mongodb源代码分析创建db流程分析

mongo/db/catalog/create_collection.cpp中reateCollection构建元数据文档：将集合信息（命名空间、选项、键前缀等）序列化为 BSON 格式，{ ns: "db.conca", ident: "collection-0--8262702921578327518", md: }；
_rs->insertRecord将元数据文档插入到Catalog的记录存储（RecordStore）中，写入到系统表_uri:table:_mdb_catalog中。_mdb_catalog存储集合和索引的元数据信息，_mdb_catalog.wt在数据库文件夹下面：

数据库的信息在哪个文件存储呢？

/mongo/db/catalog/create_collection.cpp是创建集合的核心代码，后面重点分析其中获取数据库的代码。

mongo/db/catalog/create_collection.cpp中_createCollection是命令db.createCollection('conca', { })核心方法。AutoGetOrCreateDb autoDb先获取到对应的数据库。

Status _createCollection(OperationContext* opCtx,const NamespaceString& nss,const CollectionOptions& collectionOptions,const BSONObj& idIndex) {...AutoGetOrCreateDb autoDb(opCtx, nss.db(), MODE_IX);Lock::CollectionLock collLock(opCtx, nss, MODE_X);...});
}

/mongo/db/catalog_raii.cpp中AutoGetOrCreateDb代码，_autoDb.getDb()获取数据库，如果没有获取到db，openDb则进行打开对应的数据库。

AutoGetOrCreateDb::AutoGetOrCreateDb(OperationContext* opCtx,StringData dbName,LockMode mode,Date_t deadline): _autoDb(opCtx, dbName, mode, deadline) {invariant(mode == MODE_IX || mode == MODE_X);_db = _autoDb.getDb();if (!_db) {auto databaseHolder = DatabaseHolder::get(opCtx);_db = databaseHolder->openDb(opCtx, dbName, &_justCreated);}auto dss = DatabaseShardingState::get(opCtx, dbName);auto dssLock = DatabaseShardingState::DSSLock::lockShared(opCtx, dss);dss->checkDbVersion(opCtx, dssLock);
}

 /mongo/db/catalog/database_holder_impl.cpp中openDb代码：

 std::make_unique<DatabaseImpl>(dbname, ++_epoch)创建数据库实现类DatabaseImpl， newDb->init初始化数据库实现类。auto it = _dbs.find(dbname);再次获取是否存在相同名称的数据库，如果存在则返回，如果不存在则设置进去。

Database* DatabaseHolderImpl::openDb(OperationContext* opCtx, StringData ns, bool* justCreated) {const StringData dbname = _todb(ns);invariant(opCtx->lockState()->isDbLockedForMode(dbname, MODE_IX));if (justCreated)*justCreated = false;  // Until proven otherwise.stdx::unique_lock<SimpleMutex> lk(_m);// The following will insert a nullptr for dbname, which will treated the same as a non-// existant database by the get method, yet still counts in getNamesWithConflictingCasing.if (auto db = _dbs[dbname])return db;// We've inserted a nullptr entry for dbname: make sure to remove it on unsuccessful exit.auto removeDbGuard = makeGuard([this, &lk, dbname] {if (!lk.owns_lock())lk.lock();_dbs.erase(dbname);});// Check casing in lock to avoid transient duplicates.auto duplicates = _getNamesWithConflictingCasing_inlock(dbname);uassert(ErrorCodes::DatabaseDifferCase,str::stream() << "db already exists with different case already have: ["<< *duplicates.cbegin() << "] trying to create [" << dbname.toString()<< "]",duplicates.empty());// Do the catalog lookup and database creation outside of the scoped lock, because these may// block.lk.unlock();if (CollectionCatalog::get(opCtx).getAllCollectionUUIDsFromDb(dbname).empty()) {audit::logCreateDatabase(opCtx->getClient(), dbname);if (justCreated)*justCreated = true;}auto newDb = std::make_unique<DatabaseImpl>(dbname, ++_epoch);newDb->init(opCtx);// Finally replace our nullptr entry with the new Database pointer.removeDbGuard.dismiss();lk.lock();auto it = _dbs.find(dbname);// Dropping a database requires a MODE_X lock, so the entry in the `_dbs` map cannot disappear.invariant(it != _dbs.end());if (it->second) {// Creating databases only requires a DB lock in MODE_IX. Thus databases can concurrently// created. If this thread "lost the race", return the database object that was persisted in// the `_dbs` map.return it->second;}it->second = newDb.release();invariant(_getNamesWithConflictingCasing_inlock(dbname.toString()).empty());return it->second;
}

/mongo/db/catalog/database_impl.cpp中DatabaseImpl::init代码：

void DatabaseImpl::init(OperationContext* const opCtx) const {Status status = validateDBName(_name);if (!status.isOK()) {warning() << "tried to open invalid db: " << _name;uasserted(10028, status.toString());}auto& catalog = CollectionCatalog::get(opCtx);for (const auto& uuid : catalog.getAllCollectionUUIDsFromDb(_name)) {auto collection = catalog.lookupCollectionByUUID(uuid);invariant(collection);// If this is called from the repair path, the collection is already initialized.if (!collection->isInitialized())collection->init(opCtx);}// At construction time of the viewCatalog, the CollectionCatalog map wasn't initialized yet,// so no system.views collection would be found. Now that we're sufficiently initialized, reload// the viewCatalog to populate its in-memory state. If there are problems with the catalog// contents as might be caused by incorrect mongod versions or similar, they are found right// away.auto views = ViewCatalog::get(this);Status reloadStatus = views->reload(opCtx, ViewCatalogLookupBehavior::kValidateDurableViews);if (!reloadStatus.isOK()) {warning() << "Unable to parse views: " << redact(reloadStatus)<< "; remove any invalid views from the " << _viewsName<< " collection to restore server functionality." << startupWarningsLog;}
}

DatabaseImpl::init这段代码是 MongoDB 中DatabaseImpl类的init方法实现，主要功能是完成数据库的初始化工作，包括验证数据库名合法性、初始化集合以及加载视图目录等。