SQLite入门与分析(四)---Page Cache之事务处理(1)

写在前面：从本章开始，将对SQLite的每个模块进行讨论。讨论的顺序按照我阅读SQLite的顺序来进行，由于项目的需要，以及时间关系，不能给出一个完整的计划，但是我会先讨论我认为比较重要的内容。本节讨论SQLite的事务处理技术，事务处理是DBMS中最关键的技术，对SQLite也一样，它涉及到并发控制，以及故障恢复，由于内容较多，分为两节。好了，下面进入正题。

本节通过一个具体的例子来分析SQLite原子提交的实现（基于Version 3.3.6的代码）。
CREATE TABLE episodes( id integer primary key,name text, cid int) ；
插入一条记录：insert into episodes(name,cid) values("cat",1) ;
它经过编译器处理后生成的虚拟机代码如下：
sqlite> explain insert into episodes(name,cid) values("cat",1);
0|Trace|0|0|0|explain insert into episodes(name,cid) values("cat",1);|00|
1|Goto|0|12|0||00|
2|SetNumColumns|0|3|0||00|
3|OpenWrite|0|2|0||00|
4|NewRowid|0|2|0||00|
5|Null|0|3|0||00|
6|String8|0|4|0|cat|00|
7|Integer|1|5|0||00|
8|MakeRecord|3|3|6|dad|00|
9|Insert|0|6|2|episodes|0b|
10|Close|0|0|0||00|
11|Halt|0|0|0||00|
12|Transaction|0|1|0||00|
13|VerifyCookie|0|1|0||00|
14|Transaction|1|1|0||00|
15|VerifyCookie|1|0|0||00|

16|TableLock|0|2|1|episodes|00|

17|Goto|0|2|0||00|

1、初始状态（Initial State）
当一个数据库连接第一次打开时，状态如图所示。图中最右边（“Disk”标注）表示保存在存储设备中的内容。每个方框代表一个扇区。蓝色的块表示这个扇区保存了原始数据。图中中间区域是操作系统的磁盘缓冲区。开始的时候，这些缓存是还没有被使用，因此这些方框是空白的。图中左边区域显示SQLite用户进程的内存。因为这个数据库连接刚刚打开，所以还没有任何数据记录被读入，所以这些内存也是空的。

//事务指令的实现
//p1为数据库文件的索引号---0为main database;1为temporary tables使用的文件
//p2 不为0，一个写事务开始
case OP_Transaction: {
//数据库的索引号
int i = pOp->p1;
//指向数据库对应的btree
Btree *pBt;

assert( i>=0 && i<db->nDb );
assert( (p->btreeMask & (1<<i))!=0 );
//设置btree指针
pBt = db->aDb[i].pBt;

if( pBt ){
//从这里btree开始事务,主要给文件加锁,并设置btree事务状态
rc = sqlite3BtreeBeginTrans(pBt, pOp->p2);

if( rc==SQLITE_BUSY ){
p->pc = pc;
p->rc = rc = SQLITE_BUSY;
goto vdbe_return;
}
if( rc!=SQLITE_OK && rc!=SQLITE_READONLY /* && rc!=SQLITE_BUSY */ ){
goto abort_due_to_error;
}
}
break;
}

//开始一个事务,如果第二个参数不为0,则一个写事务开始,否则是一个读事务
//如果wrflag>=2,一个exclusive事务开始,此时别的连接不能访问数据库
int sqlite3BtreeBeginTrans(Btree *p, int wrflag){
BtShared *pBt = p->pBt;
int rc = SQLITE_OK;

btreeIntegrity(p);

/* If the btree is already in a write-transaction, or it
** is already in a read-transaction and a read-transaction
** is requested, this is a no-op.
*/
//如果b-tree处于一个写事务;或者处于一个读事务,一个读事务又请求,则返回SQLITE_OK
if( p->inTrans==TRANS_WRITE || (p->inTrans==TRANS_READ && !wrflag) ){
return SQLITE_OK;
}

/* Write transactions are not possible on a read-only database */
//写事务不能访问只读数据库
if( pBt->readOnly && wrflag ){
return SQLITE_READONLY;
}

/* If another database handle has already opened a write transaction
** on this shared-btree structure and a second write transaction is
** requested, return SQLITE_BUSY.
*/
//如果数据库已存在一个写事务,则该写事务请求时返回SQLITE_BUSY
if( pBt->inTransaction==TRANS_WRITE && wrflag ){
return SQLITE_BUSY;
}

do {
//如果数据库对应btree的第一个页面还没读进内存
//则把该页面读进内存，数据库也相应的加read lock
if( pBt->pPage1==0 ){
//加read lock，并读页面到内存
rc = lockBtree(pBt);
}

if( rc==SQLITE_OK && wrflag ){
//对数据库文件加RESERVED_LOCK锁
rc = sqlite3pager_begin(pBt->pPage1->aData, wrflag>1);
if( rc==SQLITE_OK ){
rc = newDatabase(pBt);
}
}

if( rc==SQLITE_OK ){
if( wrflag ) pBt->inStmt = 0;
}else{
unlockBtreeIfUnused(pBt);
}
}while( rc==SQLITE_BUSY && pBt->inTransaction==TRANS_NONE &&
sqlite3InvokeBusyHandler(pBt->pBusyHandler) );

if( rc==SQLITE_OK ){
if( p->inTrans==TRANS_NONE ){
//btree的事务数加1
pBt->nTransaction++;
}
//设置btree事务状态
p->inTrans = (wrflag?TRANS_WRITE:TRANS_READ);
if( p->inTrans>pBt->inTransaction ){
pBt->inTransaction = p->inTrans;
}
}

btreeIntegrity(p);
return rc;
}

/*
**获取数据库的写锁,发生以下情况时去除写锁:
** * sqlite3pager_commit() is called.
** * sqlite3pager_rollback() is called.
** * sqlite3pager_close() is called.
** * sqlite3pager_unref() is called to on every outstanding page.
** pData指向数据库的打开的页面,此时并不修改,仅仅只是获取
** 相应的pager,检查它是否处于read-lock状态。
**如果打开的不是临时文件,则打开日志文件.
**如果数据库已经处于写状态，则do nothing
*/
int sqlite3pager_begin(void *pData, int exFlag){
PgHdr *pPg = DATA_TO_PGHDR(pData);
Pager *pPager = pPg->pPager;
int rc = SQLITE_OK;
assert( pPg->nRef>0 );
assert( pPager->state!=PAGER_UNLOCK );
//pager已经处于share状态
if( pPager->state==PAGER_SHARED ){
assert( pPager->aInJournal==0 );
if( MEMDB ){
pPager->state = PAGER_EXCLUSIVE;
pPager->origDbSize = pPager->dbSize;
}else{
//对文件加 RESERVED_LOCK
rc = sqlite3OsLock(pPager->fd, RESERVED_LOCK);
if( rc==SQLITE_OK ){
//设置pager的状态
pPager->state = PAGER_RESERVED;
if( exFlag ){
rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
}
}
if( rc!=SQLITE_OK ){
return rc;
}
pPager->dirtyCache = 0;
TRACE2("TRANSACTION %d\n", PAGERID(pPager));
//使用日志,不是临时文件,则打开日志文件
if( pPager->useJournal && !pPager->tempFile ){
//为pager打开日志文件,pager应该处于RESERVED或EXCLUSIVE状态
//会向日志文件写入header
rc = pager_open_journal(pPager);
}
}
}
return rc;
}

//创建日志文件,pager应该处于RESERVED或EXCLUSIVE状态
static int pager_open_journal(Pager *pPager){
int rc;
assert( !MEMDB );
assert( pPager->state>=PAGER_RESERVED );
assert( pPager->journalOpen==0 );
assert( pPager->useJournal );
assert( pPager->aInJournal==0 );
sqlite3pager_pagecount(pPager);
//日志文件页面位图
pPager->aInJournal = sqliteMalloc( pPager->dbSize/8 + 1 );
if( pPager->aInJournal==0 ){
rc = SQLITE_NOMEM;
goto failed_to_open_journal;
}
//打开日志文件
rc = sqlite3OsOpenExclusive(pPager->zJournal, &pPager->jfd,
pPager->tempFile);
//日志文件的位置指针
pPager->journalOff = 0;
pPager->setMaster = 0;
pPager->journalHdr = 0;
if( rc!=SQLITE_OK ){
goto failed_to_open_journal;
}
/*一般来说，os此时创建的文件位于磁盘缓存，并没有实际
**存在于磁盘,下面三个操作就是为了把结果写入磁盘,而对于
**windows系统来说,并没有提供相应API，所以实际上没有意义.
*/
//fullSync操作对windows没有意义
sqlite3OsSetFullSync(pPager->jfd, pPager->full_fsync);
sqlite3OsSetFullSync(pPager->fd, pPager->full_fsync);
/* Attempt to open a file descriptor for the directory that contains a file.
**This file descriptor can be used to fsync() the directory
**in order to make sure the creation of a new file is actually written to disk.
*/
sqlite3OsOpenDirectory(pPager->jfd, pPager->zDirectory);
pPager->journalOpen = 1;
pPager->journalStarted = 0;
pPager->needSync = 0;
pPager->alwaysRollback = 0;
pPager->nRec = 0;
if( pPager->errCode ){
rc = pPager->errCode;
goto failed_to_open_journal;
}
pPager->origDbSize = pPager->dbSize;
//写入日志文件的header---24个字节
rc = writeJournalHdr(pPager);

if( pPager->stmtAutoopen && rc==SQLITE_OK ){
rc = sqlite3pager_stmt_begin(pPager);
}
if( rc!=SQLITE_OK && rc!=SQLITE_NOMEM ){
rc = pager_unwritelock(pPager);
if( rc==SQLITE_OK ){
rc = SQLITE_FULL;
}
}
return rc;

failed_to_open_journal:
sqliteFree(pPager->aInJournal);
pPager->aInJournal = 0;
if( rc==SQLITE_NOMEM ){
/* If this was a malloc() failure, then we will not be closing the pager
** file. So delete any journal file we may have just created. Otherwise,
** the system will get confused, we have a read-lock on the file and a
** mysterious journal has appeared in the filesystem.
*/
sqlite3OsDelete(pPager->zJournal);
}else{
sqlite3OsUnlock(pPager->fd, NO_LOCK);
pPager->state = PAGER_UNLOCK;
}
return rc;
}

/*写入日志文件头
**journal header的格式如下:
** - 8 bytes: 标志日志文件的魔数
** - 4 bytes: 日志文件中记录数
** - 4 bytes: Random number used for page hash.
** - 4 bytes: 原来数据库的大小(kb)
** - 4 bytes: 扇区大小512byte
*/
static int writeJournalHdr(Pager *pPager){
//日志文件头
char zHeader[sizeof(aJournalMagic)+16];

int rc = seekJournalHdr(pPager);
if( rc ) return rc;

pPager->journalHdr = pPager->journalOff;
if( pPager->stmtHdrOff==0 ){
pPager->stmtHdrOff = pPager->journalHdr;
}
//设置文件指针指向header之后
pPager->journalOff += JOURNAL_HDR_SZ(pPager);

/* FIX ME:
**
** Possibly for a pager not in no-sync mode, the journal magic should not
** be written until nRec is filled in as part of next syncJournal().
**
** Actually maybe the whole journal header should be delayed until that
** point. Think about this.
*/
memcpy(zHeader, aJournalMagic, sizeof(aJournalMagic));
/* The nRec Field. 0xFFFFFFFF for no-sync journals. */
put32bits(&zHeader[sizeof(aJournalMagic)], pPager->noSync ? 0xffffffff : 0);
/* The random check-hash initialiser */
sqlite3Randomness(sizeof(pPager->cksumInit), &pPager->cksumInit);
put32bits(&zHeader[sizeof(aJournalMagic)+4], pPager->cksumInit);
/* The initial database size */
put32bits(&zHeader[sizeof(aJournalMagic)+8], pPager->dbSize);
/* The assumed sector size for this process */
put32bits(&zHeader[sizeof(aJournalMagic)+12], pPager->sectorSize);
//写入文件头
rc = sqlite3OsWrite(pPager->jfd, zHeader, sizeof(zHeader));

/* The journal header has been written successfully. Seek the journal
** file descriptor to the end of the journal header sector.
*/
if( rc==SQLITE_OK ){
rc = sqlite3OsSeek(pPager->jfd, pPager->journalOff-1);
if( rc==SQLITE_OK ){
rc = sqlite3OsWrite(pPager->jfd, "\000", 1);
}
}
return rc;
}

其实现过程如下图所示：



主要参考：http://www.sqlite.org/atomiccommit.html

转自：http://www.cnblogs.com/hustcat/