【IT168 技术】之前的文章中介绍了如何基于Mongodb进行关系型数据的分布式存储，有了存储就会牵扯到查询。虽然用普通的方式也可以进行查询，但今天要介绍的是如何使用MONGODB中提供的MapReduce功能进行查询。
今天介绍如何基于sharding机制进行mapreduce查询。在MongoDB的官方文档中，这么一句话：
Sharded Environments
In sharded environments, data processing of map/reduce operations runs in parallel on all shards.
In sharded environments, data processing of map/reduce operations runs in parallel on all shards.
即: map/reduce操作会并行运行在所有的shards上。
下面我们就用之前这篇文章中白搭建的环境来构造mapreduce查询：
首先要说的是，基于sharding的mapreduce与非sharding的数据在返回结构上有一些区别，我目前注意到的主要是不支持定制式的json格式的返回数据，也就是下面方式可能会出现问题：
return { count : total };
注意：上面的情况目前出现在了我的测试环境下，如下图：
就需要改成 return count;
下面是测试代码，首先是按帖子id来查询相应数量(基于分组查询实例方式)：
Code highlighting produced by Actipro CodeHighlighter (freeware)http://www.CodeHighlighter.com/-->public partial class getfile : System.Web.UI.Page
{
public Mongo Mongo { get; set; }
public IMongoDatabase DB
{
get
{
return this.Mongo["dnt_mongodb"];
}
}
/// <summary>
/// Sets up the test environment. You can either override this OnInit to add custom initialization.
/// </summary>
public virtual void Init()
{
string ConnectionString = "Server=10.0.4.85:27017;ConnectTimeout=30000;ConnectionLifetime=300000;MinimumPoolSize=512;MaximumPoolSize=51200;Pooled=true";
if (String.IsNullOrEmpty(ConnectionString))
throw new ArgumentNullException("Connection string not found.");
this.Mongo = new Mongo(ConnectionString);
this.Mongo.Connect();
}
string mapfunction = "function(){\n" +
" if(this._id=='548111') { emit(this._id, 1); } \n" +
"};";
string reducefunction = "function(key, current ){" +
" var count = 0;" +
" for(var i in current) {" +
" count+=current[i];" +
" }" +
" return count ;\n" +
"};";
protected void Page_Load(object sender, EventArgs e)
{
Init();
var mrb = DB["posts1"].MapReduce();//attach_gfstream.files
int groupCount = 0;
using (var mr = mrb.Map(mapfunction).Reduce(reducefunction))
{
foreach (Document doc in mr.Documents)
{
groupCount = int.Parse(doc["value"].ToString());
}
}
this.Mongo.Disconnect();
}
}
{
public Mongo Mongo { get; set; }
public IMongoDatabase DB
{
get
{
return this.Mongo["dnt_mongodb"];
}
}
/// <summary>
/// Sets up the test environment. You can either override this OnInit to add custom initialization.
/// </summary>
public virtual void Init()
{
string ConnectionString = "Server=10.0.4.85:27017;ConnectTimeout=30000;ConnectionLifetime=300000;MinimumPoolSize=512;MaximumPoolSize=51200;Pooled=true";
if (String.IsNullOrEmpty(ConnectionString))
throw new ArgumentNullException("Connection string not found.");
this.Mongo = new Mongo(ConnectionString);
this.Mongo.Connect();
}
string mapfunction = "function(){\n" +
" if(this._id=='548111') { emit(this._id, 1); } \n" +
"};";
string reducefunction = "function(key, current ){" +
" var count = 0;" +
" for(var i in current) {" +
" count+=current[i];" +
" }" +
" return count ;\n" +
"};";
protected void Page_Load(object sender, EventArgs e)
{
Init();
var mrb = DB["posts1"].MapReduce();//attach_gfstream.files
int groupCount = 0;
using (var mr = mrb.Map(mapfunction).Reduce(reducefunction))
{
foreach (Document doc in mr.Documents)
{
groupCount = int.Parse(doc["value"].ToString());
}
}
this.Mongo.Disconnect();
}
}
