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93 lines
4.1 KiB
Markdown
93 lines
4.1 KiB
Markdown
# Go SQLite VFS API
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This package implements the SQLite [OS Interface](https://sqlite.org/vfs.html) (aka VFS).
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It replaces the default SQLite VFS with a **pure Go** implementation,
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and exposes [interfaces](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#VFS)
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that should allow you to implement your own custom VFSes.
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Since it is a from scratch reimplementation,
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there are naturally some ways it deviates from the original.
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The main differences are [file locking](#file-locking) and [WAL mode](#write-ahead-logging) support.
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### File Locking
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POSIX advisory locks, which SQLite uses on Unix, are
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[broken by design](https://github.com/sqlite/sqlite/blob/b74eb0/src/os_unix.c#L1073-L1161).
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On Linux and macOS, this module uses
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[OFD locks](https://www.gnu.org/software/libc/manual/html_node/Open-File-Description-Locks.html)
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to synchronize access to database files.
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OFD locks are fully compatible with POSIX advisory locks.
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This module can also use
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[BSD locks](https://man.freebsd.org/cgi/man.cgi?query=flock&sektion=2),
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albeit with reduced concurrency (`BEGIN IMMEDIATE` behaves like `BEGIN EXCLUSIVE`).
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On BSD, macOS, and illumos, BSD locks are fully compatible with POSIX advisory locks;
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on Linux and z/OS, they are fully functional, but incompatible;
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elsewhere, they are very likely broken.
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BSD locks are the default on BSD and illumos,
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but you can opt into them with the `sqlite3_flock` build tag.
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On Windows, this module uses `LockFileEx` and `UnlockFileEx`,
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like SQLite.
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Otherwise, file locking is not supported, and you must use
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[`nolock=1`](https://sqlite.org/uri.html#urinolock)
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(or [`immutable=1`](https://sqlite.org/uri.html#uriimmutable))
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to open database files.
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To use the [`database/sql`](https://pkg.go.dev/database/sql) driver
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with `nolock=1` you must disable connection pooling by calling
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[`db.SetMaxOpenConns(1)`](https://pkg.go.dev/database/sql#DB.SetMaxOpenConns).
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You can use [`vfs.SupportsFileLocking`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#SupportsFileLocking)
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to check if your build supports file locking.
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### Write-Ahead Logging
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On 64-bit Unix, this module uses `mmap` to implement
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[shared-memory for the WAL-index](https://sqlite.org/wal.html#implementation_of_shared_memory_for_the_wal_index),
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like SQLite.
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To allow `mmap` to work, each connection needs to reserve up to 4GB of address space.
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To limit the address space each connection reserves,
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use [`WithMemoryLimitPages`](../tests/testcfg/testcfg.go).
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With [BSD locks](https://man.freebsd.org/cgi/man.cgi?query=flock&sektion=2)
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a WAL database can only be accessed by a single proccess.
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Other processes that attempt to access a database locked with BSD locks,
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will fail with the `SQLITE_PROTOCOL` error code.
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Otherwise, [WAL support is limited](https://sqlite.org/wal.html#noshm),
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and `EXCLUSIVE` locking mode must be set to create, read, and write WAL databases.
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To use `EXCLUSIVE` locking mode with the
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[`database/sql`](https://pkg.go.dev/database/sql) driver
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you must disable connection pooling by calling
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[`db.SetMaxOpenConns(1)`](https://pkg.go.dev/database/sql#DB.SetMaxOpenConns).
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You can use [`vfs.SupportsSharedMemory`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#SupportsSharedMemory)
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to check if your build supports shared memory.
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### Batch-Atomic Write
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On 64-bit Linux, this module supports [batch-atomic writes](https://sqlite.org/cgi/src/technote/714)
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on the F2FS filesystem.
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### Build Tags
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The VFS can be customized with a few build tags:
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- `sqlite3_flock` forces the use of BSD locks; it can be used on z/OS to enable locking,
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and elsewhere to test BSD locks.
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- `sqlite3_nosys` prevents importing [`x/sys`](https://pkg.go.dev/golang.org/x/sys);
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disables locking _and_ shared memory on all platforms.
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- `sqlite3_noshm` disables shared memory on all platforms.
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> [!IMPORTANT]
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> The default configuration of this package is compatible with the standard
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> [Unix and Windows SQLite VFSes](https://sqlite.org/vfs.html#multiple_vfses);
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> `sqlite3_flock` builds are compatible with the
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> [`unix-flock` VFS](https://sqlite.org/compile.html#enable_locking_style).
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> If incompatible file locking is used, accessing databases concurrently with
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> _other_ SQLite libraries will eventually corrupt data.
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