[ mongoDB ] MongoDB 连接池

一.mongoDB中的连接池

刚上手MongoDB，在做应用时，受以前使用关系型数据库的影响，会考虑数据库连接池的问题！

关系型数据库中，我们做连接池无非就是事先建立好N个连接（connection），并构建成一个连接池（connection pool），提供去连接和归还连接等操作。

而在MongoDB中，我们先来看看怎么进行操作，以insert为例：

Mongo m = new Mongo( "localhost" , 27017 );
DB db = m.getDB( "mydb" );
//get collection
DBCollection coll = db.getCollection("testCollection")
//insert
BasicDBObject doc = new BasicDBObject();
...
coll.insert(doc);
[伪代码]

如果套用以前的经验，可能会想偏，引用官方文档的一句话可能会豁然开朗。

Note: The Mongo object instance actually represents a pool of connections to the database; you will only need one object of class Mongo even with multiple threads.  See the concurrency doc page for more information.

The Mongo class is designed to be thread safe and shared among threads. Typically you create only 1 instance for a given DB cluster and use it across your app. If for some reason you decide to create many mongo intances, note that:

• all resource usage limits (max connections, etc) apply per mongo instance
• to dispose of an instance, make sure you call mongo.close() to clean up resources

mongo实例其实已经是一个现成的连接池了，而且线程安全。这个内置的连接池默认初始了10个连接，每一个操作（增删改查等）都会获取一个连接，执行操作后释放连接。

二.连接池的重要参数

内置连接池有多个重要参数，分别是：

• connectionsPerHost：每个主机的连接数
• threadsAllowedToBlockForConnectionMultiplier：线程队列数，它以上面connectionsPerHost值相乘的结果就是线程队列最大值。如果连接线程排满了队列就会抛出“Out of semaphores to get db”错误。
• maxWaitTime:最大等待连接的线程阻塞时间
• connectTimeout：连接超时的毫秒。0是默认和无限
• socketTimeout：socket超时。0是默认和无限
• autoConnectRetry：这个控制是否在一个连接时，系统会自动重试

其设置方式如下：

MongoOptions opt = mongo.getMongoOptions();
opt.connectionsPerHost = 10 ;//poolsize
opt.threadsAllowedToBlockForConnectionMultiplier = 10;
//其他参数类似

详情参考mongoDB API：

Field Summary
boolean	autoConnectRetry            If true, the driver will keep trying to connect to the same server in case that the socket cannot be established.
int	connectionsPerHost            The maximum number of connections allowed per host for this Mongo instance.
int	connectTimeout            The connection timeout in milliseconds.
DBDecoderFactory	dbDecoderFactory            Override the DBCallback factory.
DBEncoderFactory	dbEncoderFactory            Override the encoding factory.
String	description            The description for Mongo instances created with these options.
boolean	fsync            The "fsync" value of the global WriteConcern.
boolean	j            The "j" value of the global WriteConcern.
long	maxAutoConnectRetryTime            The maximum amount of time in MS to spend retrying to open connection to the same server.
int	maxWaitTime            The maximum wait time in ms that a thread may wait for a connection to become available.
boolean	safe            If true the driver will use a WriteConcern of WriteConcern.SAFE for all operations.
boolean	slaveOk            Deprecated. Replaced in MongoDB 2.0/Java Driver 2.7 with ReadPreference.SECONDARY
SocketFactory	socketFactory            sets the socket factory for creating sockets to mongod Default is SocketFactory.getDefault()
boolean	socketKeepAlive            This flag controls the socket keep alive feature that keeps a connection alive through firewalls Socket.setKeepAlive(boolean) Default is false.
int	socketTimeout            The socket timeout in milliseconds It is used for I/O socket read and write operations Socket.setSoTimeout(int) Default is 0 and means no timeout.
int	threadsAllowedToBlockForConnectionMultiplier            this multiplier, multiplied with the connectionsPerHost setting, gives the maximum number of threads that may be waiting for a connection to become available from the pool.
int	w            The "w" value of the global WriteConcern.
int	wtimeout            The "wtimeout" value of the global WriteConcern.

三.连接池实践

package com.bts.dao.mongodb;

import java.net.UnknownHostException;

import com.bts.util.ConfTool;
import com.mongodb.DB;
import com.mongodb.Mongo;
import com.mongodb.MongoException;
import com.mongodb.MongoOptions;

/**
 * @author huangfox
 * @data 2012-4-1
 * @email huangfox009@126.com
 * @desc
 */
public class MongoManager {
	private static Mongo mongo = null;

	private MongoManager() {

	}

	/**
	 * 根据名称获取DB，相当于是连接
	 * 
	 * @param dbName
	 * @return
	 */
	public static DB getDB(String dbName) {
		if (mongo == null) {
			// 初始化
			init();
		}
		return mongo.getDB(dbName);
	}

	/**
	 * 初始化连接池，设置参数。
	 */
	private static void init() {
		String confFilePath = "";
		ConfTool conf = new ConfTool(confFilePath);
		String host = conf.getValue("host");// 主机名
		int port = new Integer(conf.getValue("port"));// 端口
		int poolSize = new Integer(conf.getValue("poolSize"));// 连接数量
		int blockSize = new Integer(conf.getValue("blockSize")); // 等待队列长度
		// 其他参数根据实际情况进行添加
		try {
			mongo = new Mongo(host, port);
			MongoOptions opt = mongo.getMongoOptions();
			opt.connectionsPerHost = poolSize;
			opt.threadsAllowedToBlockForConnectionMultiplier = blockSize;
		} catch (UnknownHostException e) {
			// log error
		} catch (MongoException e) {
			// log error
		}
	}
}