sql server - Windows Azure SQL Database - Identity Auto increment column skips values

Question

Currently working on an ASP.Net MVC 4 application using Entity Framework 5. Used CodeFirst for initial development phase. But have now disabled the Automatic Migrations and designing new tables directly using SSMS and writing POCO. Everything is working good.

Recently, identified a weird issue in Production. The records in one of the initially designed tables skipped auto-increment identity value by more than 900 numbers. This has happened 3 times within last 3 months. Debugged the application locally but could not reproduce. There isn't any pattern or trend observed.

Model:

public class Enquiry
{
    [Key]
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
    public Int64 EnquiryId { get; set; }

    [Required]
    public int UserId { get; set; }

    [Required]
    public byte Bid { get; set; }

    ...

    [Required]
    public DateTime Created { get; set; }

    [Required]
    public DateTime Modified { get; set; }
}

public class EnquiryDetail
{
    [Key]
    public Int64 EnquiryId { get; set; }

    [Required]
    public int CreditScore { get; set; }

    [Required]
    public byte BidMode { get; set; }

    public virtual Enquiry Enquiry { get; set; }
}

DBContext:

public class EscrowDb : DbContext
{

    public EscrowDb()
        : base("name=DefaultConnection")
    {

    }
    public DbSet<Enquiry> Enquiries { get; set; }
    public DbSet<EnquiryDetail> EnquiryDetails { get; set; }

    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
        modelBuilder.Entity<EnquiryDetail>()
            .HasRequired<Enquiry>(ed => ed.Enquiry)
            .WithRequiredDependent(e => e.EnquiryDetail);
    }
}

Controller:

[Authorize]
public class EnquiryController : Controller
{
    private EscrowDb _db = new EscrowDb();

    [HttpPost]
    [ValidateAntiForgeryToken]
    public ActionResult Create(EnquiryViewModel core)
    {
       var enquiry = new Enquiry();
       // Some code to set properties using passed ViewModel
       ...

       var enquiryDetail = new EnquiryDetail();
       // Some code to set properties using passed ViewModel
       ...

       enquiry.EnquiryDetail = enquiryDetail;

       _db.Enquiries.Add(enquiry);
       _db.SaveChanges();
    }
}

All this code has been working fine so far except the identity value getting skipped sporadically by large gaps of almost 1000 numbers.

Has anybody come across such kind of issue? Please share your thoughts.

Answer 1

You may be out of luck here if you need to eliminate these gaps.

I hit this issue myself as I am developing/testing a new application. I'm intuiting what's happening here in sql azure based on what I've read about sql server 2012. I have not been able to find any documentation about this for sql azure.

From what I've read this is a feature that comes across as a bug IMO. In Sql server 2012 Microsoft added the ability to create sequences. Sequences record what values have been used in blocks of 1000. So lets say your sequence was progressing... 1, 2, 3, 4, 5... and then your sql server restarts. Well the sequence has already saved the fact that the block 1-1000 have already been used so it jumps you to the next 1000.... so your next value are 1001, 1002, 1003, 1004.... This improves performance of inserts when using sequences, but can result in unusual gaps. There is a solution to this for your sequence. When specifying you sequence add the "NOCACHE" parameter so that it doesn't save blocks of 1000 at a time. See here for more documentation.

Where this becomes an issue is that the Identity columns seem to have been changed to use this same paradigm. So when your server, or in this case your sql azure instance restarts you can get large gaps (1000's) in your identity columns because it is caching large blocks as "used". There is a solution to this for sql server 2012. You can specify a startup flag t272 to revert your identity to using the old sql server 2008 r2 paradigm. The problem is that I'm unaware (it may not be possible) of how to specify this in sql Azure. Can't find documentation. See this thread for more details on sql server 2012.

Check the documentation of identity here in the msdn. Specifically the section "Consecutive values after server restart or other failures". Here is what it says:

Consecutive values after server restart or other failures –SQL Server might cache identity values for performance reasons and some of the assigned values can be lost during a database failure or server restart. This can result in gaps in the identity value upon insert. If gaps are not acceptable then the application should use a sequence generator with the NOCACHE option or use their own mechanism to generate key values.

So if you need to have consecutive values you could try specifying a sequence with nocache instead of relying on your identity column. Haven't tried this myself, but sounds like you'll have trouble getting this to work with entity framework.

Sorry if this doesn't help much, but at least it's some info on what your experiencing.