Racket LMDB Wrapper

Version 0.2: Please note future API-breaking changes are possible!

This module is a direct port of the LMDB API, with the following exceptions:

1. The names have been adjusted to Scheme standards. See below.
2. All keys and data are passed around as byte strings, not MDB_vals, or #f in place of NULL.
3. All functions that return a status, unless otherwise noted, now throw failures as exn:mdb structs.
4. In the cases where a function takes a single out parameter and returns a status (e.g. dbi-open, cursor-count), that out parameter is returned instead since failures are thrown.
5. mdb-get returns the value that was read or #f if it was not found. Any error other than MDB_NOTFOUND will be raised.
6. cursor-get takes two mutable boxes for the key and data. These boxes will be modified to contain the returned key and data if applicable. #t is returned on success and #f is returned if the status is MDB_NOTFOUND. Any error other than MDB_NOTFOUND will be raised.
7. Racket booleans are used for the int parameters of dbi-drop, env-set-flags, and env-sync.
8. All the mdb_env_copy* functions have been condensed down to env-copy, which takes flags as an optional argument and dispatches to the copy or copyfd version based on the type of the second argument.
9. Two forms, with-txn and with-cursor, are provided to assist with resource management.

As a very direct port, this library gives you plenty of leeway to shoot yourself in the foot.

Naming

The names in this library have been updated to fit a bit better into the Racket ecosystem. Underscores in procedure names have been replaced with dashes and in most places, the MDB_/mdb_ prefix have been stripped off, except where the stripped names wouldn't be informative enough on their own.

Examples:

• The mdb_env_create procedure is now just env-create.
• The MDB_FIRST cursor op is now just the symbol 'FIRST.
• The MDB_DUPSORT database flag is now just the symbol 'DUPSORT.

Exceptions:

• mdb_drop has been renamed to dbi-drop. This aligns it with the other dbi- procedures and saves us from conflicting with Racket's built-in drop that operates on lists.
• mdb_version has been renamed to mdb-version so that it doesn't conflict with Racket's version function.
• mdb_strerror has been renamed to mdb-strerror because strerror is too general on its own.
• All error constants still have the MDB_ prefix.

Limitations

• Unsupported Features:
  • The MDB_RESERVE and MDB_MULTIPLE write flags are not supported.
  • The following functions have not been exposed:
    • mdb_env_set_userctx
    • mdb_env_get_userctx
    • mdb_env_set_assert
    • mdb_set_compare
    • mdb_set_dupsort
    • mdb_set_relfunc
    • mdb_set_relctx
    • mdb_cmp
    • mdb_dcmp
• Performance:
  • This library copies a lot of memory around when converting to and from byte strings.
  • Memory mapping, as used by LMDB, involves cache misses which block the entire OS thread. This means that the entire Racket place can be blocked, since Racket uses green threads. (i.e. Code in another Racket thread can be blocked by LMDB operations if they are in the same place!)

Usage Notes

• You'll likely want to open the database with MDB_NOTLS since all Racket threads within the same place are run on the same OS thread. If you don't, LMDB will use thread-local storage to track transactions and you won't be able to perform multiple read-only transactions within the same place.
• This library does no extra bookkeeping, so as with regular LMDB, it's up to you to clean up what you create, be they environments, transactions, or cursors. The with-txn and with-cursor forms can assist in cases.

Bundled Libraries

This package bundles LMDB dynamic libraries for various platforms which were sourced from these locations:

• Windows 64-bit: https://mirror.msys2.org/mingw/mingw64/mingw-w64-x86_64-lmdb-0.9.29-1-any.pkg.tar.zst (MSYS2)
• Linux 64-bit: https://mirror.pkgbuild.com/extra/os/x86_64/lmdb-0.9.29-1-x86_64.pkg.tar.zst (Arch Linux)

License

LMDB is released under the OpenLDAP license. The wrapper itself is licensed under the MIT license.