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



        ///
/// Generates a byte array ObjectId.
///
///
///
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



    ///
/// Represents a Mongo document's ObjectId
///
[System.ComponentModel.TypeConverter(typeof(ObjectIdTypeConverter))]
public class ObjectId
{        
private string _string;
///
/// Initializes a new instance of the  class.
///
public ObjectId()
{
}
///
/// Initializes a new instance of the  class.
///
///
/// The value.
///
public ObjectId(string value)
: this(DecodeHex(value))
{
}
///
/// Initializes a new instance of the  class.
///
///
/// The value.
///
internal ObjectId(byte[] value)
{
this.Value = value;
}
///
/// Provides an empty ObjectId (all zeros).
///
public static ObjectId Empty
{
get { return new ObjectId("000000000000000000000000"); }
}
///
/// Gets the value.
///
/// The value.
public byte[] Value { get; private set; }
///
/// Generates a new unique oid for use with MongoDB Objects.
///
///
///
public static ObjectId NewObjectId()
{
// TODO: generate random-ish bits.
return new ObjectId { Value = ObjectIdGenerator.Generate() };
}
///
/// Tries the parse.
///
///
/// The value.
///
///
/// The id.
///
///
/// The try parse.
///
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;
}
}
///
/// Implements the operator ==.
///
/// A.
/// The b.
/// The result of the operator.
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);
}
///
/// Implements the operator !=.
///
/// A.
/// The b.
/// The result of the operator.
public static bool operator !=(ObjectId a, ObjectId b)
{
return !(a == b);
}
///
/// Returns a hash code for this instance.
///
///
/// A hash code for this instance, suitable for use in hashing algorithms and data structures like a hash table.
///
public override int GetHashCode()
{
return this.Value != null ? this.ToString().GetHashCode() : 0;
}
///
/// Returns a  that represents this instance.
///
///
/// A  that represents this instance.
///
public override string ToString()
{
if (this._string == null && this.Value != null)
{
this._string = BitConverter.ToString(this.Value).Replace("-", string.Empty).ToLower();
}
return this._string;
}
///
/// Determines whether the specified  is equal to this instance.
///
///
/// The  to compare with this instance.
///
///
/// true if the specified  is equal to this instance; otherwise, false.
///
public override bool Equals(object o)
{
var other = o as ObjectId;
return this.Equals(other);
}
///
/// Equalses the specified other.
///
///
/// The other.
///
///
/// The equals.
///
public bool Equals(ObjectId other)
{
return other != null && this.ToString() == other.ToString();
}
///
/// Decodes a HexString to bytes.
///
///
/// The hex encoding string that should be converted to bytes.
///
///
///
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;
}
/// TODO::Description.
public static implicit operator string(ObjectId oid)
{
return oid == null ? null : oid.ToString();
}
/// TODO::Description.
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



    ///
/// Shameless-ly ripped off, then slightly altered from samus' implementation on GitHub
/// http://github.com/samus/mongodb-csharp/blob/f3bbb3cd6757898a19313b1af50eff627ae93c16/MongoDBDriver/ObjectIdGenerator.cs
///
internal static class ObjectIdGenerator
{
///
/// The epoch.
///
private static readonly DateTime epoch = new DateTime(1970, 1, 1, 0, 0, 0, DateTimeKind.Utc);
///
/// The inclock.
///
private static readonly object inclock = new object();
///
/// The inc.
///
private static int inc;
///
/// The machine hash.
///
private static byte[] machineHash;
///
/// The proc id.
///
private static byte[] procID;
///
/// Initializes static members of the  class.
///
static ObjectIdGenerator()
{
GenerateConstants();
}
///
/// Generates a byte array ObjectId.
///
///
///
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;
}
///
/// Generates time.
///
///
/// The time.
///
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));
}
///
/// Generate an increment.
///
///
/// The increment.
///
private static int GenerateInc()
{
lock (inclock)
{
return inc++;
}
}
///
/// Generates constants.
///
private static void GenerateConstants()
{
machineHash = GenerateHostHash();
procID = BitConverter.GetBytes(GenerateProcId());
}
///
/// Generates a host hash.
///
///
///
private static byte[] GenerateHostHash()
{
using (var md5 = MD5.Create())
{
var host = Dns.GetHostName();
return md5.ComputeHash(Encoding.Default.GetBytes(host));
}
}
///
/// Generates a proc id.
///
///
/// Proc id.
///
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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