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LMDB
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LMDB
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LMDB is compact, fast, powerful, and robust and implements a simplified variant of the BerkeleyDB (BDB) API. (BDB is also very powerful, and verbosely documented in its own right.) After reading this page, the main LMDB API documentation should make sense. Thanks to Bert Hubert for creating the initial version of this writeup.
Everything starts with an environment, created by mdb_env_create(). Once created, this environment must also be opened with mdb_env_open().
mdb_env_open() gets passed a name which is interpreted as a directory path. Note that this directory must exist already, it is not created for you. Within that directory, a lock file and a storage file will be generated. If you don't want to use a directory, you can pass the MDB_NOSUBDIR option, in which case the path you provided is used directly as the data file, and another file with a "-lock" suffix added will be used for the lock file.
Once the environment is open, a transaction can be created within it using mdb_txn_begin(). Transactions may be read-write or read-only, and read-write transactions may be nested. A transaction must only be used by one thread at a time. Transactions are always required, even for read-only access. The transaction provides a consistent view of the data.
Once a transaction has been created, a database can be opened within it using mdb_dbi_open(). If only one database will ever be used in the environment, a NULL can be passed as the database name. For named databases, the MDB_CREATE flag must be used to create the database if it doesn't already exist. Also, mdb_env_set_maxdbs() must be called after mdb_env_create() and before mdb_env_open() to set the maximum number of named databases you want to support.
Note: a single transaction can open multiple databases. Generally databases should only be opened once, by the first transaction in the process. After the first transaction completes, the database handles can freely be used by all subsequent transactions.
Within a transaction, mdb_get() and mdb_put() can store single key/value pairs if that is all you need to do (but see Cursors below if you want to do more).
A key/value pair is expressed as two MDB_val structures. This struct has two fields, mv_size and mv_data. The data is a void pointer to an array of mv_size bytes.
Because LMDB is very efficient (and usually zero-copy), the data returned in an MDB_val structure may be memory-mapped straight from disk. In other words look but do not touch (or free() for that matter). Once a transaction is closed, the values can no longer be used, so make a copy if you need to keep them after that.
To do more powerful things, we must use a cursor.
Within the transaction, a cursor can be created with mdb_cursor_open(). With this cursor we can store/retrieve/delete (multiple) values using mdb_cursor_get(), mdb_cursor_put(), and mdb_cursor_del().
mdb_cursor_get() positions itself depending on the cursor operation requested, and for some operations, on the supplied key. For example, to list all key/value pairs in a database, use operation MDB_FIRST for the first call to mdb_cursor_get(), and MDB_NEXT on subsequent calls, until the end is hit.
To retrieve all keys starting from a specified key value, use MDB_SET. For more cursor operations, see the LMDB API docs.
When using mdb_cursor_put(), either the function will position the cursor for you based on the key, or you can use operation MDB_CURRENT to use the current position of the cursor. Note that key must then match the current position's key.
So we have a cursor in a transaction which opened a database in an environment which is opened from a filesystem after it was separately created.
Or, we create an environment, open it from a filesystem, create a transaction within it, open a database within that transaction, and create a cursor within all of the above.
Got it?
LMDB uses POSIX locks on files, and these locks have issues if one process opens a file multiple times. Because of this, do not mdb_env_open() a file multiple times from a single process. Instead, share the LMDB environment that has opened the file across all threads. Otherwise, if a single process opens the same environment multiple times, closing it once will remove all the locks held on it, and the other instances will be vulnerable to corruption from other processes.
Also note that a transaction is tied to one thread by default using Thread Local Storage. If you want to pass read-only transactions across threads, you can use the MDB_NOTLS option on the environment.
To actually get anything done, a transaction must be committed using mdb_txn_commit(). Alternatively, all of a transaction's operations can be discarded using mdb_txn_abort(). In a read-only transaction, any cursors will not automatically be freed. In a read-write transaction, all cursors will be freed and must not be used again.
For read-only transactions, obviously there is nothing to commit to storage. The transaction still must eventually be aborted to close any database handle(s) opened in it, or committed to keep the database handles around for reuse in new transactions.
In addition, as long as a transaction is open, a consistent view of the database is kept alive, which requires storage. A read-only transaction that no longer requires this consistent view should be terminated (committed or aborted) when the view is no longer needed (but see below for an optimization).
There can be multiple simultaneously active read-only transactions but only one that can write. Once a single read-write transaction is opened, all further attempts to begin one will block until the first one is committed or aborted. This has no effect on read-only transactions, however, and they may continue to be opened at any time.
mdb_get() and mdb_put() respectively have no and only some support for multiple key/value pairs with identical keys. If there are multiple values for a key, mdb_get() will only return the first value.
When multiple values for one key are required, pass the MDB_DUPSORT flag to mdb_dbi_open(). In an MDB_DUPSORT database, by default mdb_put() will not replace the value for a key if the key existed already. Instead it will add the new value to the key. In addition, mdb_del() will pay attention to the value field too, allowing for specific values of a key to be deleted.
Finally, additional cursor operations become available for traversing through and retrieving duplicate values.
If you frequently begin and abort read-only transactions, as an optimization, it is possible to only reset and renew a transaction.
mdb_txn_reset() releases any old copies of data kept around for a read-only transaction. To reuse this reset transaction, call mdb_txn_renew() on it. Any cursors in this transaction must also be renewed using mdb_cursor_renew().
Note that mdb_txn_reset() is similar to mdb_txn_abort() and will close any databases you opened within the transaction.
To permanently free a transaction, reset or not, use mdb_txn_abort().
For read-only transactions, any cursors created within it must be closed using mdb_cursor_close().
It is very rarely necessary to close a database handle, and in general they should just be left open.
The full LMDB API documentation lists further details, like how to:
1.8.2-20120930
