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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 11:11:40 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-27 11:11:40 +0000 |
commit | 7731832751ab9f3c6ddeb66f186d3d7fa1934a6d (patch) | |
tree | e91015872543a59be2aad26c2fea02e41b57005d /libraries/liblmdb/intro.doc | |
parent | Initial commit. (diff) | |
download | openldap-1a00339458686feff01d4a8e157da4800292a9af.tar.xz openldap-1a00339458686feff01d4a8e157da4800292a9af.zip |
Adding upstream version 2.4.57+dfsg.upstream/2.4.57+dfsgupstream
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to '')
-rw-r--r-- | libraries/liblmdb/intro.doc | 192 |
1 files changed, 192 insertions, 0 deletions
diff --git a/libraries/liblmdb/intro.doc b/libraries/liblmdb/intro.doc new file mode 100644 index 0000000..4853af7 --- /dev/null +++ b/libraries/liblmdb/intro.doc @@ -0,0 +1,192 @@ +/* + * Copyright 2015-2020 Howard Chu, Symas Corp. + * All rights reserved. + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted only as authorized by the OpenLDAP + * Public License. + * + * A copy of this license is available in the file LICENSE in the + * top-level directory of the distribution or, alternatively, at + * <http://www.OpenLDAP.org/license.html>. + */ +/** @page starting Getting Started + +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 +\ref mdb documentation should make sense. Thanks to Bert Hubert +for creating the +<a href="https://github.com/ahupowerdns/ahutils/blob/master/lmdb-semantics.md"> +initial version</a> 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 \ref Cursors +below if you want to do more). + +A key/value pair is expressed as two #MDB_val structures. This struct +has two fields, \c mv_size and \c mv_data. The data is a \c void pointer to +an array of \c 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 <b>look but do not touch</b> (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. + +@section Cursors Cursors + +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 \ref mdb docs. + +When using #mdb_cursor_put(), either the function will position the +cursor for you based on the \b key, or you can use operation +#MDB_CURRENT to use the current position of the cursor. Note that +\b key must then match the current position's key. + +@subsection summary Summarizing the Opening + +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? + +@section thrproc Threads and Processes + +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. + +@section txns Transactions, Rollbacks, etc. + +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 \b 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. + +@section dupkeys Duplicate Keys + +#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. + +@section optim Some Optimization + +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(). + +@section cleanup Cleaning Up + +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. + +@section onward The Full API + +The full \ref mdb documentation lists further details, like how to: + + \li size a database (the default limits are intentionally small) + \li drop and clean a database + \li detect and report errors + \li optimize (bulk) loading speed + \li (temporarily) reduce robustness to gain even more speed + \li gather statistics about the database + \li define custom sort orders + +*/ |