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作为这个系列的开篇，本人特此声明，因为本人技术功力有限，且对mongodb源码目前也在研究探索中，可能会对mongodb内部某些实现机制及原作者的意图领会不够精确，因此错误再所难免，希望大家批评指正。另外本文所使用的mongodb源码为1.8 rc1，同时如果有条件的话，大家可以安装vs2010，用C++来编译调试mongodb源码，以便通过运行过程中的数据和流程来验证自己的判断。 VS2010 C++下编译调试MongoDB源码 http://www.cnblogs.com/daizhj/archive/2011/03/07/1973764.html 好了，开始今天的正文吧。 为了理解mongodb整体的运行机制，首先我们需要对其主要运行流程有一个大概的理解，而主入口函数main无疑是最佳突破口。首先我们在VS2010中打开db.sln文件，并打开db.cpp文件，找到主入口函数（位于文件613行），如下：
int main(int argc, char* argv[]) {
static StaticObserver staticObserver; getcurns = ourgetns; po::options_description general_options("General options");//常规选项 #if defined(_WIN32) po::options_description windows_scm_options("Windows Service Control Manager options");//windows服务控制管理选项仅限windows平台 #endif po::options_description replication_options("Replication options");//Replication选项 po::options_description ms_options("Master/slave options");//主从选项 po::options_description rs_options("Replica set options");//Replica设置选项 po::options_description sharding_options("Sharding options");//数据分片选项 po::options_description visible_options("Allowed options");//可见选项 po::options_description hidden_options("Hidden options");//隐藏选项 po::positional_options_description positional_options; general_options.add_options() ("auth", "run with security") ("cpu", "periodically show cpu and iowait utilization") ("dbpath", po::value<string>() , "directory for datafiles") ("diaglog", po::value<int>(), "0=off 1=W 2=R 3=both 7=W+some reads") ("directoryperdb", "each database will be stored in a separate directory") ..... 该方法的开头代码（上面）主要是绑定一个配置操作选项的说明，包括命令行模式下的参数说明，因为内容较长，这里就不做过多描述了，需要说明options_description的是这些内容被放到了boost库（一个C++开源库）的options_description对象中，其类型结构可以理解为key/value模式，主要用于记录一系列的选项描述（符）信息，以便于通过名称查询相应选项信息。同时mongodb将选项大致归为8类，如上所述。 接下说看一下其初始化时命令行参数的操作，如下：
if( argc == 1 )
cout << dbExecCommand << " --help for help and startup options" << endl; { po::variables_map params; string error_message = arg_error_check(argc, argv); if (error_message != "") { cout << error_message << endl << endl; show_help_text(visible_options); return 0; } if ( ! CmdLine::store( argc , argv , visible_options , hidden_options , positional_options , params ) ) return 0; 上面方法对main主函数参数argc，argv及上面的那些选项实例进行存储并以此绑定到params实例上，因为接下来会通过params来设置cmdLine对象（CmdLine类型），并最终以该对象做为最终在mongodb内部标记相应启动命令参数信息的对象。形如：
if (params.count("version")) {
cout << mongodVersion() << endl; printGitVersion(); return 0; } if ( params.count( "dbpath" ) ) { dbpath = params["dbpath"].as<string>(); if ( params.count( "fork" ) && dbpath[0] != '/' ) { // we need to change dbpath if we fork since we change // cwd to "/" // fork only exists on *nix // so '/' is safe dbpath = cmdLine.cwd + "/" + dbpath; } } else { dbpath = "d:/data/db/";//我在此处改了源码 } if ( params.count("directoryperdb")) { directoryperdb = true; } if (params.count("cpu")) { cmdLine.cpu = true; } ...... 当搜集到足够的启动信息（参数）后，mongodb开启执行下面两行代码：
Module::configAll( params );
上面用params来配置加载的模块信息，而就目前而言，mongodb中的模块有两个：其类模式和MMS模块，后者是当mongodb监视服务有效情况下，以后台线程方式（BackgroundJob）运行的程序，类定义如下：
/** Mongo Monitoring Service
if enabled, this runs in the background ands pings mss */ class MMS : public BackgroundJob , Module { .... } 因为相关代码比较简单，这里就不多作说明了，如果大家感兴趣的话，以后会专门写一篇介绍Module，BackgroundJob的文章 。 回到正文，模块实始化完成了，就会运行如下代码：
dataFileSync.go();
这里要说明的是dataFileSync类也派生自BackgroundJob类，而BackgroundJob的功能就是生成一个后台线程并执行相应任务。而当前dataFileSync的任务就是在一段时间后（cmdLine.syncdelay）将内存中的数据flash到磁盘上（因为mongodb使用mmap方式将数据先放入内存中），代码如下：
class DataFileSync : public BackgroundJob {
...... void run() { if( cmdLine.syncdelay == 0 ) log() << "warning: --syncdelay 0 is not recommended and can have strange performance" << endl; else if( cmdLine.syncdelay == 1 ) log() << "--syncdelay 1" << endl; else if( cmdLine.syncdelay != 60 ) log(1) << "--syncdelay " << cmdLine.syncdelay << endl; int time_flushing = 0; while ( ! inShutdown() ) { flushDiagLog(); if ( cmdLine.syncdelay == 0 ) { // in case at some point we add an option to change at runtime sleepsecs(5); continue; } sleepmillis( (long long) std::max(0.0, (cmdLine.syncdelay * 1000) - time_flushing) ); if ( inShutdown() ) { // occasional issue trying to flush during shutdown when sleep interrupted break; } Date_t start = jsTime(); int numFiles = MemoryMappedFile::flushAll( true );//使用系统提供的内存映射文件方法 time_flushing = (int) (jsTime() - start); globalFlushCounters.flushed(time_flushing); log(1) << "flushing mmap took " << time_flushing << "ms " << " for " << numFiles << " files" << endl; } } ...... main主函数完成上面方法后，就会启动侦听方法，开始侦听客户端的链接请求，如下：
initAndListen(cmdLine.port, appsrvPath);
该侦听方法会最终调用db.cpp (467行)的如下方法，我们来看一下该方法做了些什么：
void _initAndListen(int listenPort, const char *appserverLoc = NULL) {
首先是初始化一个名称“initandlisten”线程用于侦听客户端传来的操作信息(可能有误)：
Client::initThread("initandlisten");
接着判断当前系统是32或64位系统？并获取当前进程ID并输出进程ID及数据库路径，端口信息以及当前mongodb及系统信息（这些信息也就是我们在命令行下经常看到的启动mongodb信息）
bool is32bit = sizeof(int*) == 4;
{ #if !defined(_WIN32) pid_t pid = getpid(); #else DWORD pid=GetCurrentProcessId(); #endif Nullstream& l = log(); l << "MongoDB starting : pid=" << pid << " port=" << cmdLine.port << " dbpath=" << dbpath; if( replSettings.master ) l << " master=" << replSettings.master; if( replSettings.slave ) l << " slave=" << (int) replSettings.slave; l << ( is32bit ? " 32" : " 64" ) << "-bit " << endl; } DEV log() << "_DEBUG build (which is slower)" << endl; show_warnings(); log() << mongodVersion() << endl; printGitVersion(); printSysInfo(); 完成这一步之后，接下来mongodb就会对相应路径下的数据文件进行检查，如出现文件错误（文件不存在等）：
stringstream ss;
ss << "dbpath (" << dbpath << ") does not exist"; uassert( 10296 , ss.str().c_str(), boost::filesystem::exists( dbpath ) ); stringstream ss; ss << "repairpath (" << repairpath << ") does not exist"; uassert( 12590 , ss.str().c_str(), boost::filesystem::exists( repairpath ) ); 同时使用"路径锁"方式来移除指定路径下的临时文件夹信息，如下：
acquirePathLock();
remove_all( dbpath + "/_tmp/" ); 接着，mongodb还会启动持久化功能，该功能貌似是1.7版本后引入到系统中的，主要用于解决因系统宕机时，内存中的数据未写入磁盘而造成的数据丢失。其机制主要是通过log方式定时将操作日志（如cud操作等）记录到db的journal文件夹下，这样当系统再次重启时从该文件夹下恢复丢失的（内存）数据。有关这部分内容我会专门写文章加以介绍。
dur::startup();
if( cmdLine.durOptions & CmdLine::DurRecoverOnly ) return; 注：其命令行枚举定义如下
enum { // bits to be ORed
DurDumpJournal = 1, // dump diagnostics on the journal during recovery DurScanOnly = 2, // don't do any real work, just scan and dump if dump specified DurRecoverOnly = 4, // terminate after recovery step DurParanoid = 8, // paranoid mode enables extra checks DurAlwaysCommit = 16 // do a group commit every time the writelock is released }; int durOptions; // --durOptions <n> for debugging 完成这一步之后，系统还会初始化脚本引擎，因为mongodb支持脚本语法做为其操作数据库的语言，如下：
if ( scriptingEnabled ) {
ScriptEngine::setup(); globalScriptEngine->setCheckInterruptCallback( jsInterruptCallback ); globalScriptEngine->setGetInterruptSpecCallback( jsGetInterruptSpecCallback ); } 当这些主要工作做完之后，最后系统会调用下面方法正式启动侦听操作： 
void listen(int port) {
log() << "waiting for connections on port " << port << endl; OurListener l(cmdLine.bind_ip, port); l.setAsTimeTracker(); startReplication(); if ( !noHttpInterface ) boost::thread web( boost::bind(&webServerThread, new RestAdminAccess() /* takes ownership */)); #if(TESTEXHAUST) boost::thread thr(testExhaust); #endif l.initAndListen(); } 注意上面的OurListener类其initAndListen()方法位于message.cpp中，因为mongodb采用message相关类来封装c/s双在的数据和操作：
void Listener::initAndListen() {
checkTicketNumbers(); vector<SockAddr> mine = ipToAddrs(_ip.c_str(), _port); vector<int> socks; SOCKET maxfd = 0; // needed for select() for (vector<SockAddr>::iterator it=mine.begin(), end=mine.end(); it != end; ++it) { SockAddr& me = *it; SOCKET sock = ::socket(me.getType(), SOCK_STREAM, 0); if ( sock == INVALID_SOCKET ) { log() << "ERROR: listen(): invalid socket? " << errnoWithDescription() << endl; } if (me.getType() == AF_UNIX) { #if !defined(_WIN32) if (unlink(me.getAddr().c_str()) == -1) { int x = errno; if (x != ENOENT) { log() << "couldn't unlink socket file " << me << errnoWithDescription(x) << " skipping" << endl; continue; } } #endif } else if (me.getType() == AF_INET6) { // IPv6 can also accept IPv4 connections as mapped addresses (::ffff:127.0.0.1) // That causes a conflict if we don't do set it to IPV6_ONLY const int one = 1; setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, (const char*) &one, sizeof(one)); } prebindOptions( sock ); if ( ::bind(sock, me.raw(), me.addressSize) != 0 ) { int x = errno; log() << "listen(): bind() failed " << errnoWithDescription(x) << " for socket: " << me.toString() << endl; if ( x == EADDRINUSE ) log() << " addr already in use" << endl; closesocket(sock); return; } #if !defined(_WIN32) if (me.getType() == AF_UNIX) { if (chmod(me.getAddr().c_str(), 0777) == -1) { log() << "couldn't chmod socket file " << me << errnoWithDescription() << endl; } ListeningSockets::get()->addPath( me.getAddr() ); } #endif if ( ::listen(sock, 128) != 0 ) { log() << "listen(): listen() failed " << errnoWithDescription() << endl; closesocket(sock); return; } ListeningSockets::get()->add( sock ); socks.push_back(sock); if (sock > maxfd) maxfd = sock; } static long connNumber = 0; struct timeval maxSelectTime; while ( ! inShutdown() ) { fd_set fds[1]; FD_ZERO(fds); for (vector<int>::iterator it=socks.begin(), end=socks.end(); it != end; ++it) { FD_SET(*it, fds); } maxSelectTime.tv_sec = 0; maxSelectTime.tv_usec = 10000; const int ret = select(maxfd+1, fds, NULL, NULL, &maxSelectTime); if (ret == 0) { #if defined(__linux__) _elapsedTime += ( 10000 - maxSelectTime.tv_usec ) / 1000; #else _elapsedTime += 10; #endif continue; } _elapsedTime += ret; // assume 1ms to grab connection. very rough if (ret < 0) { int x = errno; #ifdef EINTR if ( x == EINTR ) { log() << "select() signal caught, continuing" << endl; continue; } #endif if ( ! inShutdown() ) log() << "select() failure: ret=" << ret << " " << errnoWithDescription(x) << endl; return; } for (vector<int>::iterator it=socks.begin(), end=socks.end(); it != end; ++it) { if (! (FD_ISSET(*it, fds))) continue; SockAddr from; int s = accept(*it, from.raw(), &from.addressSize); if ( s < 0 ) { int x = errno; // so no global issues if ( x == ECONNABORTED || x == EBADF ) { log() << "Listener on port " << _port << " aborted" << endl; return; } if ( x == 0 && inShutdown() ) { return; // socket closed } if( !inShutdown() ) log() << "Listener: accept() returns " << s << " " << errnoWithDescription(x) << endl; continue; } if (from.getType() != AF_UNIX) disableNagle(s); if ( _logConnect && ! cmdLine.quiet ) log() << "connection accepted from " << from.toString() << " #" << ++connNumber << endl; accepted(s, from); } } } 上面方法基本上就是一个无限循环（ while ( ! inShutdown() ) ）的侦听服务端，它调用操作系统的底层socket api接口，并将侦听到的结果使用accepted（）方法进行接收。这里要注意的是因为最终我们使用的是OurListener进行的侦听，所以最终系统会调用OurListener所实现的虚（virtual）方法，如下：
class OurListener : public Listener {
public: OurListener(const string &ip, int p) : Listener(ip, p) { } virtual void accepted(MessagingPort *mp) { if ( ! connTicketHolder.tryAcquire() ) { log() << "connection refused because too many open connections: " << connTicketHolder.used() << " of " << connTicketHolder.outof() << endl; // TODO: would be nice if we notified them... mp->shutdown(); delete mp; return; } try { boost::thread thr(boost::bind(&connThread,mp)); } catch ( boost::thread_resource_error& ) { log() << "can't create new thread, closing connection" << endl; mp->shutdown(); delete mp; } catch ( ... ) { log() << "unkonwn exception starting connThread" << endl; mp->shutdown(); delete mp; } } }; 上面方法中的try{}语句中包含的是boost库中的thread方法，其主要提供了跨操作系统的线程创建方式及相关并行操作（相关信息参数boost官方网站），我们这里只要知道，通过该语句，我们最终用一个线程来运行connThread方法及其所需参数mp即可。下面看一下connThread方法的代码：
void connThread( MessagingPort * inPort ) {
TicketHolderReleaser connTicketReleaser( &connTicketHolder ); /* todo: move to Client object */ LastError *le = new LastError(); lastError.reset(le); inPort->_logLevel = 1; auto_ptr<MessagingPort> dbMsgPort( inPort ); Client& c = Client::initThread("conn", inPort); try { c.getAuthenticationInfo()->isLocalHost = dbMsgPort->farEnd.isLocalHost(); Message m; while ( 1 ) { inPort->clearCounters(); if ( !dbMsgPort->recv(m) ) { if( !cmdLine.quiet ) log() << "end connection " << dbMsgPort->farEnd.toString() << endl; dbMsgPort->shutdown(); break; } sendmore: if ( inShutdown() ) { log() << "got request after shutdown()" << endl; break; } lastError.startRequest( m , le ); DbResponse dbresponse; assembleResponse( m, dbresponse, dbMsgPort->farEnd ); if ( dbresponse.response ) { dbMsgPort->reply(m, *dbresponse.response, dbresponse.responseTo); if( dbresponse.exhaust ) { ...出现问题时 } } networkCounter.hit( inPort->getBytesIn() , inPort->getBytesOut() ); m.reset(); } } ...... // thread ending... { Client * c = currentClient.get(); if( c ) c->shutdown(); } globalScriptEngine->threadDone(); } 上面代码主要工作就是不断循环[while ( 1 )]获取当前客户端发来的信息（上面已封装成了message）并将其信息进行分析，并根据相应操作标志位确定当前操作是CRUD或构建索引等[assembleResponse()]，如果一些正常，则向客户端发送应答信息：
void connThread( MessagingPort * inPort ) {
TicketHolderReleaser connTicketReleaser( &connTicketHolder ); /* todo: move to Client object */ LastError *le = new LastError(); lastError.reset(le); inPort->_logLevel = 1; auto_ptr<MessagingPort> dbMsgPort( inPort ); Client& c = Client::initThread("conn", inPort); try { c.getAuthenticationInfo()->isLocalHost = dbMsgPort->farEnd.isLocalHost(); Message m; while ( 1 ) { inPort->clearCounters(); if ( !dbMsgPort->recv(m) ) { if( !cmdLine.quiet ) log() << "end connection " << dbMsgPort->farEnd.toString() << endl; dbMsgPort->shutdown(); break; } sendmore: if ( inShutdown() ) { log() << "got request after shutdown()" << endl; break; } lastError.startRequest( m , le ); DbResponse dbresponse; assembleResponse( m, dbresponse, dbMsgPort->farEnd ); if ( dbresponse.response ) { dbMsgPort->reply(m, *dbresponse.response, dbresponse.responseTo); if( dbresponse.exhaust ) { ...出现问题时 } } networkCounter.hit( inPort->getBytesIn() , inPort->getBytesOut() ); m.reset(); } } ...... // thread ending... { Client * c = currentClient.get(); if( c ) c->shutdown(); } globalScriptEngine->threadDone(); }
运行到这里，main函数的使命就完成了，本来想用一张时序图来大致回顾一下，只有等有时间再补充了。
好了，今天的内容到这里就告一段落了，在接下来的文章中，将会介绍客户端发起查询操作时，Mongodb的执行流程和运行机制。 原文链接:http://www.cnblogs.com/daizhj/archive/2011/03/17/1987311.html |