file from a wrong place, namely from system32. I checked that this is identical to what would happen if @[Template::FolderPackages] variable were empty, but because the very same variable is used for every child package and works for some but doesn't work sometimes for others, I have no expalnation to this fact.

Anything obvious, or time to raise a support call with Microsoft?

回答1:

Are you using Expressions on the SSIS variables directly? Variables with Expressions are calculated each time the variable is referenced by the consuming object which needs to use it. That is where the race condition bug exists, because sometimes the expression doesn't get evaluated if another thread is already evaluating a different variable, and the default value for the variable is provided to the consumer object.

If that matches your design, these two bugs on the connect site discuss the problem, and the workarounds:

https://connect.microsoft.com/SQLServer/feedback/details/332372/ssis-variable-expressions-dont-always-evaluate

A second one at connect.microsoft.com/SQLServer/feedback/details/406534/ssis-2008-variable-expressions-dont-always-evaluate

A summary of workarounds is { - Note the parallel tasks that could run in you SSIS control flow and utilize these expression variables. If you have two tasks side-by-side if each relies upon the same variable, and that variable has an Expression to set its value, then you could hit this. Manually sequentialize such tasks, so that they don't run in parallel. Ie. Add a green arrow on the control flow, so that the tasks occur in order Task1, Task2, Task3, rather than side-by-side on parallel paths and rather than inside the same container with no paths.

• You could avoid variable expressions: Assigning local variables in the required order using a home-made script task that does the same kind of work, so that variables are not evaluated using expressions (ie. the thing which can hit this race condition). In other words, manually assign the variable values at a point in time in your control flow just before they are used. The point of using expressions on variables is to dynamically set a value based on another value whenever it is used, so this acheives a similar design goal but in a manual way.

• Reduce threads to minimize potential: Setting the Dataflow task EngineThreads to 1 and MaxConcurrentExecutables to 1. This will help sequentalize execution of your package to one task at a time, but that has the side effect which may cause slower performance.

• Create and set values on distinct copies of variables at different scope levels in the design, so that they evaluate in different parallel execution scopes and avoid the expression evaluation on parallel threads. Master::Var1, Child1::Var1, Child2::Var1

}

回答2:

A bit of a stab in the dark but...

I've had a similar issue with variables where readonly=false and multiple components were reading the variable at the same time and causing locking issues.

I consistently recreated the problem by running a pair of dataflows that did nothing but reference the variable inside a for loop container and changed the variable to be read only and this resolved the problem.

If you temporarily hardcode the package name does this resolve the issue?

回答3:

Turns out after sending trace info to Microsoft that we are encountering heap corruption. I'll update this question if we get to the bottom of it. The current suggestion is to disable heap lookaside for dtexec.exe.

回答4:

The official answer to this issue is that it is a bug in SQL 2005 and 2008. Many tasks accessing the same variable cause a race condition, and some tasks get the default value for the expression instead of the evaluated value.

The workaround is to ensure that the default value (the value defined in the property sheet for whatever property you are having trouble with) should be the value that will work in your production environment.

This way, when the race condition happens in prod, SSIS will fall back to the package value, which will still work.

In dev? Well you're just going to have to deal with that manually until we get a bug fix from Microsoft.

回答5:

There is a KB article relating to this issue: http://support.microsoft.com/kb/2448991 which states when and where this was fixed.