Crate lmdb

Idiomatic and safe APIs for interacting with the Symas Lightning Memory-Mapped Database (LMDB).

Structs

Database	A handle to an individual database in an environment.
DatabaseFlags	Database Options
Environment	An LMDB environment.
EnvironmentBuilder	Options for opening or creating an environment.
EnvironmentFlags	Environment Options
InactiveTransaction	An inactive read-only transaction.
Iter
IterDup
RoCursor	A read-only cursor for navigating the items within a database.
RoTransaction	An LMDB read-only transaction.
RwCursor	A read-only cursor for navigating items within a database.
RwTransaction	An LMDB read-write transaction.
WriteFlags	Write Options

Enums

Error

Constants

APPEND	Append the given item to the end of the database. No key comparisons are performed. This option allows fast bulk loading when keys are already known to be in the correct order. Loading unsorted keys with this flag will cause data corruption.
APPEND_DUP	Same as APPEND, but for sorted dup data.
CURRENT	For Cursor::put. Replace the item at the current cursor position. The key parameter must match the current position. If using sorted duplicates (DUP_SORT) the data item must still sort into the same position. This is intended to be used when the new data is the same size as the old. Otherwise it will simply perform a delete of the old record followed by an insert.
DUP_FIXED	This flag may only be used in combination with DUP_SORT. This option tells the library that the data items for this database are all the same size, which allows further optimizations in storage and retrieval. When all data items are the same size, the GET_MULTIPLE and NEXT_MULTIPLE cursor operations may be used to retrieve multiple items at once.
DUP_SORT	Duplicate keys may be used in the database. (Or, from another perspective, keys may have multiple data items, stored in sorted order.) By default keys must be unique and may have only a single data item.
FIXED_MAP	Use a fixed address for the mmap region. This flag must be specified when creating the environment, and is stored persistently in the environment. If successful, the memory map will always reside at the same virtual address and pointers used to reference data items in the database will be constant across multiple invocations. This option may not always work, depending on how the operating system has allocated memory to shared libraries and other uses. The feature is highly experimental.
INTEGER_DUP	This option specifies that duplicate data items are also integers, and should be sorted as such.
INTEGER_KEY	Keys are binary integers in native byte order. Setting this option requires all keys to be the same size, typically 32 or 64 bits.
MAP_ASYNC	When using WRITE_MAP, use asynchronous flushes to disk. As with NO_SYNC, a system crash can then corrupt the database or lose the last transactions. Calling Environment::sync ensures on-disk database integrity until next commit.
NO_DUP_DATA	Insert the new item only if it does not already appear in the database. This flag may only be specified if the database was opened with DUP_SORT. The function will return LmdbError::KeyExist if the item already appears in the database.
NO_LOCK	Do not do any locking. If concurrent access is anticipated, the caller must manage all concurrency themself. For proper operation the caller must enforce single-writer semantics, and must ensure that no readers are using old transactions while a writer is active. The simplest approach is to use an exclusive lock so that no readers may be active at all when a writer begins.
NO_MEM_INIT	Do not initialize malloc'd memory before writing to unused spaces in the data file. By default, memory for pages written to the data file is obtained using malloc. While these pages may be reused in subsequent transactions, freshly malloc'd pages will be initialized to zeroes before use. This avoids persisting leftover data from other code (that used the heap and subsequently freed the memory) into the data file. Note that many other system libraries may allocate and free memory from the heap for arbitrary uses. E.g., stdio may use the heap for file I/O buffers. This initialization step has a modest performance cost so some applications may want to disable it using this flag. This option can be a problem for applications which handle sensitive data like passwords, and it makes memory checkers like Valgrind noisy. This flag is not needed with WRITE_MAP, which writes directly to the mmap instead of using malloc for pages. The initialization is also skipped if writing with reserve; the caller is expected to overwrite all of the memory that was reserved in that case.
NO_META_SYNC	Flush system buffers to disk only once per transaction, omit the metadata flush. Defer that until the system flushes files to disk, or next non-READ_ONLY commit or Environment::sync. This optimization maintains database integrity, but a system crash may undo the last committed transaction. I.e. it preserves the ACI (atomicity, consistency, isolation) but not D (durability) database property.
NO_OVERWRITE	Insert the new item only if the key does not already appear in the database. The function will return LmdbError::KeyExist if the key already appears in the database, even if the database supports duplicates (DUP_SORT).
NO_READAHEAD	Turn off readahead. Most operating systems perform readahead on read requests by default. This option turns it off if the OS supports it. Turning it off may help random read performance when the DB is larger than RAM and system RAM is full. The option is not implemented on Windows.
NO_SUB_DIR	By default, LMDB creates its environment in a directory whose pathname is given in path, and creates its data and lock files under that directory. With this option, path is used as-is for the database main data file. The database lock file is the path with -lock appended.
NO_SYNC	Don't flush system buffers to disk when committing a transaction. This optimization means a system crash can corrupt the database or lose the last transactions if buffers are not yet flushed to disk. The risk is governed by how often the system flushes dirty buffers to disk and how often Environment::sync is called. However, if the filesystem preserves write order and the WRITE_MAP flag is not used, transactions exhibit ACI (atomicity, consistency, isolation) properties and only lose D (durability). I.e. database integrity is maintained, but a system crash may undo the final transactions. Note that (NO_SYNC | WRITE_MAP) leaves the system with no hint for when to write transactions to disk, unless Environment::sync is called. (MAP_ASYNC | WRITE_MAP) may be preferable.
NO_TLS	Don't use thread-local storage. Tie reader locktable slots to transaction objects instead of to threads. I.e. RoTransaction::reset keeps the slot reseved for the transaction object. A thread may use parallel read-only transactions. A read-only transaction may span threads if the user synchronizes its use. Applications that multiplex many the user synchronizes its use. Applications that multiplex many user threads over individual OS threads need this option. Such an application must also serialize the write transactions in an OS thread, since LMDB's write locking is unaware of the user threads.
READ_ONLY	Open the environment in read-only mode. No write operations will be allowed. When opening an environment, LMDB will still modify the lock file - except on read-only filesystems, where LMDB does not use locks.
REVERSE_DUP	This option specifies that duplicate data items should be compared as strings in reverse order.
REVERSE_KEY	Keys are strings to be compared in reverse order, from the end of the strings to the beginning. By default, Keys are treated as strings and compared from beginning to end.
WRITE_MAP	Use a writeable memory map unless READ_ONLY is set. This is faster and uses fewer mallocs, but loses protection from application bugs like wild pointer writes and other bad updates into the database. Incompatible with nested transactions. Processes with and without WRITE_MAP on the same environment do not cooperate well.

Traits

Cursor	An LMDB cursor.
Transaction	An LMDB transaction.

Type Definitions

Result