go/sqlite3
1
0
Fork 0
mirror of https://github.com/ncruces/go-sqlite3.git synced 2026-01-12 05:59:14 +00:00
Code Issues Packages Projects Releases Wiki Activity

Update README.md

Browse Source
This commit is contained in:
Nuno Cruces
2024-05-06 00:41:56 +01:00
committed by GitHub
parent 12111a619a
commit 1a223fa69f
1 changed files with 26 additions and 30 deletions
Download Patch File Download Diff File Expand all files Collapse all files

56
vfs/README.md
View File

@@ -2,15 +2,14 @@
This package implements the SQLite [OS Interface](https://sqlite.org/vfs.html) (aka VFS).
It replaces the default SQLite VFS with a **pure Go** implementation.
It also exposes [interfaces](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#VFS)
It replaces the default SQLite VFS with a **pure Go** implementation,
and exposes [interfaces](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#VFS)
that should allow you to implement your own custom VFSes.
Since it is a from scratch reimplementation,
there are naturally some ways it deviates from the original.
The main differences are [file locking](#file-locking) and [WAL mode](write-ahead-logging) support.
The main differences are [file locking](#file-locking) and [WAL mode](#write-ahead-logging) support.
### File Locking
@@ -22,21 +21,22 @@ On Linux and macOS, this module uses
to synchronize access to database files.
OFD locks are fully compatible with POSIX advisory locks.
On BSD Unixes, this module uses
[BSD locks](https://man.freebsd.org/cgi/man.cgi?query=flock&sektion=2).
On BSD, macOS, and illumos, these locks are fully compatible with POSIX advisory locks;
on Linux and z/OS, they are fully functional, but incompatible with POSIX advisory locks.
However, concurrency is always reduced with BSD locks
(`BEGIN IMMEDIATE` behaves the same as `BEGIN EXCLUSIVE`).
You can opt into BSD locks with the `sqlite3_flock` build tag.
This module can also use
[BSD locks](https://man.freebsd.org/cgi/man.cgi?query=flock&sektion=2),
albeit with reduced concurrency (`BEGIN IMMEDIATE` behaves like `BEGIN EXCLUSIVE`).
On BSD, macOS, and illumos, BSD locks are fully compatible with POSIX advisory locks;
on Linux and z/OS, they are fully functional, but incompatible;
elsewhere, they are very likely broken.
BSD locks are the default on BSD and illumos,
but you can opt into them with the `sqlite3_flock` build tag.
On Windows, this module uses `LockFileEx` and `UnlockFileEx`,
like SQLite.
On all other platforms, file locking is not supported, and you must use
Otherwise, file locking is not supported, and you must use
[`nolock=1`](https://sqlite.org/uri.html#urinolock)
(or [`immutable=1`](https://sqlite.org/uri.html#uriimmutable))
to open database files.\
to open database files.
To use the [`database/sql`](https://pkg.go.dev/database/sql) driver
with `nolock=1` you must disable connection pooling by calling
[`db.SetMaxOpenConns(1)`](https://pkg.go.dev/database/sql#DB.SetMaxOpenConns).
@@ -50,41 +50,37 @@ On 64-bit Linux and macOS, this module uses `mmap` to implement
[shared-memory for the WAL-index](https://sqlite.org/wal.html#implementation_of_shared_memory_for_the_wal_index),
like SQLite.
To allow `mmap` to work, each connection needs to reserve up to 4GB of address space.\
To limit the amount of address space each connection needs,
To allow `mmap` to work, each connection needs to reserve up to 4GB of address space.
To limit the address space each connection reserves,
use [`WithMemoryLimitPages`](../tests/testcfg/testcfg.go).
On Windows, and with BSD locks, [WAL](https://sqlite.org/wal.html) support is
[limited](https://sqlite.org/wal.html#noshm).
`EXCLUSIVE` locking mode can be set to create, read, and write WAL databases.\
Otherwise, [WAL support is limited](https://sqlite.org/wal.html#noshm),
and `EXCLUSIVE` locking mode must be set to create, read, and write WAL databases.
To use `EXCLUSIVE` locking mode with the
[`database/sql`](https://pkg.go.dev/database/sql) driver
you must disable connection pooling by calling
[`db.SetMaxOpenConns(1)`](https://pkg.go.dev/database/sql#DB.SetMaxOpenConns).
On all other platforms, where file locking is not supported, WAL mode is disabled.
You can use [`vfs.SupportsSharedMemory`](https://pkg.go.dev/github.com/ncruces/go-sqlite3/vfs#SupportsSharedMemory)
to check if your build supports shared memory.
### Batch-Atomic Write
On 64-bit Linux, this module supports [batch-atomic writes](https://sqlite.org/cgi/src/technote/714)
with the F2FS filesystem.
on the F2FS filesystem.
### Build Tags
The VFS can be customized with a few build tags:
- `sqlite3_flock` forces the use of BSD locks; it can be used on z/OS for locking support,
and elsewhere to test the BSD locks.
- `sqlite3_flock` forces the use of BSD locks; it can be used on z/OS to enable locking,
and elsewhere to test BSD locks.
- `sqlite3_nosys` prevents importing [`x/sys`](https://pkg.go.dev/golang.org/x/sys);
disables locking _and_ shared memory on all platforms.
- `sqlite3_noshm` disables shared memory on all platforms.
> [!CAUTION]
> If file locking is incompatible with POSIX advisory locks,
> accessing databases concurrently with this package and other SQLite implementations
> is unsafe, and will eventually corrupt data.
> This will only be the case if you explicitly opt into BSD locks with `sqlite3_flock`.
> The SQLite [`unix-flock`](https://sqlite.org/compile.html#enable_locking_style) VFS
> is always compatible with `sqlite3_flock`.
> [!IMPORTANT]
> The default configuration of this package is compatible with
> the standard [Unix and Windows SQLite VFSes](https://sqlite.org/vfs.html#multiple_vfses);
> `sqlite3_flock` is compatible with the [`unix-flock` VFS](https://sqlite.org/compile.html#enable_locking_style).
> If incompatible file locking is used, accessing databases concurrently with _other_ SQLite libraries
> will eventually corrupt data.