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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.</p> + +<p>The document only describes locking for the older rollback-mode +transaction mechanism. Locking for the newer <a href="wal.html">write-ahead log</a> or <a href="wal.html">WAL mode</a> +is described separately.</p> +</font> + +<h2>1.0 File Locking And Concurrency In SQLite Version 3</h2> + +<p>SQLite <a href="releaselog/3_0_0.html">Version 3.0.0</a> 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. +</p> + +<a name="overview"></a> +<h2>2.0 Overview</h2> + +<p> +Locking and concurrency control are handled by the +<a href="https://www.sqlite.org/src/finfo?name=src/pager.c"> +pager module</a>. +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.</p> + +<p>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: +<a href="https://www.sqlite.org/src/finfo?name=src/os_unix.c"> +os_unix.c</a>, +<a href="https://www.sqlite.org/src/finfo?name=src/os_win.c"> +os_win.c</a>) +that provides a uniform abstraction for operating system services. +</p> + +<p>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.</p> + +<a name="locking"></a> +<h2>3.0 Locking</h2> + +<p> +From the point of view of a single process, a database file +can be in one of five locking states: +</p> + +<p> +<table cellpadding="20"> +<tr><td valign="top">UNLOCKED</td> +<td valign="top"> +No locks are held on the database. The database may be neither read nor +written. Any internally cached data is considered suspect and subject to +verification against the database file before being used. Other +processes can read or write the database as their own locking states +permit. This is the default state. +</td></tr> + +<tr><td valign="top">SHARED</td> +<td valign="top"> +<a name="shared_lock"></a> + +The database may be read but not written. Any number of +processes can hold SHARED locks at the same time, hence there can be +many simultaneous readers. But no other thread or process is allowed +to write to the database file while one or more SHARED locks are active. +</td></tr> + +<tr><td valign="top">RESERVED</td> +<td valign="top"> +<a name="reserved_lock"></a> + +A RESERVED lock means that the process is planning on writing to the +database file at some point in the future but that it is currently just +reading from the file. Only a single RESERVED lock may be active at one +time, though multiple SHARED locks can coexist with a single RESERVED lock. +RESERVED differs from PENDING in that new SHARED locks can be acquired +while there is a RESERVED lock. +</td></tr> + +<tr><td valign="top">PENDING</td> +<td valign="top"> +<a name="pending_lock"></a> + +A PENDING lock means that the process holding the lock wants to write +to the database as soon as possible and is just waiting on all current +SHARED locks to clear so that it can get an EXCLUSIVE lock. No new +SHARED locks are permitted against the database if +a PENDING lock is active, though existing SHARED locks are allowed to +continue. +</td></tr> + +<tr><td valign="top">EXCLUSIVE</td> +<td valign="top"> +<a name="excl_lock"></a> + +An EXCLUSIVE lock is needed in order to write to the database file. +Only one EXCLUSIVE lock is allowed on the file and no other locks of +any kind are allowed to coexist with an EXCLUSIVE lock. In order to +maximize concurrency, SQLite works to minimize the amount of time that +EXCLUSIVE locks are held. +</td></tr> +</table> +</p> + +<p> +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. +</p> + +<a name="rollback"></a> +<h2>4.0 The Rollback Journal</h2> + +<p>When a process wants to change a database file (and it is not +in <a href="wal.html">WAL</a> mode), it +first records the original unchanged database content +in a <em>rollback journal</em>. 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 <tt>-journal</tt> +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.</p> + +<p>If SQLite is working with multiple databases at the same time +(using the <a href="lang_attach.html">ATTACH</a> command) then each database has its own rollback journal. +But there is also a separate aggregate journal +called the <em>super-journal</em>. +The super-journal does not contain page data used for rolling back +changes. Instead the super-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 super-journal. +If there are no ATTACHed databases (or if none of the ATTACHed database +is participating in the current transaction) no super-journal is +created and the normal rollback journal contains an empty string +in the place normally reserved for recording the name of the +super-journal.</p> + +<p>A rollback journal is said to be <a href="fileformat2.html#hotjrnl">hot</a> +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. +</p> + +<p> +If no super-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 super-journal is named in the file journal, then the file journal +is hot if its super-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: +</p> + +<ul> +<li>A journal is hot if... + <ul> + <li>It exists, and</li> + <li>Its size is greater than 512 bytes, and</li> + <li>The journal header is non-zero and well-formed, and</li> + <li>Its super-journal exists or the super-journal name is an + empty string, and</li> + <li>There is no RESERVED lock on the corresponding database file.</li> + </ul> +</li> +</ul> + +<a name="hot_journals"></a> +<h3>4.1 Dealing with hot journals</h3> + +<p> +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. +</p> + +<p>When a process wants to read from a database file, it followed +the following sequence of steps: +</p> + +<ol> +<li>Open the database file and obtain a SHARED lock. If the SHARED lock + cannot be obtained, fail immediately and return SQLITE_BUSY.</li> +<li>Check to see if the database file has a hot journal. If the file + does not have a hot journal, we are done. Return immediately. + If there is a hot journal, that journal must be rolled back by + the subsequent steps of this algorithm.</li> +<li>Acquire a PENDING lock then an EXCLUSIVE lock on the database file. + (Note: Do not acquire a RESERVED lock because that would make + other processes think the journal was no longer hot.) If we + fail to acquire these locks it means another process + is already trying to do the rollback. In that case, + drop all locks, close the database, and return SQLITE_BUSY. </li> +<li>Read the journal file and roll back the changes.</li> +<li>Wait for the rolled back changes to be written onto + persistent storage. This protects the integrity of the database + in case another power failure or crash occurs.</li> +<li>Delete the journal file (or truncate the journal to zero bytes in + length if <a href="pragma.html#pragma_journal_mode">PRAGMA journal_mode=TRUNCATE</a> is + set, or zero the journal header if + <a href="pragma.html#pragma_journal_mode">PRAGMA journal_mode=PERSIST</a> is set).</li> +<li>Delete the super-journal file if it is safe to do so. + This step is optional. It is here only to prevent stale + super-journals from cluttering up the disk drive. + See the discussion below for details.</li> +<li>Drop the EXCLUSIVE and PENDING locks but retain the SHARED lock.</li> +</ol> + +<p>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.</p> + +<a name="stale_super_journals"></a> +<h3>4.2 Deleting stale super-journals</h3> + +<p>A stale super-journal is a super-journal that is no longer being +used for anything. There is no requirement that stale super-journals +be deleted. The only reason for doing so is to free up disk space.</p> + +<p>A super-journal is stale if no individual file journals are pointing +to it. To figure out if a super-journal is stale, we first read the +super-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 super-journal exists and points back to the super-journal, then +the super-journal is not stale. If all file journals are either missing +or refer to other super-journals or no super-journal at all, then the +super-journal we are testing is stale and can be safely deleted.</p> + +<a name="writing"></a> +<h2>5.0 Writing to a database file</h2> + +<p>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. +</p> + +<p>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.</p> + +<p>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 super-journal name, though the super-journal +name is initially empty.</p> + +<p>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.</p> + +<p>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:</p> + +<ol> +<li>Make sure all rollback journal data has actually been written to + the surface of the disk (and is not just being held in the operating + system's or disk controllers cache) so that if a power failure occurs + the data will still be there after power is restored.</li> +<li>Obtain a PENDING lock and then an EXCLUSIVE lock on the database file. + If other processes still have SHARED locks, the writer might have + to wait until those SHARED locks clear before it is able to obtain + an EXCLUSIVE lock.</li> +<li>Write all page modifications currently held in memory out to the + original database disk file.</li> +</ol> + +<p> +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. +</p> + +<p> +When a writer is ready to commit its changes, it executes the following +steps: +</p> + +<ol> +<li value="4"> + Obtain an EXCLUSIVE lock on the database file and + make sure all memory changes have been written to the database file + using the algorithm of steps 1-3 above.</li> +<li>Flush all database file changes to the disk. Wait for those changes + to actually be written onto the disk surface.</li> +<li>Delete the journal file. (Or if the <a href="pragma.html#pragma_journal_mode">PRAGMA journal_mode</a> is TRUNCATE or + PERSIST, truncate the journal file or zero the header of the journal file, + respectively.) This is the instant when the changes are + committed. Prior to deleting the journal file, if a power failure + or crash occurs, the next process to open the database will see that + it has a hot journal and will roll the changes back. + After the journal is deleted, there will no longer be a hot journal + and the changes will persist. + </li> +<li>Drop the EXCLUSIVE and PENDING locks from the database file. + </li> +</ol> + +<p>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.</p> + +<p>If a transaction involves multiple databases, then a more complex +commit sequence is used, as follows:</p> + +<ol> +<li value="4"> + Make sure all individual database files have an EXCLUSIVE lock and a + valid journal. +<li>Create a super-journal. The name of the super-journal is arbitrary. + (The current implementation appends random suffixes to the name of the + main database file until it finds a name that does not previously exist.) + Fill the super-journal with the names of all the individual journals + and flush its contents to disk. +<li>Write the name of the super-journal into + all individual journals (in space set aside for that purpose in the + headers of the individual journals) and flush the contents of the + individual journals to disk and wait for those changes to reach the + disk surface. +<li>Flush all database file changes to the disk. Wait for those changes + to actually be written onto the disk surface.</li> +<li>Delete the super-journal file. This is the instant when the changes are + committed. Prior to deleting the super-journal file, if a power failure + or crash occurs, the individual file journals will be considered hot + and will be rolled back by the next process that + attempts to read them. After the super-journal has been deleted, + the file journals will no longer be considered hot and the changes + will persist. + </li> +<li>Delete all individual journal files. +<li>Drop the EXCLUSIVE and PENDING locks from all database files. + </li> +</ol> + +<a name="writer_starvation"></a> +<h3>5.1 Writer starvation</h3> + +<p>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 <em>writer starvation</em>.</p> + +<p>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.</p> + +<a name="how_to_corrupt"></a> +<h2>6.0 How To Corrupt Your Database Files</h2> + +<p>The pager module is very robust but it can be subverted. +This section attempts to identify and explain the risks. +(See also the <a href="atomiccommit.html#sect_9_0">Things That Can Go Wrong</a> section of the article +on <a href="atomiccommit.html">Atomic Commit</a>.</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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 some network filesystems or 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. +</p> + +<a name="ext3-barrier-problem"></a> + +<p>If a Linux <a href="http://en.wikipedia.org/wiki/Ext3">ext3</a> +filesystem is mounted without the "barrier=1" option +in the <a href="http://en.wikipedia.org/wiki/fstab">/etc/fstab</a> +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 +<a href="http://www.redhat.com/archives/ext3-users/2010-July/msg00001.html"> +confirm this behavior</a>. +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. +<a href="http://ozlabs.org/~rusty/index.cgi/tech/2009-10-20.html"> +Other database engines</a> have also run into this same problem.</p> + +<p> +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: +</p> + +<ul> +<li>An administrator might be cleaning up after an OS crash or power failure, + see the journal file, think it is junk, and delete it.</li> +<li>Someone (or some process) might rename the database file but fail to + also rename its associated journal.</li> +<li>If the database file has aliases (hard or soft links) and the file + is opened by a different alias than the one used to create the journal, + then the journal will not be found. To avoid this problem, you should + not create links to SQLite database files.</li> +<li>Filesystem corruption following a power failure might cause the + journal to be renamed or deleted.</li> +</ul> + +<p> +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. +</p> + +<p> +For a commit involving multiple databases and a super-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 +super-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. +</p> + +<a name="transaction_control"></a> +<h2>7.0 Transaction Control At The SQL Level</h2> + +<p> +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 <em>autocommit</em> mode. +In autocommit mode, +all changes to the database are committed as soon as all operations associated +with the current database connection complete.</p> + +<p>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 until the +last possible moment. +</p> + +<p>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.</p> + +<p>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.</p> + +<p>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. +</p> + + |