diff --git a/src/Volo.Abp.EntityFrameworkCore/Volo/Abp/Domain/Repositories/EntityFrameworkCore/EfCoreRepository.cs b/src/Volo.Abp.EntityFrameworkCore/Volo/Abp/Domain/Repositories/EntityFrameworkCore/EfCoreRepository.cs
index 5f57032272..8cb88d38f0 100644
--- a/src/Volo.Abp.EntityFrameworkCore/Volo/Abp/Domain/Repositories/EntityFrameworkCore/EfCoreRepository.cs
+++ b/src/Volo.Abp.EntityFrameworkCore/Volo/Abp/Domain/Repositories/EntityFrameworkCore/EfCoreRepository.cs
@@ -89,19 +89,14 @@ namespace Volo.Abp.Domain.Repositories.EntityFrameworkCore
 
         public override TEntity Update(TEntity entity)
         {
-            //TODO: This code is got from UserStore.UpdateAsync and revised Update method based on that, but we should be sure that it's valid
-            //Context.Attach(user);
-            //user.ConcurrencyStamp = Guid.NewGuid().ToString();
-            //Context.Update(user);
-            
-            DbContext.Attach(entity); //TODO: What is different for DbSet.Attach(entity)?
+            DbContext.Attach(entity);
 
             if (entity is IHasConcurrencyStamp)
             {
                 (entity as IHasConcurrencyStamp).ConcurrencyStamp = Guid.NewGuid().ToString(); //TODO: Use IGuidGenerator!
             }
 
-            return DbContext.Update(entity).Entity; //TODO: or DbSet.Update(entity) ?
+            return DbContext.Update(entity).Entity;
         }
 
         public override void Delete(TEntity entity)
diff --git a/src/Volo.Abp/Volo/Abp/Domain/Entities/DefaultIdGenerator.cs b/src/Volo.Abp/Volo/Abp/Domain/Entities/DefaultIdGenerator.cs
new file mode 100644
index 0000000000..bb075812c1
--- /dev/null
+++ b/src/Volo.Abp/Volo/Abp/Domain/Entities/DefaultIdGenerator.cs
@@ -0,0 +1,26 @@
+using System;
+using Volo.Abp.Guids;
+using Volo.DependencyInjection;
+
+namespace Volo.Abp.Domain.Entities
+{
+    public class DefaultIdGenerator : IIdGenerator, ITransientDependency
+    {
+        private readonly IGuidGenerator _guidGenerator;
+
+        public DefaultIdGenerator(IGuidGenerator guidGenerator)
+        {
+            _guidGenerator = guidGenerator;
+        }
+
+        public string GenerateStringId()
+        {
+            return GenerateGuid().ToString("D");
+        }
+
+        public Guid GenerateGuid()
+        {
+            return _guidGenerator.Create();
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Volo.Abp/Volo/Abp/Domain/Entities/IdGenerator.cs b/src/Volo.Abp/Volo/Abp/Domain/Entities/IdGenerator.cs
new file mode 100644
index 0000000000..4c629874c0
--- /dev/null
+++ b/src/Volo.Abp/Volo/Abp/Domain/Entities/IdGenerator.cs
@@ -0,0 +1,11 @@
+using System;
+
+namespace Volo.Abp.Domain.Entities
+{
+    public interface IIdGenerator
+    {
+        string GenerateStringId();
+
+        Guid GenerateGuid();
+    }
+}
diff --git a/src/Volo.Abp/Volo/Abp/Guids/IGuidGenerator.cs b/src/Volo.Abp/Volo/Abp/Guids/IGuidGenerator.cs
new file mode 100644
index 0000000000..fcddc00e10
--- /dev/null
+++ b/src/Volo.Abp/Volo/Abp/Guids/IGuidGenerator.cs
@@ -0,0 +1,15 @@
+using System;
+
+namespace Volo.Abp.Guids
+{
+    /// <summary>
+    /// Used to generate Ids.
+    /// </summary>
+    public interface IGuidGenerator
+    {
+        /// <summary>
+        /// Creates a new <see cref="Guid"/>.
+        /// </summary>
+        Guid Create();
+    }
+}
diff --git a/src/Volo.Abp/Volo/Abp/Guids/SequentialGuidGenerator.cs b/src/Volo.Abp/Volo/Abp/Guids/SequentialGuidGenerator.cs
new file mode 100644
index 0000000000..22db24cde2
--- /dev/null
+++ b/src/Volo.Abp/Volo/Abp/Guids/SequentialGuidGenerator.cs
@@ -0,0 +1,103 @@
+using System;
+using System.Security.Cryptography;
+using Microsoft.Extensions.Options;
+using Volo.Abp.Threading;
+
+namespace Volo.Abp.Guids
+{
+    /* This code is taken from jhtodd/SequentialGuid https://github.com/jhtodd/SequentialGuid/blob/master/SequentialGuid/Classes/SequentialGuid.cs */
+
+    /// <summary>
+    /// Implements <see cref="IGuidGenerator"/> by creating sequential Guids.
+    /// Use <see cref="SequentialGuidGeneratorOptions"/> to configure.
+    /// </summary>
+    public class SequentialGuidGenerator : IGuidGenerator
+    {
+        public SequentialGuidGeneratorOptions Options { get; }
+
+        private static readonly RandomNumberGenerator RandomNumberGenerator = RandomNumberGenerator.Create();
+
+        public SequentialGuidGenerator(IOptions<SequentialGuidGeneratorOptions> options)
+        {
+            Options = options.Value;
+        }
+
+        public Guid Create()
+        {
+            return Create(Options.DefaultSequentialGuidType);
+        }
+
+        public Guid Create(SequentialGuidType guidType)
+        {
+            // We start with 16 bytes of cryptographically strong random data.
+            var randomBytes = new byte[10];
+            RandomNumberGenerator.Locking(r => r.GetBytes(randomBytes));
+
+            // An alternate method: use a normally-created GUID to get our initial
+            // random data:
+            // byte[] randomBytes = Guid.NewGuid().ToByteArray();
+            // This is faster than using RNGCryptoServiceProvider, but I don't
+            // recommend it because the .NET Framework makes no guarantee of the
+            // randomness of GUID data, and future versions (or different
+            // implementations like Mono) might use a different method.
+
+            // Now we have the random basis for our GUID.  Next, we need to
+            // create the six-byte block which will be our timestamp.
+
+            // We start with the number of milliseconds that have elapsed since
+            // DateTime.MinValue.  This will form the timestamp.  There's no use
+            // being more specific than milliseconds, since DateTime.Now has
+            // limited resolution.
+
+            // Using millisecond resolution for our 48-bit timestamp gives us
+            // about 5900 years before the timestamp overflows and cycles.
+            // Hopefully this should be sufficient for most purposes. :)
+            long timestamp = DateTime.UtcNow.Ticks / 10000L;
+
+            // Then get the bytes
+            byte[] timestampBytes = BitConverter.GetBytes(timestamp);
+
+            // Since we're converting from an Int64, we have to reverse on
+            // little-endian systems.
+            if (BitConverter.IsLittleEndian)
+            {
+                Array.Reverse(timestampBytes);
+            }
+
+            byte[] guidBytes = new byte[16];
+
+            switch (guidType)
+            {
+                case SequentialGuidType.SequentialAsString:
+                case SequentialGuidType.SequentialAsBinary:
+
+                    // For string and byte-array version, we copy the timestamp first, followed
+                    // by the random data.
+                    Buffer.BlockCopy(timestampBytes, 2, guidBytes, 0, 6);
+                    Buffer.BlockCopy(randomBytes, 0, guidBytes, 6, 10);
+
+                    // If formatting as a string, we have to compensate for the fact
+                    // that .NET regards the Data1 and Data2 block as an Int32 and an Int16,
+                    // respectively.  That means that it switches the order on little-endian
+                    // systems.  So again, we have to reverse.
+                    if (guidType == SequentialGuidType.SequentialAsString && BitConverter.IsLittleEndian)
+                    {
+                        Array.Reverse(guidBytes, 0, 4);
+                        Array.Reverse(guidBytes, 4, 2);
+                    }
+
+                    break;
+
+                case SequentialGuidType.SequentialAtEnd:
+
+                    // For sequential-at-the-end versions, we copy the random data first,
+                    // followed by the timestamp.
+                    Buffer.BlockCopy(randomBytes, 0, guidBytes, 0, 10);
+                    Buffer.BlockCopy(timestampBytes, 2, guidBytes, 10, 6);
+                    break;
+            }
+
+            return new Guid(guidBytes);
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Volo.Abp/Volo/Abp/Guids/SequentialGuidGeneratorOptions.cs b/src/Volo.Abp/Volo/Abp/Guids/SequentialGuidGeneratorOptions.cs
new file mode 100644
index 0000000000..913a7a059b
--- /dev/null
+++ b/src/Volo.Abp/Volo/Abp/Guids/SequentialGuidGeneratorOptions.cs
@@ -0,0 +1,15 @@
+namespace Volo.Abp.Guids
+{
+    public class SequentialGuidGeneratorOptions
+    {
+        /// <summary>
+        /// Default value: <see cref="SequentialGuidType.SequentialAtEnd"/>.
+        /// </summary>
+        public SequentialGuidType DefaultSequentialGuidType { get; set; }
+
+        public SequentialGuidGeneratorOptions()
+        {
+            DefaultSequentialGuidType = SequentialGuidType.SequentialAtEnd;
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Volo.Abp/Volo/Abp/Guids/SequentialGuidType.cs b/src/Volo.Abp/Volo/Abp/Guids/SequentialGuidType.cs
new file mode 100644
index 0000000000..bc83923359
--- /dev/null
+++ b/src/Volo.Abp/Volo/Abp/Guids/SequentialGuidType.cs
@@ -0,0 +1,28 @@
+using System;
+
+namespace Volo.Abp.Guids
+{
+    /// <summary>
+    /// Describes the type of a sequential GUID value.
+    /// </summary>
+    public enum SequentialGuidType
+    {
+        /// <summary>
+        /// The GUID should be sequential when formatted using the <see cref="Guid.ToString()" /> method.
+        /// Used by MySql and PostgreSql.
+        /// </summary>
+        SequentialAsString,
+
+        /// <summary>
+        /// The GUID should be sequential when formatted using the <see cref="Guid.ToByteArray" /> method.
+        /// Used by Oracle.
+        /// </summary>
+        SequentialAsBinary,
+
+        /// <summary>
+        /// The sequential portion of the GUID should be located at the end of the Data4 block.
+        /// Used by SqlServer.
+        /// </summary>
+        SequentialAtEnd
+    }
+}
\ No newline at end of file
diff --git a/src/Volo.Abp/Volo/Abp/Guids/SimpleGuidGenerator.cs b/src/Volo.Abp/Volo/Abp/Guids/SimpleGuidGenerator.cs
new file mode 100644
index 0000000000..265cd52d81
--- /dev/null
+++ b/src/Volo.Abp/Volo/Abp/Guids/SimpleGuidGenerator.cs
@@ -0,0 +1,15 @@
+using System;
+
+namespace Volo.Abp.Guids
+{
+    /// <summary>
+    /// Implements <see cref="IGuidGenerator"/> by using <see cref="Guid.NewGuid"/>.
+    /// </summary>
+    public class SimpleGuidGenerator : IGuidGenerator
+    {
+        public virtual Guid Create()
+        {
+            return Guid.NewGuid();
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/Volo.Abp/Volo/Abp/Threading/LockExtensions.cs b/src/Volo.Abp/Volo/Abp/Threading/LockExtensions.cs
new file mode 100644
index 0000000000..463cca7cc5
--- /dev/null
+++ b/src/Volo.Abp/Volo/Abp/Threading/LockExtensions.cs
@@ -0,0 +1,68 @@
+using System;
+
+namespace Volo.Abp.Threading
+{
+    /// <summary>
+    /// Extension methods to make locking easier.
+    /// </summary>
+    public static class LockExtensions
+    {
+        /// <summary>
+        /// Executes given <paramref name="action"/> by locking given <paramref name="source"/> object.
+        /// </summary>
+        /// <param name="source">Source object (to be locked)</param>
+        /// <param name="action">Action (to be executed)</param>
+        public static void Locking(this object source, Action action)
+        {
+            lock (source)
+            {
+                action();
+            }
+        }
+
+        /// <summary>
+        /// Executes given <paramref name="action"/> by locking given <paramref name="source"/> object.
+        /// </summary>
+        /// <typeparam name="T">Type of the object (to be locked)</typeparam>
+        /// <param name="source">Source object (to be locked)</param>
+        /// <param name="action">Action (to be executed)</param>
+        public static void Locking<T>(this T source, Action<T> action) where T : class
+        {
+            lock (source)
+            {
+                action(source);
+            }
+        }
+
+        /// <summary>
+        /// Executes given <paramref name="func"/> and returns it's value by locking given <paramref name="source"/> object.
+        /// </summary>
+        /// <typeparam name="TResult">Return type</typeparam>
+        /// <param name="source">Source object (to be locked)</param>
+        /// <param name="func">Function (to be executed)</param>
+        /// <returns>Return value of the <paramref name="func"/></returns>
+        public static TResult Locking<TResult>(this object source, Func<TResult> func)
+        {
+            lock (source)
+            {
+                return func();
+            }
+        }
+
+        /// <summary>
+        /// Executes given <paramref name="func"/> and returns it's value by locking given <paramref name="source"/> object.
+        /// </summary>
+        /// <typeparam name="T">Type of the object (to be locked)</typeparam>
+        /// <typeparam name="TResult">Return type</typeparam>
+        /// <param name="source">Source object (to be locked)</param>
+        /// <param name="func">Function (to be executed)</param>
+        /// <returns>Return value of the <paramnref name="func"/></returns>
+        public static TResult Locking<T, TResult>(this T source, Func<T, TResult> func) where T : class
+        {
+            lock (source)
+            {
+                return func(source);
+            }
+        }
+    }
+}