Quirks that make it prone to misuse and failure. Unfortunately, POSIX advisory locking has design The default locking mechanism used by SQLite on unix platforms is Posix advisory locks canceled by a separate thread doing close() If SQLite is used on a filesystem where the locking primitives containīugs, and if two or more threads or processes try to access the sameĭatabase at the same time, then database corruption might result. This is especially true of network filesystems and NFS in particular. Locking logic such that the locks do not always behave as advertised. But some filesystems contain bugs in their SQLite depends on the underlying filesystem to do locking as theĭocumentation says it will. Filesystems with broken or missing lock implementations To make incompatible changes to a database file at the same time, Without coordination, two threads or processes might try Write-ahead log or WAL file, to coordinate access between concurrent SQLite uses file locks on the database file, and on the Overwriting a database file with another without alsoĭeleting any hot journal associated with the original database.Copying a database file without also copying its journal.Moving a journal file from one database to another.Overwritting a journal file with a different journal file.Swapping journal files between two different databases.The followingĪctions are all likely to lead to corruption: Kept together with the database to avoid corruption. In a quiescent state, the journalįile does not exist and only the database file matters.īut if the journal file does exist, it must be The state of an SQLite database is controlled by both theĭatabase file and the journal file. The previous example is a specific case of a more general problem: Mispairing database files and hot journals If the hot journal files are moved, deleted, or renamedĪfter a crash or power failure, then automatic recovery will not workĭatabase corruption caused by inconsistent use of 8+3 filenames. SQLite must see the journal files in order to recover from a crash The journal files have the same name as the Such journal files are described as "hot". While performing a transaction, information necessary to recover theĭatabase following a crash or power failure is stored in auxiliary SQLite normally stores all content in a single disk file. (the *-journal file) or write-ahead log (the *-wal file)īe copied together with the database file itself. Transaction failed, then it is important that any rollback journal Is to make use of the backup API that is part of the SQLite library.įailing that, it is safe to make a copy of an SQLite database file as longĪs there are no transactions in progress by any process. The best approach to make reliable backup copies of an SQLite database The backup copy then might contain some old and some Make a backup copy of an SQLite database file while it is in the middle Systems that run automatic backups in the background might try to Backup or restore while a transaction is active Reported by facebook engineers in a blog post on. (See SQLITE_MINIMUM_FILE_DESCRIPTOR for additional information.)Īnother example of corruption caused by using a closed file To guard against this kind of problem,Īnd later refuse to use low-numbered file descriptors Now connected to a database file, the error message Later, an applicationĪn error message by invoking write(2.). Sqlite3_open_v2() so that the file descriptor used for the One example of this occurred circa on the canonical repositoryįile descriptor 2 (standard error) was being erroneously closed (by Overwriting parts of the SQLite database, leading to corruption of the The information that was intended to go into the original file ended up But because the file descriptor had been reopened by SQLite, Old file descriptor, not realizing that the original file had been closedĪlready. Later, some other thread continued to write into the Then that file descriptor was closed and reopened on an SQLite database. We have seen multiple cases where a file descriptor was open on a file, Continuing to use a file descriptor after it has been closed That means that any process can open the file and SQLite database files are ordinary disk files. File overwrite by a rogue thread or process This document describes the various ways that an SQLite database might go corrupt.ġ. Though SQLite is resistant to database corruption, it is not immune. The recovery process is fully automatic and does not require any action on the part of the user or the application. If an application crash, or an operating-system crash, or even a power failure occurs in the middle of a transaction, the partially written transaction should be automatically rolled back the next time the database file is accessed. An SQLite database is highly resistant to corruption.
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