How to avoid "Violation of UNIQUE KEY constraint" when doing LOTS of concurrent INSERTs

c# dapper database sql sql-server


I am performing MANY concurrent SQL INSERT statements which are colliding on a UNIQUE KEY constraint, even though I am also checking for existing records for the given key inside of a single transaction. I am looking for a way to eliminate, or minimize, the amount of collisions I am getting without hurting the performance (too much).


I am working on an ASP.NET MVC4 WebApi project which receives A LOT of HTTP POST requests to INSERT records. It gets about 5K - 10K requests a second. The project's sole responsibility is de-duplicating and aggregating records. It is very write heavy; it has a relatively small amount of read requests; all of which use a Transaction with IsolationLevel.ReadUncommitted.

Database schema

Here is the DB table:

CREATE TABLE [MySchema].[Records] ( 
    RecordType TINYINT NOT NULL, 
    CONSTRAINT [UQ_MySchemaRecords_UserIdRecordTypeOtherId] UNIQUE CLUSTERED ( 
        [UserID], [RecordType], [OtherID] 
        [Id] ASC 

Repository Code

Here is the code for the Upsert method which is causing the Exception:

using System;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using Dapper;

namespace MyProject.DataAccess
    public class MyRepo
        public void Upsert(MyRecord record)
            var dbConnectionString = "MyDbConnectionString";
            using (var connection = new SqlConnection(dbConnectionString))
                using (var transaction = connection.BeginTransaction(IsolationLevel.ReadCommitted))
                        var existingRecord = FindByByUniqueKey(transaction, record.RecordType, record.UserID, record.OtherID);

                        if (existingRecord == null)
                            const string sql = @"INSERT INTO [MySchema].[Records] 
                                                 ([UserID], [RecordType], [OtherID], [TimestampUtc]) 
                                                 VALUES (@UserID, @RecordType, @OtherID, @TimestampUtc) 
                                                 SELECT CAST(SCOPE_IDENTITY() AS BIGINT";
                            var results = transaction.Connection.Query<long>(sql, record, transaction);
                            record.Id = results.Single();
                        else if (existingRecord.TimestampUtc <= record.TimestampUtc)
                            // UPDATE

                    catch (Exception e)
                        throw e;

        // all read-only methods use explicit transactions with IsolationLevel.ReadUncommitted

        private static MyRecord FindByByUniqueKey(SqlTransaction transaction, RecordType recordType, long userID, short? otherID)
            const string sql = @"SELECT * from [MySchema].[Records] 
                                 WHERE [UserID] = @UserID
                                 AND [RecordType] = @RecordType
                                 AND [OtherID] = @OtherID";
            var paramz = new {
                UserID = userID,
                RecordType = recordType,
                OtherID = otherID
            var results = transaction.Connection.Query<MyRecord>(sql, paramz, transaction);
            return results.SingleOrDefault();

    public class MyRecord
        public long ID { get; set; }
        public RecordType RecordType { get; set; }
        public long UserID { get; set; }
        public short? OtherID { get; set; }
        public DateTimeOffset TimestampUtc { get; set; }

    public enum RecordType : byte
        TypeOne = 1,
        TypeTwo = 2,
        TypeThree = 3

The Problem

When the server is under heavy enough load, I am seeing many of these Exceptions occurring:

Violation of UNIQUE KEY constraint 'UQ_MySchemaRecords_UserIdRecordTypeOtherId'. Cannot insert duplicate key in object 'MySchema.Records'. The duplicate key value is (1234567890, 1, 123). The statement has been terminated.

This Exception occurs often, as many as 10 times in a minute.

What I have tried

  • I tried changing the IsolationLevel to Serializable. The Exception occured much less often but still occured. Also, the performance of the code suffered greatly; the system could only handle 2K requests a second. I suspect that this decrease in throughput was actually the cause of the reduced Exceptions so I concluded that this didn't solve my problem.
  • I have considered using the UPDLOCK Table Hint but I don't fully understand how it cooperates with isolation levels or how to apply it to my code. It does seem like it might be the best solution though, from my current understanding.
  • I also tried adding the initial SELECT statement (for existing records) to be part of the INSERT statement, like shown here but this attempt still had the same problem.
  • I tried implementing my Upsert method by using the SQL MERGE statement but this also suffered from the same problem.

My Question(s)

  • Is there anything I can do to prevent this type of UNIQUE key constraint collisions?
  • If I should be using the UPDLOCK table hint (or any other table hint for that matter), how would I add that to my code? Would I add it to the INSERT? The SELECT? Both?

Accepted Answer

Make the validating read take a lock:


This serializes accesses on a single key, allowing for concurrency on all others.

HOLDLOCK ( = SERIALIZABLE) protects a range of values. This ensures a row that doesn't exist continues to not exist so the INSERT succeeds.

UPDLOCK ensures any existing row is not changed or deleted by another concurrent transaction so the UPDATE succeeds.

ROWLOCK encourages the engine to take a row-level lock.

These changes may increase the chances of a deadlock.

Popular Answer

It may be faster to permit and suppress the errors in your scenario than to attempt to eliminate them. If you're consolidating multiple sources synchronously with overlapping data you will need to create a bottleneck somewhere to manage the race condition.

You could create a singleton manager class that held the unique constraints of the records in a hashset so you would automatically drop duplicates when they're added to the set. Records get added prior to submitting to the DB and removed upon statement completion. That way either the hashset eats the duplicate or the existing record check you do at the top of your try detects the committed duplicate record.

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Is this KB legal? Yes, learn why