MongoDB学习笔记~ObjectId主键的设计

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说一些关于ObjectId的事

MongoDB确实是最像关系型数据库的NoSQL，这在它主键设计上可以体现的出来，它并没有采用自动增长主键，因为在分布式服务器之间做数据同步很麻烦，而是采用了一种ObjectId的方式，它生成方便，占用空间比long多了4个字节，（12个字节）在数据表现层面也说的过去，它是一种以时间，机器，进程和自增几个因素组合的方式来体现的，可以近似看成是按时间的先后进行排序的，对于ObjectId的生成我们可以通过MongoDB服务端去获得，或者在客户端也有对它的集成，使用方便，一般情况下，在客户端实体类中只要定义一个ObjectId类型的属性，这个属性就默认被赋上值了，应该说，还是比较方便的，由于它存储是一种字符串，所以，一般客户端，像NoRM都为我们实现了对string类型的隐藏转换，应该说，还是比较友好的！

ObjectId的组成

为何选择十六进制表示法

为什么在ObjectId里，将byte[]数组转为字符串时，使用十六进制而没有使用默认的十进制呢，居 我的研究，它应该是考虑字符串的长度一致性吧，因为byte取值为(0~255)，如果使用默认的十进制那么它的值长度非常不规范，有1位，2位和3位， 而如果使用十六进制表示，它的长度都为2位，2位就可以表示0到255中的任何数字了，0对应0x00，255对应0xFF，呵呵，将它们转为字符串后，即可 以保证数据的完整性，又可以让它看上去长度是一致的，何乐不为呢，哈哈！

漂亮的设计，原自于扎实的基础！

 在C#版的NoRM这样设计ObjectId

        /// <summary>
        /// Generates a byte array ObjectId.
        /// </summary>
        /// <returns>
        /// </returns>
        public static byte[] Generate()
        {
            var oid = new byte[12];
            var copyidx = 0;
            //时间差
            Array.Copy(BitConverter.GetBytes(GenerateTime()), 0, oid, copyidx, 4);
            copyidx += 4;
            //机器码
            Array.Copy(machineHash, 0, oid, copyidx, 3);
            copyidx += 3;
            //进程码
            Array.Copy(procID, 0, oid, copyidx, 2);
            copyidx += 2;
            //自增值
            Array.Copy(BitConverter.GetBytes(GenerateInc()), 0, oid, copyidx, 3);
            return oid;
        }

完整的ObjectId类型源代码

它重写的ToString()方法，为的是实现byte[]到string串之间的类型转换，并且为string和ObjectId对象实现 implicit的隐式类型转换，方便开发人员在实际中最好的使用它们，需要注意的是在byte[]中存储的数据都是以十六进制的形式体现的

    /// <summary>
    /// Represents a Mongo document's ObjectId
    /// </summary>
    [System.ComponentModel.TypeConverter(typeof(ObjectIdTypeConverter))]
    public class ObjectId
    {        
        private string _string;

        /// <summary>
        /// Initializes a new instance of the <see cref="ObjectId"/> class.
        /// </summary>
        public ObjectId()
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="ObjectId"/> class.
        /// </summary>
        /// <param retval="value">
        /// The value.
        /// </param>
        public ObjectId(string value)
            : this(DecodeHex(value))
        {
        }

        /// <summary>
        /// Initializes a new instance of the <see cref="ObjectId"/> class.
        /// </summary>
        /// <param retval="value">
        /// The value.
        /// </param>
        internal ObjectId(byte[] value)
        {
            this.Value = value;
        }

        /// <summary>
        /// Provides an empty ObjectId (all zeros).
        /// </summary>
        public static ObjectId Empty
        {
            get { return new ObjectId("000000000000000000000000"); }
        }

        /// <summary>
        /// Gets the value.
        /// </summary>
        /// <value>The value.</value>
        public byte[] Value { get; private set; }

        /// <summary>
        /// Generates a new unique oid for use with MongoDB Objects.
        /// </summary>
        /// <returns>
        /// </returns>
        public static ObjectId NewObjectId()
        {
            // TODO: generate random-ish bits.
            return new ObjectId { Value = ObjectIdGenerator.Generate() };
        }

        /// <summary>
        /// Tries the parse.
        /// </summary>
        /// <param retval="value">
        /// The value.
        /// </param>
        /// <param retval="id">
        /// The id.
        /// </param>
        /// <returns>
        /// The try parse.
        /// </returns>
        public static bool TryParse(string value, out ObjectId id)
        {
            id = Empty;
            if (value == null || value.Length != 24)
            {
                return false;
            }

            try
            {
                id = new ObjectId(value);
                return true;
            }
            catch (FormatException)
            {
                return false;
            }
        }

        /// <summary>
        /// Implements the operator ==.
        /// </summary>
        /// <param retval="a">A.</param>
        /// <param retval="b">The b.</param>
        /// <returns>The result of the operator.</returns>
        public static bool operator ==(ObjectId a, ObjectId b)
        {
            if (ReferenceEquals(a, b))
            {
                return true;
            }

            if (((object)a == null) || ((object)b == null))
            {
                return false;
            }

            return a.Equals(b);
        }

        /// <summary>
        /// Implements the operator !=.
        /// </summary>
        /// <param retval="a">A.</param>
        /// <param retval="b">The b.</param>
        /// <returns>The result of the operator.</returns>
        public static bool operator !=(ObjectId a, ObjectId b)
        {
            return !(a == b);
        }

        /// <summary>
        /// Returns a hash code for this instance.
        /// </summary>
        /// <returns>
        /// A hash code for this instance, suitable for use in hashing algorithms and data structures like a hash table. 
        /// </returns>
        public override int GetHashCode()
        {
            return this.Value != null ? this.ToString().GetHashCode() : 0;
        }

        /// <summary>
        /// Returns a <see cref="System.String"/> that represents this instance.
        /// </summary>
        /// <returns>
        /// A <see cref="System.String"/> that represents this instance.
        /// </returns>
        public override string ToString()
        {
            if (this._string == null && this.Value != null)
            {
                this._string = BitConverter.ToString(this.Value).Replace("-", string.Empty).ToLower();
            }

            return this._string;
        }

        /// <summary>
        /// Determines whether the specified <see cref="System.Object"/> is equal to this instance.
        /// </summary>
        /// <param retval="o">
        /// The <see cref="System.Object"/> to compare with this instance.
        /// </param>
        /// <returns>
        /// <c>true</c> if the specified <see cref="System.Object"/> is equal to this instance; otherwise, <c>false</c>.
        /// </returns>
        public override bool Equals(object o)
        {
            var other = o as ObjectId;
            return this.Equals(other);
        }

        /// <summary>
        /// Equalses the specified other.
        /// </summary>
        /// <param retval="other">
        /// The other.
        /// </param>
        /// <returns>
        /// The equals.
        /// </returns>
        public bool Equals(ObjectId other)
        {
            return other != null && this.ToString() == other.ToString();
        }

        /// <summary>
        /// Decodes a HexString to bytes.
        /// </summary>
        /// <param retval="val">
        /// The hex encoding string that should be converted to bytes.
        /// </param>
        /// <returns>
        /// </returns>
        protected static byte[] DecodeHex(string val)
        {
            var chars = val.ToCharArray();
            var numberChars = chars.Length;
            var bytes = new byte[numberChars / 2];

            for (var i = 0; i < numberChars; i += 2)
            {
                bytes[i / 2] = Convert.ToByte(new string(chars, i, 2), 16);
            }

            return bytes;
        }

        /// <summary>TODO::Description.</summary>
        public static implicit operator string(ObjectId oid)
        {
            return oid == null ? null : oid.ToString();
        }

        /// <summary>TODO::Description.</summary>
        public static implicit operator ObjectId(String oidString)
        {
            ObjectId retval = ObjectId.Empty;
            if(!String.IsNullOrEmpty(oidString))
            {
                retval = new ObjectId(oidString);
            }
            return retval;
        }
    }

ObjectIdGenerator源代码

它主要实现了ObjectId串生成的规则及方式

    /// <summary>
    /// Shameless-ly ripped off, then slightly altered from samus' implementation on GitHub
    /// http://github.com/samus/mongodb-csharp/blob/f3bbb3cd6757898a19313b1af50eff627ae93c16/MongoDBDriver/ObjectIdGenerator.cs
    /// </summary>
    internal static class ObjectIdGenerator
    {
        /// <summary>
        /// The epoch.
        /// </summary>
        private static readonly DateTime epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);

        /// <summary>
        /// The inclock.
        /// </summary>
        private static readonly object inclock = new object();

        /// <summary>
        /// The inc.
        /// </summary>
        private static int inc;

        /// <summary>
        /// The machine hash.
        /// </summary>
        private static byte[] machineHash;

        /// <summary>
        /// The proc id.
        /// </summary>
        private static byte[] procID;

        /// <summary>
        /// Initializes static members of the <see cref="ObjectIdGenerator"/> class. 
        /// </summary>
        static ObjectIdGenerator()
        {
            GenerateConstants();
        }

        /// <summary>
        /// Generates a byte array ObjectId.
        /// </summary>
        /// <returns>
        /// </returns>
        public static byte[] Generate()
        {
            var oid = new byte[12];
            var copyidx = 0;
            //时间差
            Array.Copy(BitConverter.GetBytes(GenerateTime()), 0, oid, copyidx, 4);
            copyidx += 4;
            //机器码
            Array.Copy(machineHash, 0, oid, copyidx, 3);
            copyidx += 3;
            //进程码
            Array.Copy(procID, 0, oid, copyidx, 2);
            copyidx += 2;
            //自增值
            Array.Copy(BitConverter.GetBytes(GenerateInc()), 0, oid, copyidx, 3);
            return oid;
        }

        /// <summary>
        /// Generates time.
        /// </summary>
        /// <returns>
        /// The time.
        /// </returns>
        private static int GenerateTime()
        {
            var now = DateTime.Now.ToUniversalTime();

            var nowtime = new DateTime(epoch.Year, epoch.Month, epoch.Day, now.Hour, now.Minute, now.Second, now.Millisecond);
            var diff = nowtime - epoch;
            return Convert.ToInt32(Math.Floor(diff.TotalMilliseconds));
        }

        /// <summary>
        /// Generate an increment.
        /// </summary>
        /// <returns>
        /// The increment.
        /// </returns>
        private static int GenerateInc()
        {
            lock (inclock)
            {
                return inc++;
            }
        }

        /// <summary>
        /// Generates constants.
        /// </summary>
        private static void GenerateConstants()
        {
            machineHash = GenerateHostHash();
            procID = BitConverter.GetBytes(GenerateProcId());
        }

        /// <summary>
        /// Generates a host hash.
        /// </summary>
        /// <returns>
        /// </returns>
        private static byte[] GenerateHostHash()
        {
            using (var md5 = MD5.Create())
            {
                var host = Dns.GetHostName();
                return md5.ComputeHash(Encoding.Default.GetBytes(host));
            }
        }

        /// <summary>
        /// Generates a proc id.
        /// </summary>
        /// <returns>
        /// Proc id.
        /// </returns>
        private static int GenerateProcId()
        {
            var proc = Process.GetCurrentProcess();
            return proc.Id;
        }
    }

事实上，通过对NoRm这个MongoDB客户端的学习，让我们的眼界放宽了许多，可能在思考问题时不局限于眼前，对于同一个问题可以会有更多的解决方法了，呵呵！

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