CODEWITHSUNDEEP
asp.netasp.net-mvcmultithreadingconcurrencymdriven
Kjell Rilbe
Kjell Rilbe

Reputation: 1509

MDriven ECO_ID duplicates

We appear to have a problem with MDriven generating the same ECO_ID for multiple objects. For the most part it seems to happen in conjunction with unexpected process shutdowns and/or server shutdowns, but it does also happen during normal activity.

Our system consists of one ASP.NET application and one WinForms application. The ASP.NET app is setup in IIS to use a single worker process. We have a mixture of WebForms and MVC, including ApiControllers. We're using a rather old version of the ECO packages: 7.0.0.10021. We're on VS 2017, target framework is 4.7.1.

We have it configured to use 64 bit integers for object id:s. Database is Firebird. SQL configuration is set to use ReadCommitted transaction isolation.

As far as I can tell we have configured EcoSpaceStrategyHandler with EcoSpaceStrategyHandler.SessionStateMode.Never, which should mean that EcoSpaces are not reused at all, right? (Why would I even use EcoSpaceStrategyHandler in this case, instead of just creating EcoSpace normally with the new keyword?)

We have created MasterController : Controller and MasterApiController : ApiController classes that we use for all our controllers. These have a EcoSpace property that simply does this:

if (ecoSpace == null)
{
    if (ecoSpaceStrategyHandler == null)
        ecoSpaceStrategyHandler = new EcoSpaceStrategyHandler(
            EcoSpaceStrategyHandler.SessionStateMode.Never,
            typeof(DiamondsEcoSpace),
            null,
            false
        );
    ecoSpace = (DiamondsEcoSpace)ecoSpaceStrategyHandler.GetEcoSpace();
}
return ecoSpace;

I.e. if no strategy handler has been created, create one specifying no pooling and no session state persisting of eco spaces. Then, if no ecospace has been fetched, fetch one from the strategy handler. Return the ecospace. Is this an acceptable approach? Why would it be better than simply doing this:

if (ecoSpace = null)
    ecoSpace = new DiamondsEcoSpace();
return ecoSpace;

In aspx we have a master page that has an EcoSpaceManager. It has been configured to use a pool but SessionStateMode is Never. It has EnableViewState set to true. Is this acceptable? Does it mean that EcoSpaces will be pooled but inactivated between round trips?

It is possible that we receive multiple incoming API calls in tight succession, so that one API call hasn't been completed before the next one comes in. I assume that this means that multiple instances of MasterApiController can execute simultaneously but in separate threads. There may of course also be MasterController instances executing MVC requests and also the WinForms app may be running some batch job or other.

But as far as I understand id reservation is made at the beginning of any UpdateDatabase call, in this way:

update "ECO_ID" set "BOLD_ID" = "BOLD_ID" + :N;
select "BOLD_ID" from "ECO_ID";

If the returned value is K, this will reserve N new id:s ranging from K - N to K - 1. Using ReadCommitted transactions everywhere should ensure that the update locks the id data row, forcing any concurrent save operations to wait, then fetches the update result without interference from other transactions, then commits. At that point any other pending save operation can proceed with its own id reservation. I fail to see how this could result in the same ID being used for multiple objects.

I should note that it does seem like it sometimes produces id duplicates within one single UpdateDatabase, i.e. when saving a set of new related objects, some of them end up with the same id. I haven't really confirmed this though.

Any ideas what might be going on here? What should I look for?

Upvotes: 1

Views: 103

Answers (1)

Hans Karlsen
Hans Karlsen

Reputation: 2435

The issue is most likely that you use ReadCommitted isolation. This allows for 2 systems to simultaneously start a transaction, read the current value, increase the batch, and then save after each other.

You must use Serializable isolation for key generation; ie only read things not currently in a write operation.

MDriven use 2 settings for isolation level UpdateIsolationLevel and FetchIsolationLevel.

Set your UpdateIsolationLevel to Serializable

Upvotes: 1

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