c++ 连接 MongoDB replica-set

MongoDB的官方文档描述了如何使用连接字符串的正确格式来支持single server和replica-set server，并且声称官方驱动都支持。

http://docs.mongodb.org/manual/reference/connection-string/

但实际上c++驱动仍然沿用旧的方式，而且文档描述相当少。可以通过这个讨论让大家看到MongoDB c++驱动有多讨厌。

http://stackoverflow.com/questions/14629281/c-driver-mongodb-connection-options

我使用的是MongoDB 2.4.1的C++驱动，通过检查源代码，找到了使用的方法。

在mongo-cxx-driver-v2.4.1/src/mongo/client/dbclientinterface.h文件中，在ConnectionString类头部有注释，很奇怪，官方站点的文档中也能看到，不过没有换行，看起来特别恶心。http://api.mongodb.org/cplusplus/2.4.3/classmongo_1_1_connection_string.html#details

    /**
     * ConnectionString handles parsing different ways to connect to mongo and determining method
     * samples:
     *    server
     *    server:port
     *    foo/server:port,server:port   SET
     *    server,server,server          SYNC
     *                                    Warning - you usually don't want "SYNC", it's used 
     *                                    for some special things such as sharding config servers.
     *                                    See syncclusterconnection.h for more info.
     *
     * tyipcal use
     * string errmsg,
     * ConnectionString cs = ConnectionString::parse( url , errmsg );
     * if ( ! cs.isValid() ) throw "bad: " + errmsg;
     * DBClientBase * conn = cs.connect( errmsg );
     */
    class ConnectionString {
    public:
        enum ConnectionType { INVALID , MASTER , PAIR , SET , SYNC, CUSTOM };

        ConnectionString() {
            _type = INVALID;
        }

这里SET就是指Replica-Set，用法是通过ConnctionString::parse解析字符串。

那么ConnectionString::parse的源代码呢？在dbclient.cpp文件中：

    ConnectionString ConnectionString::parse( const string& host , string& errmsg ) {

        string::size_type i = host.find( '/' );
        if ( i != string::npos && i != 0) {
            // replica set
            return ConnectionString( SET , host.substr( i + 1 ) , host.substr( 0 , i ) );
        }

        int numCommas = str::count( host , ',' );

        if( numCommas == 0 )
            return ConnectionString( HostAndPort( host ) );

        if ( numCommas == 1 )
            return ConnectionString( PAIR , host );

        if ( numCommas == 2 )
            return ConnectionString( SYNC , host );

        errmsg = (string)"invalid hostname [" + host + "]";
        return ConnectionString(); // INVALID
    }

也就是先找第一个斜线，然后前面的作为replica-set名称，后面的用,分割开作为主机名。

我需要一种通过配置就能自如的从single server到replica-set切换的方式，这很简单，因为唯一的变动仅仅是配置文件中的字符串。

现在先看一下single server的情况，我有配置如下：

		"uri" : "localhost:27017"

然后创建一个自己的mongo_session对象。

shared_ptr<mongo_session> mongo_session_factory::get_session() {
    configuration& config = app_singleton_holder::Instance().config();
    string error_message;
    mongo::ConnectionString host = mongo::ConnectionString::parse(config.mongo_uri, error_message);
    if (!host.isValid()) {
      throw "bad: " + error_message;
    }
    shared_ptr<mongo_session> s(new mongo_session(host));
    return s;
}

我的mongo_session类以前博客中提过，这里修改了一点，看一下.h文件：

#ifndef _MONGO_SESSION_H
#define _MONGO_SESSION_H
#include <string>
#include <mongo/client/connpool.h>

class mongo_session {
 public:
    /**
     * construct ScopedDBConnection instance and keep it as member variable
     */
    mongo_session(mongo::ConnectionString const& host, double socketTimeout=0);

    /**
     * release connction back to pool
     */
    ~mongo_session();
    
    /**
     * Get one connection from pool
     */
    mongo::DBClientBase& get();

 private:
    std::auto_ptr<mongo::ScopedDbConnection> con_;
};


#endif

在看一下.cpp文件：

#include "helper/mongo_session.h"

using namespace mongo;

mongo_session::mongo_session(mongo::ConnectionString const& host, double socketTimeout):
  con_(mongo::ScopedDbConnection::getScopedDbConnection(host, socketTimeout)) {
    
}

mongo_session::~mongo_session(){
  con_->done();
}

DBClientBase& mongo_session::get(){
  return con_->conn();
}

测试一下，能够正常的连接single server。

现在修改一下配置：

		"uri" : "rs1/d1:27017,d2:27017,d3:27017"

rs1是我的replica-set名称

d1, d2,和d3是三个mongodb instances所在的主机名。

27017是端口号。

启动后，看到终端显示信息：

Thu May  9 19:37:56.947 starting new replica set monitor for replica set rs1 with seed of d1:27017,d2:27017,d3:27017
Thu May  9 19:37:56.949 successfully connected to seed d1:27017 for replica set rs1
Thu May  9 19:37:56.950 changing hosts to { 0: "d1:27017", 1: "d3:27017", 2: "d2:27017" } from rs1/
Thu May  9 19:37:56.950 trying to add new host d1:27017 to replica set rs1
Thu May  9 19:37:56.952 successfully connected to new host d1:27017 in replica set rs1
Thu May  9 19:37:56.952 trying to add new host d2:27017 to replica set rs1
Thu May  9 19:37:56.953 successfully connected to new host d2:27017 in replica set rs1
Thu May  9 19:37:56.953 trying to add new host d3:27017 to replica set rs1
Thu May  9 19:37:56.956 successfully connected to new host d3:27017 in replica set rs1
Thu May  9 19:37:56.960 Primary for replica set rs1 changed to d1:27017
Thu May  9 19:37:56.967 replica set monitor for replica set rs1 started, address is rs1/d1:27017,d2:27017,d3:27017
Thu May  9 19:37:56.967 [ReplicaSetMonitorWatcher] starting

上面说明当前连接成功，并且正在使用d1为primary。

测试程序，一切工作正常。

结论是，MongoDB C++的官方文档真恶心，这帮C++程序员也许是天才，但是和规范化相当好的Java社区有老大的距离。只能去看他们的代码和注释才能找到如何使用。