Hi

I've just recently started working with NHibernate , and have encountered a few problems. Is it possible that NHibernate might save two objects with same timestamps ?
In my case , I have a .Net 2.0 windows application which has a class called 'Message' which has two properties :
ModifiedDT CreateDT . They are mapped to NHibernate Type : TimeStamp.

NHibernate successfully saves the objects to the databases , but on some occassions ( whch are quite random) , I have found two records being inserted in my Message table with exactly same ModifiedDT and CreateDT values. .. as in same value right down to the millisecond.

The same timestamp records cause a lot of problem as they are later picked and processed by another application in ascending order of their ModifiedDT. The order in which the records get processed is very critical ....and with same timestamps records the order in which they get processed is not certain.

Here are the details :

NHibernate Version : 1.0.2.0

Mapping File :
<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.0">

<class name="Shared.Core.Messaging.Message" table="Message">

<id name="Id" column="MessageId" access="nosetter.camelcase">
<generator class="assigned"/></id>

<property name="Body" column="Message" type="Shared.Core.NHibernate.Messaging.UserTypes.MessageType, Shared.Core" not-null="true" update="false" access="nosetter.camelcase" />

<property name="Status" column="Status" type="Shared.Core.NHibernate.Messaging.UserTypes.MessageStatusType, Shared.Core" not-null="true" access="nosetter.camelcase" />

<property name="ModifiedDT" column="ModifiedDT" type="Timestamp" not-null="true" update="false" access="field.camelcase" />

<property name="CreateDT" column="CreateDT" type="Timestamp" not-null="true" update="false" access="field.camelcase" />

<component name="Header" class="Shared.Core.Messaging.MessageHeader, Shared.Core" access="nosetter.camelcase">

<property name="To" column="AddressTo" type="String" length="100" not-null="false" access="nosetter.camelcase" />

<property name="From" column="AddressFrom" type="String" length="100" not-null="false" access="nosetter.camelcase" />

<property name="Subject" column="Subject" type="String" length="50" not-null="false" access="nosetter.camelcase" />

</component>
/class>
</hibernate-mapping>

The folowing method is responsible for saving the object:



Database : SQL Server 2005

Any help is welcome !
Please let me know if any other details are required

Thanks

Timestamp and property of timestamp type are 2 different things

Timestamp is declared as


Property of timestamp type is declared as



Timestamp is a value for concurrency control. Property of timestamp type has a higher precision than property of DateTime type.

Hope this helps

And I notice that you've put update="false" in ModifiedDT. The value couldn't be updated.

Thanks a lot for the reply.

Yes I understand that timestamp and "property of timestamp" type are seperate things .... Here I have two poperties of type timestamp .

I tried taking off the 'update=false' tag also .. but the problem stil persists.
Anyways the windows application which generates these records is not meant to update it ... hence the tag..

My issue is that there at times records inserted in the Message table with exactly same datetime values for these fields. Below is a representation of the outut of sql server query analyser whidh shows the modifieddt values for some records just generated... ( have manually copy pasted it ..could show only one field ..otherwise it gets all messed up )

modifieddt
-----------------------
2007-02-23 14:13:25.273
2007-02-23 14:13:13.743
2007-02-23 14:13:13.743
2007-02-23 14:11:56.350
2007-02-23 14:11:56.337

There is one thing that I don't understand. Why do you think ModifiedDt has to be unique? And which layer of the system takes the responsibility to make it unique?

Obviously NHibernate can't make ModifiedDt unique for you because NH treats it as a normal property (you defined it as a property of type timestamp). NH only persists the value you've set to the property to database.

Therefore the responsibility goes to you. How do you set values to ModifiedDt?

Machines now are very fast, 1G (1,000,000,000) instructions could be executed in 1 second. Simple math tells us about 1,000,000 instructions would take place within the same millisecond. (Of course 1 value assignment takes more than 1 instruction but definitely less than 1 million)

We require the ModifiedDT values to be unique as these records are later processed by another application ( a windows service actually ) which picks them in ascending order of ModifiedDT . The records have to get
processed in the right order .... which is not possible if I have records with similar ModifiedDt values

The windows application creates the 'Message' record and assigns the values to the createdt and modifieddt fields.

This is the constructor of the Message class :


When we instantiate a Message object , we set its ModifiedDT and CreateDT to DateTime.UtcNow. These values are not altered at any time once they are set.

The DoSave method does get called in something like a loop ... multiple times
So as you say , is it possible that within 1 millisecond , the DoSave method gets called twice ..which results in two Message objects being assigned the same datetime values ..?

You cannot guarantee unique timestamp.
Assuming that you have SQL Server, datetime is limited to 3 milliseconds accuracy.
I would suggest adding an IDENTITY field and working with that, since that is guranteed to work.