mdbx: backport - Add Getting Started doc.

Change-Id: I41847e550dc8246fca293bef3d103c35d43f3001

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+INPUT                  = lmdb.h midl.h mdb.c midl.c intro.doc
 
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intro.doc

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+/*
+ * Copyright 2015 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
+
+*/

lmdb.h

@@ -40,6 +40,9 @@
  *	corrupt the database. Of course if your application code is known to
  *	be bug-free (...) then this is not an issue.
  *
+ *	If this is your first time using a transactional embedded key/value
+ *	store, you may find the \ref starting page to be helpful.
+ *
  *	@section caveats_sec Caveats
  *	Troubleshooting the lock file, plus semaphores on BSD systems:
  *