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这篇文档最初创建于2004年之初,当时SQLite2还在广泛的使用中,然后为已经熟悉SQLite2的读者写了一篇介绍SQLite3中的新概念的文档。但是目前,大多数的读者很可能凑来没有见过SQLite2,并且只熟悉SQLite3。不过这篇文档依然可以用作SQLite3中数据库锁工作原理的权威参考。
This document was originally created in early 2004 when SQLite version 2
was still in widespread use and was written to introduce
the new concepts of SQLite version 3 to readers who were already familiar
with SQLite version 2. But these days, most readers of this document have
probably never seen SQLite version 2 and are only familiar with SQLite
version 3. Nevertheless, this document continues to serve as an
authoritative reference to how database file locking works in SQLite
version 3.
这篇文档只介绍了老的回滚模式事务机制中的锁。较新的WAL中的锁在别处介绍。
The document only describes locking for the older rollback-mode
transaction mechanism. Locking for the newer write-ahead log or WAL mode
is described separately.
SQLite 3.0.0 版 引入了新的锁和日志机制,用来增强SQLite2中的并发能力,以及减少写入者饿死的问题。这个新的机制还实现了在多个数据库文件上事务的原子提交。本文就是介绍这个新的锁机制。本文的目标读者是一些希望理解或修改pager源码或者验证SQLite3的设计的程序员们。
SQLite Version 3.0.0 introduced a new locking and journaling
mechanism designed to improve concurrency over SQLite version 2
and to reduce the writer starvation
problem. The new mechanism also allows atomic commits of transactions
involving multiple database files.
This document describes the new locking mechanism.
The intended audience is programmers who want to understand and/or modify
the pager code and reviewers working to verify the design
of SQLite version 3.
锁和并发控制是由
pager 模块提供的。pager模块是用来保证SQLite“ACID”的(原子性、一致性、隔离性、持久性)。pager模块可以确保修改总是立刻发生的,要么所有的修改都完成了,要么任何修改都没有执行。两个或多个进程不会使用不兼容的方式同时访问数据库。一旦修改变动就会被永久的写入,直到明确的删除它。pager同时还提供了磁盘文件内容的内存缓存。
Locking and concurrency control are handled by the
pager module.
The pager module is responsible for making SQLite "ACID" (Atomic,
Consistent, Isolated, and Durable). The pager module makes sure changes
happen all at once, that either all changes occur or none of them do,
that two or more processes do not try to access the database
in incompatible ways at the same time, and that once changes have been
written they persist until explicitly deleted. The pager also provides
a memory cache of some of the contents of the disk file.
pager并不关心B-Tree、文本编码、索引等内容的细节。从pager的视点来看数据库只是由一个包含固定大小块的单一文件组成的。每个块叫做“页(page)”,其通常大小为1024字节。这些页从1开始编号。所以数据库的前1024字节叫做“页1”,第二个1024字节叫做“页2”等等。其它具体编码都是库里更高层处理的事情。pager使用某一个模块来和操作系统通讯(例如:os_unix.c、os_win.c),其伟操作系统服务提供了统一的抽象。
The pager is unconcerned
with the details of B-Trees, text encodings, indices, and so forth.
From the point of view of the pager the database consists of
a single file of uniform-sized blocks. Each block is called a
"page" and is usually 1024 bytes in size. The pages are numbered
beginning with 1. So the first 1024 bytes of the database are called
"page 1" and the second 1024 bytes are call "page 2" and so forth. All
other encoding details are handled by higher layers of the library.
The pager communicates with the operating system using one of several
modules
(Examples:
os_unix.c,
os_win.c)
that provides a uniform abstraction for operating system services.
pager模块有效的控制了不同线程或者进程的访问。在全文中,只要你看到词“进程”,就可以在不修改语句中任何词的情况下替换为词“线程”。
The pager module effectively controls access for separate threads, or
separate processes, or both. Throughout this document whenever the
word "process" is written you may substitute the word "thread" without
changing the truth of the statement.
在单独一个进程来看,一个数据库文件可以是五种锁状态中的一个:
From the point of view of a single process, a database file
can be in one of five locking states:
操作系统接口层能够理解和关注上面锁说的这五种锁状态。pager模块只关注其中的四种锁状态。PENDING锁通常是在获取EXCLUSIVE锁过程中的一个临时步奏,所以pager模块不关注PENDING锁。
The operating system interface layer understands and tracks all five
locking states described above.
The pager module only tracks four of the five locking states.
A PENDING lock is always just a temporary
stepping stone on the path to an EXCLUSIVE lock and so the pager module
does not track PENDING locks.
当一个进程想要修改数据库文件时(并且不是在WAL模式),需要首先将未修改的原始数据库内容记录到回滚日志之中。回滚日志是一个于数据库文件位于同一文件夹内的普通磁盘文件,并且命名是在数据库文件名的基础上加一个-journal后缀。回滚日志还会记录数据库的初始尺寸,以便如果数据库文件增长了,那么在回滚的时候能够截短到原始尺寸。
When a process wants to change a database file (and it is not
in WAL mode), it
first records the original unchanged database content
in a rollback journal. The rollback journal is an ordinary
disk file that is always located
in the same directory or folder as the database file and has the
same name as the database file with the addition of a -journal
suffix. The rollback journal also records the initial
size of the database so that if the database file grows it can be truncated
back to its original size on a rollback.
如果SQLite同时操作多个数据库(使用ATTACH命令),那么每个数据库都有自己的回滚日志。但是还有一个单独的聚合日志叫做主日志。主日志不包含用于回滚改动的页数据,而是包含了每个ATTACH数据库的回滚日志的名字。每个独立的数据库回滚日志中也同样包含了主日志的名字。如果没有ATTACH数据库(或者没有ATTACH数据库参与到当前事务中),那么就不会创建主日志文件,回滚日志中会使用一个空字符串来替代记录主日志名字的位置。
If SQLite is working with multiple databases at the same time
(using the ATTACH command) then each database has its own rollback journal.
But there is also a separate aggregate journal
called the master journal.
The master journal does not contain page data used for rolling back
changes. Instead the master journal contains the names of the
individual database rollback journals for each of the ATTACHed databases.
Each of the individual database rollback journals also contain the name
of the master journal.
If there are no ATTACHed databases (or if none of the ATTACHed database
is participating in the current transaction) no master journal is
created and the normal rollback journal contains an empty string
in the place normally reserved for recording the name of the master
journal.
如果一个回滚日志需要执行回滚来恢复数据库的完整性,那么就称为活跃日志。当一个进程正在进行数据库更新的过程中发生了程序或系统崩溃或者意外断电,这阻止了更新的完成,此时就会出现一个活跃日志。活跃日志是一个意外情况。活跃日志是用于从崩溃或意外断电中恢复的。如果一切都正常的运行(没有崩溃或断电)那么你永远也不会得到活跃日志。
A rollback journal is said to be hot
if it needs to be rolled back
in order to restore the integrity of its database.
A hot journal is created when a process is in the middle of a database
update and a program or operating system crash or power failure prevents
the update from completing.
Hot journals are an exception condition.
Hot journals exist to recover from crashes and power failures.
If everything is working correctly
(that is, if there are no crashes or power failures)
you will never get a hot journal.
当没有主日志的参与时,只有存在回滚日志,且日志的头部不是零,且相应的数据库文件没有RESERVED锁时,回滚日志才是活跃的。
当回滚日志中包含了主日志的名字时,只有存在主日志,且对应的数据库文件没有RESERVED锁时回滚日志才是活跃的。
理解什么时候回滚日志时活跃的是非常重要的,所以下面的条目重复了上面的规则:
If no master journal is involved, then
a journal is hot if it exists and has a non-zero header
and its corresponding database file
does not have a RESERVED lock.
If a master journal is named in the file journal, then the file journal
is hot if its master journal exists and there is no RESERVED
lock on the corresponding database file.
It is important to understand when a journal is hot so the
preceding rules will be repeated in bullets:
在读取数据库文件之前,SQLite总要检查一下看看数据库文件是否有相应的活跃日志。如果存在一个活跃日志,那么久需要在读取文件之前进行回滚。通过这种方法,我们可以确保在读取前数据库状态时一致的。
Before reading from a database file, SQLite always checks to see if that
database file has a hot journal. If the file does have a hot journal, then
the journal is rolled back before the file is read. In this way, we ensure
that the database file is in a consistent state before it is read.
当一个进程想要读取数据库文件时,需要按照下列的步奏:
When a process wants to read from a database file, it followed
the following sequence of steps:
在上面的算法成功完成之后,就可以安全的从数据库文件读取内容了。一旦所有的读取都完成,那么就释放SHARED锁。
After the algorithm above completes successfully, it is safe to
read from the database file. Once all reading has completed, the
SHARED lock is dropped.
无效的主日志是指一个再也没有任何用处的主日志。这里并不要求一定要删除主日志,做这件事的唯一目的就是释放磁盘空间。
A stale master journal is a master journal that is no longer being
used for anything. There is no requirement that stale master journals
be deleted. The only reason for doing so is to free up disk space.
如果没有其他日志文件指向这个主日志文件了,那么这个主日志文件就是无效的了。为了计算出一个主日志文件是否时失效的,我们首先要读取主日志文件,来获取所有日志文件的名字。然后检查每个日志文件,如果每个日志文件名所指的日志都还在,并且还指回了主日志文件,那么这个主日志就不是无效的。如果所有日志文件要么不在了,要么指向其它的主日志文件或没有指向主日志,那么这个日志文件就认为时失效的,可以安全的删除。
A master journal is stale if no individual file journals are pointing
to it. To figure out if a master journal is stale, we first read the
master journal to obtain the names of all of its file journals. Then
we check each of those file journals. If any of the file journals named
in the master journal exists and points back to the master journal, then
the master journal is not stale. If all file journals are either missing
or refer to other master journals or no master journal at all, then the
master journal we are testing is stale and can be safely deleted.
要想写一个数据库,进程必须首先获取一个上面所说的SHARED锁(如果有一个活跃日志,有可能还没有完成回滚)。在获取了SHARED锁之后,需要获取一个RESERVED锁。RESERVED锁标志这这个进程在未来的某个时间点打算写入数据库。同一时刻只能有一个进程持有RESERVED锁。不过其他进程在持有RESERVED锁的时候还可以继续读取数据库。
To write to a database, a process must first acquire a SHARED lock
as described above (possibly rolling back incomplete changes if there
is a hot journal).
After a SHARED lock is obtained, a RESERVED lock must be acquired.
The RESERVED lock signals that the process intends to write to the
database at some point in the future. Only one process at a time
can hold a RESERVED lock. But other processes can continue to read
the database while the RESERVED lock is held.
如果想要写入的进程无法获取RESERVED锁,那么就意味着其他进程已经获取了RESERVED锁。这种情况下,写入会失败并返回SQLITE_BUSY。
If the process that wants to write is unable to obtain a RESERVED
lock, it must mean that another process already has a RESERVED lock.
In that case, the write attempt fails and returns SQLITE_BUSY.
在获取了RESERVED锁之后,想要写入的进程会创建一个回滚日志。日志的头部会初始化为数据库文件的原始尺寸值。日志头部的空间还要保存主日志的名字,也可能主日志的名字是空字符串。
After obtaining a RESERVED lock, the process that wants to write
creates a rollback journal. The header of the journal is initialized
with the original size of the database file. Space in the journal header
is also reserved for a master journal name, though the master journal
name is initially empty.
在修改数据库的任何页之前,进程需要将这些页的原始内容写入到回滚日志中。修改的页首先是保存在内存中,而不写入到磁盘中。原始数据库文件还是未更新的,这意味着其他进程还可以继续读取数据库。
Before making changes to any page of the database, the process writes
the original content of that page into the rollback journal. Changes
to pages are held in memory at first and are not written to the disk.
The original database file remains unaltered, which means that other
processes can continue to read the database.
最后,写入进程将更新数据库文件,因为内存缓存已经慢了,也因为做好了提交的准备。在这发生之前,写入者必须确保没有其它进程还在读取数据库以及回滚日志数据已经安全的写入到磁盘中,以便当遇到断电等事件的时候可以回滚未完成的改动。具体步奏按如下所述:
Eventually, the writing process will want to update the database
file, either because its memory cache has filled up or because it is
ready to commit its changes. Before this happens, the writer must
make sure no other process is reading the database and that the rollback
journal data is safely on the disk surface so that it can be used to
rollback incomplete changes in the event of a power failure.
The steps are as follows:
如果写入磁盘文件的原因时由于内存缓存满了,那么写入者就不会立即提交,而是继续修改其它页。在后续修改写入到数据库文件之前,回滚日志必须已经同步到磁盘上。还要注意,写入者最初为了写入数据库而持有的EXCLUSIVE锁必须一直持有者,知道所有的改动都已经提交了。这意味着从内存缓存第一次溢出到事务提交期间,其它进程都无法访问数据库。
If the reason for writing to the database file is because the memory
cache was full, then the writer will not commit right away. Instead,
the writer might continue to make changes to other pages. Before
subsequent changes are written to the database file, the rollback
journal must be flushed to disk again. Note also that the EXCLUSIVE
lock that the writer obtained in order to write to the database initially
must be held until all changes are committed. That means that no other
processes are able to access the database from the
time the memory cache first spills to disk until the transaction
commits.
当一个写入者准备提交改动的时候,会执行以下步奏:
When a writer is ready to commit its changes, it executes the following
steps:
一旦从数据库文件上释放了PENDING锁,其他进程就再一次可以开始读取数据库了。在当前的实现中,RESERVED锁也会释放掉,不过这并不是正确操作所必须的。
As soon as the PENDING lock is released from the database file, other
processes can begin reading the database again. In the current implementation,
the RESERVED lock is also released, but that is not essential for
correct operation.
如果一个事务包含了多个数据库,那么会使用一个更加复杂的流程,如下:
If a transaction involves multiple databases, then a more complex
commit sequence is used, as follows:
在SQLite2中,如果许多进程都读取数据库,那么可能会出现一个情况时,永远都有活跃的读取者。并且如果总是有至少一个读取锁加在数据库上,那么就没有进程能够修改数据库了,因为不可能获取到一个写锁。这种情况叫做写饥饿。
In SQLite version 2, if many processes are reading from the database,
it might be the case that there is never a time when there are
no active readers. And if there is always at least one read lock on the
database, no process would ever be able to make changes to the database
because it would be impossible to acquire a write lock. This situation
is called writer starvation.
SQlite3中力争通过使用PENDING锁来避免写饥饿。PENDING锁允许已有的读者继续读取,但是会阻止新的读取者连接到数据库。所以,当一个进程想要写入一个繁忙的数据库时,可以先设置一个PENDING锁,用来阻止新的读取者进入。一旦已有的读取者都最终完成了读取,所有的SHARED锁最终都会释放,这时写入者就可以有机会来做修改了。
SQLite version 3 seeks to avoid writer starvation through the use of
the PENDING lock. The PENDING lock allows existing readers to continue
but prevents new readers from connecting to the database. So when a
process wants to write a busy database, it can set a PENDING lock which
will prevent new readers from coming in. Assuming existing readers do
eventually complete, all SHARED locks will eventually clear and the
writer will be given a chance to make its changes.
pager模块时非常健壮的,但是依然会被破坏。这一节主要来明确和解释这个风险。(也可以参见原子提交一章中的能够导致错误的事情节)
The pager module is very robust but it can be subverted.
This section attempts to identify and explain the risks.
(See also the Things That Can Go Wrong section of the article
on Atomic Commit.
毫无疑问,硬件或者操作系统错误导致错误的数据写入到数据库文件或日志中会导致问题。同样的,如果一个流氓程序打开数据库文件或者日志文件,并且在文件中写入一些畸形的数据,这样也会导致数据库损坏。由于对于这些类型的问题没有任何可做的事情,所以也就不再过多关注这个问题了。
Clearly, a hardware or operating system fault that introduces incorrect data
into the middle of the database file or journal will cause problems.
Likewise,
if a rogue process opens a database file or journal and writes malformed
data into the middle of it, then the database will become corrupt.
There is not much that can be done about these kinds of problems
so they are given no further attention.
SQLitezai Unix上使用POSIX推荐的锁来实现锁模块。在Windows上使用LockFile()、LockFileEx()和 UnlockFile()系统调用。SQLite假设这些系统调用都能像广告上说的一样工作。如果不是这样,那么结果可能就是数据库损坏了。需要注意的一点时,POSIX推荐的锁在许多NFS实现中是有很多bug的,甚至还有未实现的(包括当前版本的Mac OS X)。Windows上的网络文见系统也报告出许多锁的问题。最好的预防手段就是不要在网络文件系统上使用SQLite。
SQLite uses POSIX advisory locks to implement locking on Unix. On
Windows it uses the LockFile(), LockFileEx(), and UnlockFile() system
calls. SQLite assumes that these system calls all work as advertised. If
that is not the case, then database corruption can result. One should
note that POSIX advisory locking is known to be buggy or even unimplemented
on many NFS implementations (including recent versions of Mac OS X)
and that there are reports of locking problems
for network filesystems under Windows. Your best defense is to not
use SQLite for files on a network filesystem.
SQLite在Unix上使用fsync()系统调用来将数据同步到磁盘上,在Windows上则只使用FlushFileBuffers()来完成。SQLite假设这些操作系统服务函数都能稳定的工作。但是依然有报告说fsync()和FlushFileBuffers()会出现错误,特别是在廉价的IDE磁盘上。而且一些IDE磁盘设备中,当数据还在磁盘驱动中易失的内存缓存中时,就报告说数据已经写入到磁盘上了。还有报告显示Windows有时候会因为未知的原因有选择的忽略FlushFileBuffers()。作者无法验证所有的这些报告。但是如果这些时真的,这又意味着数据库可能会在意外断电中被损坏。这些都是硬件或者操作系统的漏洞,SQLite完全无法预防这些问题。
SQLite uses the fsync() system call to flush data to the disk under Unix and
it uses the FlushFileBuffers() to do the same under Windows. Once again,
SQLite assumes that these operating system services function as advertised.
But it has been reported that fsync() and FlushFileBuffers() do not always
work correctly, especially with inexpensive IDE disks. Apparently some
manufactures of IDE disks have controller chips that report
that data has reached the disk surface when in fact the data is still
in volatile cache memory in the disk drive electronics. There are also
reports that Windows sometimes chooses to ignore FlushFileBuffers() for
unspecified reasons. The author cannot verify any of these reports.
But if they are true, it means that database corruption is a possibility
following an unexpected power loss. These are hardware and/or operating
system bugs that SQLite is unable to defend against.
如果在Linux中,挂载ext3文件系统时没有在/etc/fstab中加入"barrier=1"选项,并且开启了磁盘驱动器的写入缓存,那么断电或者系统崩溃会导致文件系统发生错误。具体是否会发生错误取决于磁盘控制器的具体实现。在廉价的用户级磁盘上更容易发生错误,而使用了类似非易失写入缓存等高级特性的企业级的存储设备则很少发生错误。
许多ext3专家都
确认了这个行为告诉我们,大多数的Linux分发版本都没有使用barrier=1,并且没有禁用写入缓存,也就更容易受到这个问题的影响。注意,这些都是操作系统或者硬件的问题,对于这些问题SQLite是无能为力的。其他数据库引擎也同样会面对这些问题。
If a Linux ext3
filesystem is mounted without the "barrier=1" option
in the /etc/fstab
and the disk drive write cache is enabled
then filesystem corruption can occur following a power loss or OS crash.
Whether or not corruption can occur depends on the details of the disk control
hardware; corruption is more likely with inexpensive consumer-grade disks
and less of a problem for enterprise-class storage devices with advanced
features such as non-volatile write caches.
Various ext3 experts
confirm this behavior.
We are told that most Linux distributions do not use barrier=1 and do
not disable the write cache so most
Linux distributions are vulnerable to this problem. Note that this is an
operating system and hardware issue and that there is nothing that SQLite
can do to work around it.
Other database engines have also run into this same problem.
如果崩溃或者意外断电导致生成了活跃日志,但是这个日志被删除了,那么下一个打开数据库的进程就不知道里面的修改需要回滚。从而导致数据库出现不一致状态。回滚日志可能会因为下列原因而被删除:
If a crash or power failure occurs and results in a hot journal but that
journal is deleted, the next process to open the database will not
know that it contains changes that need to be rolled back. The rollback
will not occur and the database will be left in an inconsistent state.
Rollback journals might be deleted for any number of reasons:
上面最后一条(第四条)记录还有一些备注信息。当SQLite在Unix上创建一个日志文件时,打开包含这个文件的目录,并且对目录调用fsync(),以此将目录信息写入到磁盘中。但是假设其他一些进程正在这个目录中添加或删除无关的文件的时候发生了意外断电,这些进程的无关操作可能导致日志文件被删除或者移动到"lost+found"中。这是一个很难遇到的场景,但是还是有可能发生。最好的预防方法就是使用一个日志文件系统或者只在一个目录中保存数据库和日志文件。
The last (fourth) bullet above merits additional comment. When SQLite creates
a journal file on Unix, it opens the directory that contains that file and
calls fsync() on the directory, in an effort to push the directory information
to disk. But suppose some other process is adding or removing unrelated
files to the directory that contains the database and journal at the
moment of a power failure. The supposedly unrelated actions of this other
process might result in the journal file being dropped from the directory and
moved into "lost+found". This is an unlikely scenario, but it could happen.
The best defenses are to use a journaling filesystem or to keep the
database and journal in a directory by themselves.
对于涉及多个数据库的提交和主日志文件,如果这些数据库在不同的磁盘上,当提交过程中发生意外断电时,当机器恢复时磁盘可能会被挂载为不同的名字。或者一些磁盘可能没有挂载。当发生这个问题时,每个独立的日志文件和主日志可能无法找到对方。这种情况下最坏的结果就是提交无法保证原子性,一些数据库可能回滚了,而另一些可能没有。所有数据库都继续自一致。为了预防这个问题,应当保持所有数据库在同一个磁盘上,并且重新挂载磁盘时使用相同的名字。
For a commit involving multiple databases and a master journal, if the
various databases were on different disk volumes and a power failure occurs
during the commit, then when the machine comes back up the disks might
be remounted with different names. Or some disks might not be mounted
at all. When this happens the individual file journals and the master
journal might not be able to find each other. The worst outcome from
this scenario is that the commit ceases to be atomic.
Some databases might be rolled back and others might not.
All databases will continue to be self-consistent.
To defend against this problem, keep all databases
on the same disk volume and/or remount disks using exactly the same names
after a power failure.
SQLite3对锁和并发控制的改动还为SQL语言层的事务工作方式引入了一些细微的改动。默认情况,SQLite3运行在自动提交模式下,在自动提交模式下,一旦所有与当前数据库连接相关的操作完成,就会将所有对数据库的修改提交。
The changes to locking and concurrency control in SQLite version 3 also
introduce some subtle changes in the way transactions work at the SQL
language level.
By default, SQLite version 3 operates in autocommit mode.
In autocommit mode,
all changes to the database are committed as soon as all operations associated
with the current database connection complete.
SQL命令 "BEGIN TRANSACTION"(TRANSACTION关键词是可选的)用来时SQLite跳出自动提交模式。注意,BEGIN命令不会向数据库请求任何锁。在BEGIN命令之后,当执行第一个SELECT语句的时候会获取一个SHARED锁。当执行第一条INSERT、UPDATE或DELETE语句时会获取一个RESERVED锁。只有到内存缓存已满必须写入到磁盘或者事务提交的时候才会获取EXCLUSIVE锁。通过这个方法,系统将对读取访问的阻塞延迟到最后时刻。
The SQL command "BEGIN TRANSACTION" (the TRANSACTION keyword
is optional) is used to take SQLite out of autocommit mode.
Note that the BEGIN command does not acquire any locks on the database.
After a BEGIN command, a SHARED lock will be acquired when the first
SELECT statement is executed. A RESERVED lock will be acquired when
the first INSERT, UPDATE, or DELETE statement is executed. No EXCLUSIVE
lock is acquired until either the memory cache fills up and must
be spilled to disk or until the transaction commits. In this way,
the system delays blocking read access to the file file until the
last possible moment.
SQL命令"COMMIT"并没有真正就爱那个改动提交到磁盘上,它只是将自动提交重新打开。这样,在这个命令的末尾,标准的自动提交逻辑接管业务并且最终真正提交到磁盘上。SQL命令"ROLLBACK"也同样是将自动提交重新打开,但是还会设置一个标志来告诉自动提交逻辑进行回滚而不是提交。
The SQL command "COMMIT" does not actually commit the changes to
disk. It just turns autocommit back on. Then, at the conclusion of
the command, the regular autocommit logic takes over and causes the
actual commit to disk to occur.
The SQL command "ROLLBACK" also operates by turning autocommit back on,
but it also sets a flag that tells the autocommit logic to rollback rather
than commit.
如果SQL COMMIT命令打开了自动提交,然后自动提交逻辑尝试提交所有改动,但是,因为其他进程持有SHARED锁而导致失败了,那么自动提交会重新自动关闭。这就允许用户等SHARED可能已经释放的时候再一次尝试COMMIT。
If the SQL COMMIT command turns autocommit on and the autocommit logic
then tries to commit change but fails because some other process is holding
a SHARED lock, then autocommit is turned back off automatically. This
allows the user to retry the COMMIT at a later time after the SHARED lock
has had an opportunity to clear.
如果在同一个SQLite数据库连接上同时执行多个命令,自动提交会延时到最后一个命令完成时才打开。例如,如果执行了一个SELECT语句,这个命令的执行在每行数据都返回来之前会暂停。在这个暂停期间,其他的INSERT、UPDATE或DELETE命令可以在这个数据库上的其它表上执行。但是在最初的SELECT语句完成之前,这些改动是不会提交的。
If multiple commands are being executed against the same SQLite database
connection at the same time, the autocommit is deferred until the very
last command completes. For example, if a SELECT statement is being
executed, the execution of the command will pause as each row of the
result is returned. During this pause other INSERT, UPDATE, or DELETE
commands can be executed against other tables in the database. But none
of these changes will commit until the original SELECT statement finishes.