README

# LMDB Manual

###### \[in package LMDB\]
## LMDB ASDF System

- Version: 0.1
- Description: Bindings to LMDB, the Lightning Memory-mapped Database.
- Licence: MIT, see COPYING.
- Author: Fernando Borretti <eudoxiahp@gmail.com>, James Anderson <james.anderson@setf.de>, Gábor Melis <mega@retes.hu>
- Maintainer: Fernando Borretti <eudoxiahp@gmail.com>
- Homepage: [https://github.com/antimer/lmdb](https://github.com/antimer/lmdb)
- Bug tracker: [https://github.com/antimer/lmdb/issues](https://github.com/antimer/lmdb/issues)
- Source control: [GIT](git@github.com:antimer/lmdb.git)

## Links

Here is the [official repository](https://github.com/antimer/lmdb)
and the [HTML
documentation](http://melisgl.github.io/mgl-pax-world/lmdb-manual.html)
for the latest version.

## Introduction

[LMDB](http://www.lmdb.tech/doc/), the Lightning Memory-mapped
Database, is an [ACID](https://en.wikipedia.org/wiki/ACID) key-value
database with
[MVCC](https://en.wikipedia.org/wiki/Multiversion_concurrency_control).
It is a small C library ("C lmdb" from now on), around which LMDB is
a Common Lisp wrapper. LMDB covers most of C lmdb's functionality,
has a simplified API, much needed @LMDB/SAFETY checks, and
comprehensive documentation.

Compared to other key-value stores, LMDB's distuingishing features
are:

- Transactions span multiple keys.

- Embedded. It has no server but can be used concurrently not only
  by multiple threads but by multiple OS processes, too.

- Extremely high read performance: millions of transactions per
  second.

- Very low maintenance.

Other notable things:

- With its default - the most durable - settings, it has average
  write performance, which is bottlenecked by `fsync()`.

- Readers don't block readers or writers, but there is at most one
  writer at a time.

- Extremely simple, crash-proof design.

- The entire database (called *environment*) is backed by a single
  memory-mapped file, with a
  [copy-on-write](https://en.wikipedia.org/wiki/Copy-on-write)
  [B+ tree](https://en.wikipedia.org/wiki/B%2B_tree).

- No transaction log.

- It is very much like [Berkeley
  DB](https://en.wikipedia.org/wiki/Berkeley_DB) done right, without
  the fluff and much improved administration.

Do read the [Caveats](http://www.lmdb.tech/doc/), though. On the
Lisp side, this library **will not work with virtual threads**
because LMDB's write locking is tied to native threads.

Using LMDB is easy:

```
(with-temporary-env (*env*)
  (let ((db (get-db "test")))
    (with-txn (:write t)
      (put db "k1" #(2 3))
      (print (g3t db "k1")) ; => #(2 3)
      (del db "k1"))))
```

More typically, the environment and databases are opened once so
that multiple threads and transactions can access them:

```
(defvar *test-db*)

(unless *env*
  (setq *env* (open-env "/tmp/lmdb-test-env/" :if-does-not-exist :create))
  (setq *test-db* (get-db "test" :value-encoding :utf-8)))

(with-txn (:write t)
  (put *test-db* 1 "hello")
  (print (g3t *test-db* 1)) ; => "hello"
  (del *test-db* 1))
```

Note how :VALUE-ENCODING sneaked in above. This was so to make G3T
return a string instead of an octet vector.

LMDB treats keys and values as opaque byte arrays to be hung on a B+
tree, and only requires a comparison function to be defined over
keys. LMDB knows how to serialize the types `(UNSIGNED-BYTE 64)` and
[STRING][type] (which are often used as keys so sorting must work as
expected). Serialization of the rest of the datatypes is left to the
client. See @LMDB/ENCODINGS for more.

## Design and implementation

### Safety

The lmdb C API trusts client code to respect its rules. Being C,
managing object lifetimes is the biggest burden. There are also
rules that are documented, but not enforced. This Lisp wrapper tries
to enforce these rules itself and manage object lifetimes in a safe
way to avoid data corruption. How and what it does is described in
the following.

##### Environments

- OPEN-ENV checks that the same path is not used in multiple open
  environments to prevent locking issues documented in
  [Caveats](http://www.lmdb.tech/doc/).

- CLOSE-ENV waits until all @ACTIVE-TRANSACTIONs are finished before
  actually closing the environment. Alternatively, if OPEN-ENV was
  called with :SYNCHRONIZED NIL, to avoid the overhead of
  synchronization, the environment is closed only when garbage
  collected.

##### Transactions

- Checks are made to detect illegal operations on parent
  transactions (see LMDB-ILLEGAL-ACCESS-TO-PARENT-TXN-ERROR).

- Access to closed transactions is reliably detected.

- C LMDB allows read transactions to be used in multiple threads.
  The synchronization cost of performing this safely (i.e. without
  risking access to closed and freed transaction objects) is
  significant so this is not supported.

##### Databases

- [mdb\_dbi\_open()](http://www.lmdb.tech/doc/group__mdb.html#gac08cad5b096925642ca359a6d6f0562a)
  is wrapped by GET-DB in a transaction and is protected by a mutex
  to comply with C lmdb's requirements:

         A transaction that opens a database must finish (either
         commit or abort) before another transaction may open it.
         Multiple concurrent transactions cannot open the same
         database.

- [mdb\_dbi\_close()](http://www.lmdb.tech/doc/group__mdb.html#ga52dd98d0c542378370cd6b712ff961b5)
  is too dangerous to be exposed as explained in the GET-DB
  documentation.

- For similar reasons, DROP-DB is wrapped in WITH-ENV.

- [mdb\_env\_set\_mapsize()](http://www.lmdb.tech/doc/group__mdb.html#gaa2506ec8dab3d969b0e609cd82e619e5),
  [mdb\_env\_set\_max\_readers()](http://www.lmdb.tech/doc/group__mdb.html#gae687966c24b790630be2a41573fe40e2),
  and [mdb\_env\_set\_maxdbs()](http://www.lmdb.tech/doc/group__mdb.html#gaa2fc2f1f37cb1115e733b62cab2fcdbc)
  are only available through OPEN-ENV because they either require that
  there are no write transactions or do not work on open environments.

##### Cursors

- As even read transactions are restricted to a single thread, so
  are cursors. Using a cursor from a thread other than the one in
  which it was created (i.e. the thread of its transaction) raises
  LMDB-CURSOR-THREAD-ERROR. In return for this restriction, access
  to cursors belonging to closed transactions is reliably detected.

##### Signal handling

The C lmdb library handles system calls being interrupted (`EINTR`
and `EAGAIN`), but unwinding the stack from interrupts in the middle
of LMDB calls can leave the in-memory data structures such as
transactions inconsistent. If this happens, their further use risks
data corruption. For this reason, calls to LMDB are performed with
interrupts disabled. For SBCL, this means SB-SYS:WITHOUT-INTERRUPTS.
It is an error when compiling LMDB if an equivalent facility is not
found in the Lisp implementation. A warning is signalled if no
substitute is found for SB-SYS:WITH-INTERRUPTS because this makes
the body of WITH-ENV, WITH-TXN, WITH-CURSOR and similar
uninterruptible.

Operations that do not modify the database (G3T, CURSOR-FIRST,
CURSOR-VALUE, etc) are async unwind safe, and for performance they
are called without the above provisions.

Note that the library is not reentrant, so don't call LMDB from
signal handlers.

### Deviations from the C lmdb API

The following are the most prominent deviations and omissions from
the C lmdb API in addition to those listed in @LMDB/SAFETY.

##### Environments

- [mdb\_reader\_list()](http://www.lmdb.tech/doc/group__mdb.html#ga8550000cd0501a44f57ee6dff0188744)
  is not implemented.

- [mdb\_env\_copy()](http://www.lmdb.tech/doc/group__mdb.html#ga5d51d6130325f7353db0955dbedbc378)
  and its close kin are not yet implemented.

##### Transactions

- Read-only WITH-TXNs are turned into noops when "nested" (unless
  IGNORE-PARENT).

##### Databases

- [mdb\_set\_compare()](http://www.lmdb.tech/doc/group__mdb.html#ga68e47ffcf72eceec553c72b1784ee0fe)
  and [mdb\_set\_dupsort()](http://www.lmdb.tech/doc/group__mdb.html#gacef4ec3dab0bbd9bc978b73c19c879ae)
  are not exposed. If they are needed, implement a foreign comparison
  function and call LIBLMDB:SET-COMPARE or LIBLMDB:SET-DUPSORT
  directly or perhaps change the encoding of the data.

- Working with multiple contiguous values with DUPFIXED is not yet
  implemented. This functionality would belong in PUT, CURSOR-PUT,
  CURSOR-NEXT and CURSOR-VALUE.

- PUT, CURSOR-PUT do not support the
  [`RESERVE`](http://www.lmdb.tech/doc/group__mdb__put.html#gac0545c6aea719991e3eae6ccc686efcc)
  flag.


## Library versions

- [function] LMDB-FOREIGN-VERSION

    Return the version of the C lmdb library as a string like `0.9.26`.
    
    Wraps [mdb\_version()](http://www.lmdb.tech/doc/group__mdb.html#ga0e5d7298fc39b3c187fffbe30264c968).

- [function] LMDB-BINDING-VERSION

    Return a string representing the version of C lmdb based on which
    the CFFI bindings were created. The version string has the same
    format as LMDB-FOREIGN-VERSION.

## Environments

An environment (class ENV) is basically a single memory-mapped file
holding all the data, plus some flags determining how we interact
it. An environment can have multiple databases (class DB), each of
which is a B+ tree within the same file. An environment is like a
database in a relational db, and the databases in it are like tables
and indices. The terminology comes from [Berkeley
DB](https://docs.oracle.com/cd/E17276_01/html/programmer_reference/env.html).

### Environments reference

- [class] ENV

    An environment object through which a memory-mapped
    data file can be accessed. Always to be created by OPEN-ENV.

- [reader] ENV-PATH ENV (:PATH)

    The location of the memory-mapped file and the
    environment lock file.

- [reader] ENV-MAX-DBS ENV (:MAX-DBS)

    The maximum number of named databases in the
    environment. Currently a moderate number is cheap, but a huge
    number gets expensive: 7-120 words per transaction, and every
    GET-DB does a linear search of the opened database.

- [reader] ENV-MAX-READERS ENV (:MAX-READERS)

    The maximum number of threads/reader slots. See
    the documentation of the [reader lock
    table](http://lmdb.tech/doc/group__readers.html) for more.

- [reader] ENV-MAP-SIZE ENV (:MAP-SIZE)

    Specifies the size of the data file in bytes.

- [reader] ENV-MODE ENV (:MODE)

- [reader] ENV-FLAGS ENV (:FLAGS)

    A plist of the options as captured by OPEN-ENV.
    For example, `(:FIXED-MAP NIL :SUBDIR T ...)`.

### Opening and closing environments

- [variable] *ENV-CLASS* ENV

    The default class OPEN-ENV instaniates. Must be a subclass of ENV.
    This provides a way to associate application specific data with ENV
    objects.

- [function] OPEN-ENV PATH &KEY (CLASS \*ENV-CLASS\*) (IF-DOES-NOT-EXIST :ERROR) (SYNCHRONIZED T) (MAX-DBS 1) (MAX-READERS 126) (MAP-SIZE (\* 1024 1024)) (MODE 436) (SUBDIR T) (SYNC T) (META-SYNC T) READ-ONLY (TLS T) (READ-AHEAD T) (LOCK T) (MEM-INIT T) FIXED-MAP WRITE-MAP MAP-ASYNC

    Create an ENV object through which the LMDB environment can be
    accessed and open it. To prevent corruption, an error is signalled
    if the same data file is opened multiple times. However, the checks
    performed do not work on remote filesystems (see ENV-PATH).
    
    LMDB-ERROR is signalled if opening the environment fails for any
    other reason.
    
    Unless explicitly noted, none of the arguments persist (i.e. they
    are not saved in the data file).
    
    PATH is the filesystem location of the environment files (see SUBDIR
    below for more). Do not use LMDB data files on remote filesystems,
    even between processes on the same host. This breaks `flock()` on
    some OSes, possibly memory map sync, and certainly sync between
    programs on different hosts.
    
    IF-DOES-NOT-EXIST determines what happens if ENV-PATH does not
    exists:
    
    - :ERROR: An error is signalled.
    
    - :CREATE: A new memory-mapped file is created ensuring that all
      containing directories exist.
    
    - `NIL`: Return NIL without doing anything.
    
    See CLOSE-ENV for the description of SYNCHRONIZED.
    
    - MAX-DBS: The maximum number of named databases in the environment.
      Currently a moderate number is cheap, but a huge number gets
      expensive: 7-120 words per transaction, and every GET-DB does a
      linear search of the opened database.
    
    - MAP-SIZE: Specifies the size of the data file in bytes. The new
      size takes effect immediately for the current process, but will
      not be persisted to any others until a write transaction has been
      committed by the current process. Also, only map size increases
      are persisted into the environment. If the map size is increased
      by another process, and data has grown beyond the range of the
      current mapsize, starting a new transaction (see WITH-TXN) will
      signal LMDB-MAP-RESIZED-ERROR. If zero is specified for MAP-SIZE,
      then the persisted size is used from the data file. Also see
      LMDB-MAP-FULL-ERROR.
    
    - MODE: Unix file mode for files created. The default is `#o664`.
      Has no effect when opening an existing environment.
    
    The rest of the arguments correspond to LMDB environment flags and
    are available in the plist ENV-FLAGS.
    
    - SUBDIR: If SUBDIR, then the path is a directory which holds the
      `data.mdb` and the `lock.mdb` files. If SUBDIR is NIL, the path
      is the filename of the data file and the lock file has the same
      name plus a `-lock` suffix.
    
    - SYNC: If NIL, don't `fsync` after commit. 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 SYNC-ENV is called. However, if the filesystem preserves
      write order (very few do) and the WRITE-MAP (currently
      unsupported) 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.
    
    - META-SYNC: If NIL, flush system buffers to disk only once per
      transaction, but omit the metadata flush. Defer that until the
      system flushes files to disk, the next commit of a non-read-only
      transaction or SYNC-ENV. 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.
    
    - READ-ONLY: Map the data file in read-only mode. It is an error to
      try to modify anything in it.
    
    - TLS: Setting it to NIL allows each OS thread to have multiple
      read-only transactions (see WITH-TXN's IGNORE-PARENT argument). It
      also allows and transactions not to be tied to a single thread,
      but that's quite dangerous, see @LMDB/SAFETY.
    
    - READ-AHEAD: Turn off readahead as in `madvise(MADV_RANDOM)`. Most
      operating systems perform read-ahead 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. This option is not implemented on Windows.
    
    - LOCK: Data corruption lurks here. If NIL, don't do any locking. If
      concurrent access is anticipated, the caller must manage all
      concurrency itself. 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.
    
    - MEM-INIT: If NIL, don't initialize `malloc`ed 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`ed 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.
    
    - FIXED-MAP (experimental): This flag must be specified when
      creating the environment and is stored persistently in the data
      file. 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.
    
    Unsupported flags (an error is signalled if they are changed from
    their default values):
    
    - WRITE-MAP: Use a writable 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. This may
      be slightly faster for DBs that fit entirely in RAM, but is slower
      for DBs larger than RAM. Do not mix processes with and without
      WRITE-MAP on the same environment. This can defeat
      durability (SYNC-ENV, etc).
    
    - MAP-ASYNC: When using WRITE-MAP, use asynchronous flushes to disk.
      As with SYNC NIL, a system crash can then corrupt the database or
      lose the last transactions. Calling #sync ensures on-disk database
      integrity until next commit.
    
    Open environments have a finalizer attached to them that takes care
    of freeing foreign resources. Thus, the common idiom:
    
    ```
    (setq *env* (open-env "some-path"))
    ```
    
    is okay for development, too. No need to always do WITH-ENV,
    which does not mesh with threads anyway.
    
    Wraps [mdb\_env\_create()](http://www.lmdb.tech/doc/group__mdb.html#gaad6be3d8dcd4ea01f8df436f41d158d4)
    and [mdb\_env\_open()](http://www.lmdb.tech/doc/group__mdb.html#ga32a193c6bf4d7d5c5d579e71f22e9340).

- [function] CLOSE-ENV ENV &KEY FORCE

    Close ENV and free the memory. Closing an already closed ENV has no effect.
    
    Since accessing @LMDB/TRANSACTIONS, @LMDB/DATABASES and
    @LMDB/CURSORS after closing their environment would risk database
    curruption, CLOSE-ENV makes sure that they are not in use. There are
    two ways this can happen:
    
    - If ENV was opened :SYNCHRONIZED (see OPEN-ENV), then CLOSE-ENV
      waits until there are no @ACTIVE-TRANSACTIONs in ENV before
      closing it. This requires synchronization and introduces some
      overhead, which might be noticable for workloads involving lots of
      quick read transactions. It is an LMDB-ERROR to attempt to close
      an environment in a WITH-TXN to avoid deadlocks.
    
    - On the other hand, if SYNCHRONIZED was NIL, then - unless FORCE is
      true - calling CLOSE-ENV signals an LMDB-ERROR to avoid the
      @LMDB/SAFETY issues involved in closing the environment.
      Environments opened with :SYNCHRONIZED NIL are only closed when
      they are garbage collected and their finalizer is run. Still, for
      production it might be worth it to gain the last bit of
      performance.
    
    Wraps [mdb\_env\_close()](http://www.lmdb.tech/doc/group__mdb.html#ga4366c43ada8874588b6a62fbda2d1e95).

- [variable] *ENV* NIL

    The default ENV for macros and function that take an environment
    argument.

- [macro] WITH-ENV (ENV PATH &REST OPEN-ENV-ARGS) &BODY BODY

    Bind the variable ENV to a new enviroment returned by OPEN-ENV
    called with PATH and OPEN-ENV-ARGS, execute BODY, and CLOSE-ENV. The
    following example binds the default environment:
    
    ```
    (with-env (*env* "/tmp/lmdb-test" :if-does-not-exist :create)
      ...)
    ```


- [function] OPEN-ENV-P ENV

    See if ENV is open, i.e. OPEN-ENV has been called on it without a
    corresponding CLOSE-ENV.

### Miscellaneous environment functions

- [function] CHECK-FOR-STALE-READERS &OPTIONAL (ENV \*ENV\*)

    Check for stale entries in the reader lock table. See
    [Caveats](http://www.lmdb.tech/doc/). This function is called
    automatically by OPEN-ENV. If other OS processes or threads
    accessing ENV abort without closing read transactions, call this
    function periodically to get rid off them. Alternatively, close all
    environments accessing the data file.
    
    Wraps [mdb\_reader\_check()](http://www.lmdb.tech/doc/group__mdb.html#ga366923d08bb384b3d9580a98edf5d668).

- [function] ENV-STATISTICS &OPTIONAL (ENV \*ENV\*)

    Return statistics about ENV as a plist.
    
    - :PAGE-SIZE: The size of a database page in bytes.
    
    - :DEPTH: The height of the B-tree.
    
    - :BRANCH-PAGES: The number of internal (non-leaf) pages.
    
    - :LEAF-PAGES: The number of leaf pages.
    
    - :OVERFLOW-PAGES: The number of overflow pages.
    
    - :ENTRIES: The number of data items.
    
    Wraps [mdb\_env\_stat()](http://www.lmdb.tech/doc/group__mdb.html#gaf881dca452050efbd434cd16e4bae255).

- [function] ENV-INFO &OPTIONAL (ENV \*ENV\*)

    Return information about ENV as a plist.
    
    - :MAP-ADDRESS: Address of memory map, if fixed (see OPEN-ENV's
      FIXED-MAP).
    
    - :MAP-SIZE: Size of the memory map in bytes.
    
    - :LAST-PAGE-NUMBER: Id of the last used page.
    
    - :LAST-TXN-ID: Id of the last committed transaction.
    
    - :MAXIMUM-READERS: The number of reader slots.
    
    - :N-READERS: The number of reader slots current used.
    
    Wraps [mdb\_env\_info()](http://www.lmdb.tech/doc/group__mdb.html#ga18769362c7e7d6cf91889a028a5c5947).

- [function] SYNC-ENV &OPTIONAL (ENV \*ENV\*)

    Flush the data buffers to disk as in calling `fsync()`. When ENV
    had been opened with :SYNC NIL or :META-SYNC NIL, this may be handy
    to force flushing the OS buffers to disk, which avoids potential
    durability and integrity issues.
    
    Wraps [mdb\_env\_sync()](http://www.lmdb.tech/doc/group__mdb.html#ga85e61f05aa68b520cc6c3b981dba5037).

- [function] ENV-MAX-KEY-SIZE &OPTIONAL (ENV \*ENV\*)

    Return the maximum size of keys and @DUPSORT data in bytes. Depends
    on the compile-time constant `MDB_MAXKEYSIZE` in the C library. The
    default is 511. If this limit is exceeded LMDB-BAD-VALSIZE-ERROR is
    signalled.
    
    Wraps [mdb\_env\_get\_maxkeysize()](http://www.lmdb.tech/doc/group__mdb.html#gaaf0be004f33828bf2fb09d77eb3cef94).

- [macro] WITH-TEMPORARY-ENV (ENV &REST OPEN-ENV-ARGS) &BODY BODY

    Run BODY with an open temporary environment bound to ENV. In more
    detail, create an environment in a fresh temporary directory in an
    OS specific location. OPEN-ENV-ARGS is a list of keyword arguments
    and values for OPEN-ENV. This macro is intended for testing and
    examples.
    
    ```
    (with-temporary-env (*env*)
      (let ((db (get-db "test")))
        (with-txn (:write t)
          (put db "k1" #(2 3))
          (print (g3t db "k1")) ; => #(2 3)
          (del db "k1"))))
    ```
    
    Since data corruption in temporary environments is not a concern,
    unlike WITH-ENV, WITH-TEMPORARY-ENV closes the environment even if
    it was opened with :SYNCHRONIZED NIL (see OPEN-ENV and
    CLOSE-ENV).

## Transactions

The LMDB environment supports transactional reads and writes. By
default, these provide the standard ACID (atomicity, consistency,
isolation, durability) guarantees. Writes from a transaction are not
immediately visible to other transactions. When the transaction is
committed, all its writes become visible atomically for future
transactions even if Lisp crashes or there is power failure. If the
transaction is aborted, its writes are discarded.

Transactions span the entire environment (see ENV). All the updates
made in the course of an update transaction - writing records across
all databases, creating databases, and destroying databases - are
either completed atomically or rolled back.

Write transactions can be nested. Child transactions see the
uncommitted writes of their parent. The child transaction can commit
or abort, at which point its writes become visible to the parent
transaction or are discarded. If the parent aborts, all of the
writes performed in the context of the parent, including those from
committed child transactions, are discarded.

- [macro] WITH-TXN (&KEY (ENV '\*ENV\*) WRITE IGNORE-PARENT (SYNC T) (META-SYNC T)) &BODY BODY

    Start a transaction in ENV, execute BODY. Then, if the transaction
    is open (see OPEN-TXN-P) and BODY returned normally, attempt to
    commit the transaction. Next, if BODY performed a non-local exit or
    committing failed, but the transaction is still open, then abort it.
    It is explicitly allowed to call COMMIT-TXN or ABORT-TXN within
    WITH-TXN.
    
    Transactions provide ACID guarantees (with SYNC and META-SYNC both
    on). They span the entire environment, they are not specific to
    individual DB.
    
    - If WRITE is NIL, the transaction is read-only and no writes (e.g.
      PUT) may be performed in the transaction. On the flipside, many
      read-only transactions can run concurrently (see ENV-MAX-READERS),
      while write transactions are mutually exclusive. Furthermore, the
      single write transaction can also run concurrently with read
      transactions, just keep in mind that read transactions hold on to
      the state of the environment at the time of their creation and
      thus prevent pages since replaced from being reused.
    
    - If IGNORE-PARENT is true, then in an enclosing WITH-TXN, instead
      of creating a child transaction, start an independent transaction.
    
    - If SYNC is NIL, then no flushing of buffers will take place after
      a commit as if the environment had been opened with :SYNC NIL.
    
    - Likewise, META-SYNC is the per-transaction equivalent of the
      OPEN-ENV's META-SYNC.
    
    Also see @LMDB/NESTING-TRANSACTIONS.
    
    Wraps [mdb\_txn\_begin()](http://www.lmdb.tech/doc/group__mdb.html#gad7ea55da06b77513609efebd44b26920).

- [glossary-term] active transaction

    The active transaction in some environment and thread is the
    transaction of the innermost WITH-TXN being executed in the thread
    that belongs to the environment. In most cases, this is simply the
    enclosing WITH-TXN, but if WITH-TXNs with different :ENV arguments
    are nested, then it may not be:
    
    ```
    (with-temporary-env (env)
      (let ((db (get-db "db" :env env)))
        (with-temporary-env (inner-env)
          (with-txn (:env env :write t)
            (with-txn (:env inner-env)
              (put db #(1) #(2)))))))
    ```
    
    In the above example, DB is known to belong to ENV so although the
    immediately enclosing transaction belongs to INNER-ENV, PUT is
    executed in context of the outer, write transaction because that's
    the innermost in ENV.
    
    Operations that require a transaction always attempt to use the
    active transaction even if it is not open (see OPEN-TXN-P).

- [function] OPEN-TXN-P &OPTIONAL ENV

    See if there is an active transaction and it is open, i.e.
    COMMIT-TXN or ABORT-TXN have not been called on it. Also, RESET-TXN
    without a corresponding RENEW-TXN closes the transaction.

- [function] TXN-ID

    The ID of TXN. IDs are integers incrementing from 1. For a
    read-only transaction, this corresponds to the snapshot being read;
    concurrent readers will frequently have the same transaction ID.
    Only committed write transactions increment the ID. If a transaction
    aborts, the ID may be re-used by the next writer.

- [function] COMMIT-TXN &OPTIONAL ENV

    Commit the innermost enclosig transaction (or @ACTIVE-TRANSACTION
    belonging to ENV if ENV is specified) or signal an error if it is
    not open. If TXN is not nested in another transaction, committing
    makes updates performed visible to future transactions. If TXN is a
    child transaction, then committing makes updates visible to its
    parent only. For read-only transactions, committing releases the
    reference to a historical version environment, allowing reuse of
    pages replaced since.
    
    Wraps [mdb\_txn\_commit()](http://www.lmdb.tech/doc/group__mdb.html#ga846fbd6f46105617ac9f4d76476f6597).

- [function] ABORT-TXN &OPTIONAL ENV

    Close TXN by discarding all updates performed, which will then not
    be visible to either parent or future transactions. Aborting an
    already closed transaction is a noop. Always succeeds.
    
    Wraps [mdb\_txn\_abort()](http://www.lmdb.tech/doc/group__mdb.html#ga73a5938ae4c3239ee11efa07eb22b882).

- [function] RENEW-TXN &OPTIONAL ENV

    Renew TXN that was reset by RESET-TXN. This acquires a new reader
    lock that had been released by RESET-TXN. After renewal, it is as if
    TXN had just been started.
    
    Wraps [mdb\_txn\_renew()](http://www.lmdb.tech/doc/group__mdb.html#ga6c6f917959517ede1c504cf7c720ce6d).

- [function] RESET-TXN &OPTIONAL ENV

    Abort the open, read-only TXN, release the reference to the
    historical version of the environment, but make it faster to start
    another read-only transaction with RENEW-TXN. This is accomplished
    by not deallocating some data structures, and keeping the slot in
    the reader table. Cursors opened within the transaction must not be
    used again, except if renewed (see RENEW-CURSOR). If TXN is an open,
    read-only transaction, this function always succeeds.
    
    Wraps [mdb\_txn\_reset()](http://www.lmdb.tech/doc/group__mdb.html#ga02b06706f8a66249769503c4e88c56cd).

### Nesting transactions

When WITH-TXNs are nested (i.e. one is executed in the dynamic
extent of another), we speak of nested transactions. Transaction can
be nested to arbitrary levels. Child transactions may be committed
or aborted independently from their parent transaction (the
immediately enclosing WITH-TXN). Committing a child transaction only
makes the updates made by it visible to the parent. If the parent
then aborts, the child's updates are aborted too. If the parent
commits, all child transactions that were not aborted are committed,
too.

Actually, the C lmdb library only supports nesting write
transactions. To simplify usage, the Lisp side turns read-only
WITH-TXNs nested in another WITH-TXNs into noops.

```
(with-temporary-env (*env*)
  (let ((db (get-db "test" :value-encoding :uint64)))
    ;; Create a top-level write transaction.
    (with-txn (:write t)
      (put db "p" 0)
      ;; First child transaction
      (with-txn (:write t)
        ;; Writes of the parent are visible in children.
        (assert (= (g3t db "p") 0))
        (put db "c1" 1))
      ;; Parent sees what the child committed (but it's not visible to
      ;; unrelated transactions).
      (assert (= (g3t db "c1") 1))
      ;; Second child transaction
      (with-txn (:write t)
        ;; Sees writes from the parent that came from the first child.
        (assert (= (g3t db "c1") 1))
        (put db "c1" 2)
        (put db "c2" 2)
        (abort-txn)))
    ;; Create a top-level read transaction to check what was committed.
    (with-txn ()
      ;; Since the second child aborted, its writes are discarded.
      (assert (= (g3t db "p") 0))
      (assert (= (g3t db "c1") 1))
      (assert (null (g3t db "c2"))))))
```

COMMIT-TXN, ABORT-TXN, and RESET-TXN all close the
@ACTIVE-TRANSACTION (see OPEN-TXN-P). When the active transaction is
not open, database operations such as G3T, PUT, DEL signal
LMDB-BAD-TXN-ERROR. Furthermore, any @LMDB/CURSORS created in the
context of the transaction will no longer be valid (but see
CURSOR-RENEW).

An LMDB parent transaction and its cursors must not issue operations
other than COMMIT-TXN and ABORT-TXN while there are active child
transactions. As the Lisp side does not expose transaction objects
directly, performing @LMDB/BASIC-OPERATIONS in the parent
transaction is not possible, but it is possible with @LMDB/CURSORS
as they are tied to the transaction in which they were created.

IGNORE-PARENT true overrides the default nesting semantics of
WITH-TXN and creates a new top-level transaction, which is not a
child of the enclosing WITH-TXN.

- Since LMDB is single-writer, on nesting an IGNORE-PARENT write
  transaction in another write transaction, LMDB-BAD-TXN-ERROR is
  signalled to avoid the deadlock.

- Nesting a read-only WITH-TXN with IGNORE-PARENT in another
  read-only WITH-TXN is LMDB-BAD-RSLOT-ERROR error with the TLS
  option because it would create two read-only transactions in the
  same thread.

Nesting a read transaction in another transaction would be an
LMDB-BAD-RSLOT-ERROR according to the C lmdb library, but a
read-only WITH-TXN with IGNORE-PARENT NIL nested in another WITH-TXN
is turned into a noop so this edge case is papered over.

## Databases

### The unnamed database

LMDB has a default, unnamed database backed by a B+ tree. This db
can hold normal key-value pairs and named databases. The unnamed
database can be accessed by passing NIL as the database name to
GET-DB. There are some restrictions on the flags of the unnamed
database, see LMDB-INCOMPATIBLE-ERROR.

### DUPSORT

A prominent feature of LMDB is the ability to associate multiple
sorted values with keys, which is enabled by the DUPSORT argument of
GET-DB. Just as a named database is a B+ tree associated with a
key (its name) in the B+ tree of the unnamed database, so do these
sorted duplicates form a B+ tree under a key in a named or the
unnamed database. Among the @LMDB/BASIC-OPERATIONS, PUT and DEL are
equipped to deal with duplicate values, but G3T is too limited, and
@LMDB/CURSORS are needed to make full use of DUPSORT.

When using this feature the limit on the maximum key size applies to
duplicate data, as well. See ENV-MAX-KEY-SIZE.

### Database API

- [variable] *DB-CLASS* DB

    The default class that GET-DB instantiates. Must a subclass of DB.
    This provides a way to associate application specific data with DB
    objects.

- [function] GET-DB NAME &KEY (CLASS \*DB-CLASS\*) (ENV \*ENV\*) (IF-DOES-NOT-EXIST :CREATE) KEY-ENCODING VALUE-ENCODING INTEGER-KEY REVERSE-KEY DUPSORT INTEGER-DUP REVERSE-DUP DUPFIXED

    Open the database with NAME in the open environment ENV, and return
    a DB object. If NAME is NIL, then the @LMDB/THE-UNNAMED-DATABASE is
    opened.
    
    If GET-DB is called with the same name multiple times, the returned
    DB objects will be associated with the same database (although they
    may not be EQ). The first time GET-DB is called with any given name
    and environment, it must not be from an open transaction. This is
    because GET-DB starts a transaction itself to comply with C lmdb's
    requirements on
    [mdb\_dbi\_open()](http://www.lmdb.tech/doc/group__mdb.html#gac08cad5b096925642ca359a6d6f0562a) (see
    @LMDB/SAFETY). Since dbi handles are cached within ENV, subsequent
    calls do not involve `mdb_dbi_open()` and are thus permissible
    within transactions.
    
    CLASS designates the class which will instantiated. See *DB-CLASS*.
    
    If IF-DOES-NOT-EXIST is :CREATE, then a new named database is
    created. If IF-DOES-NOT-EXIST is :ERROR, then an error is signalled
    if the database does not exists.
    
    KEY-ENCODING and VALUE-ENCODING are both one of NIL, :UINT64,
    :OCTETS or :UTF-8. KEY-ENCODING is set to :UINT64 when INTEGER-KEY
    is true. VALUE-ENCODING is set to :UINT64 when INTEGER-DUP is true.
    Note that changing the encoding does *not* reencode already existing
    data. See @LMDB/ENCODINGS for the full semantics.
    
    GET-DB may be called more than once with the same NAME and ENV, and
    the returned DB objects will have the same underlying C lmdb
    database, but they may have different KEY-ENCODING and
    VALUE-ENCODING.
    
    The following flags are for database creation, they do not have any
    effect in subsequent calls (except for the
    @LMDB/THE-UNNAMED-DATABASE).
    
    - INTEGER-KEY: Keys in the database are C `unsigned` or `size_t`
      integers encoded in native byte order. Keys must all be either
      `unsigned` or `size_t`, they cannot be mixed in a single database.
    
    - 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.
    
    - DUPSORT: Duplicate keys may be used in the database (or, from
      another perspective, keys may have multiple values, stored in
      sorted order). By default, keys must be unique and may have only a
      single value. Also, see @DUPSORT.
    
    - INTEGER-DUP: This option specifies that duplicate data items are
      binary integers, similarly to INTEGER-KEY. Only matters if
      DUPSORT.
    
    - REVERSE-DUP: This option specifies that duplicate data items
      should be compared as strings in reverse order. Only matters if
      DUPSORT.
    
    - DUPFIXED: This flag may only be used in combination DUPSORT. When
      true, data items for this database must all be the same size,
      which allows further optimizations in storage and retrieval.
      Currently, the wrapper functions that could take advantage of
      this (e.g. PUT, CURSOR-PUT, CURSOR-NEXT and CURSOR-VALUE), do not.
    
    No function to close a database (an equivalent to
    [mdb\_dbi\_close()](http://www.lmdb.tech/doc/group__mdb.html#ga52dd98d0c542378370cd6b712ff961b5))
    is provided due to subtle races and corruption it could cause when
    an `MDB_dbi` (unsigned integer, similar to an fd) is assigned by a
    subsequent open to another named database.
    
    Wraps [mdb\_dbi\_open()](http://www.lmdb.tech/doc/group__mdb.html#gac08cad5b096925642ca359a6d6f0562a).

- [class] DB

    A database in an environment (class ENV). Always to
    be created by GET-DB.

- [reader] DB-NAME DB (:NAME)

    The name of the database.

- [reader] DB-KEY-ENCODING DB (:KEY-ENCODING)

    The ENCODING that was passed as KEY-ENCODING to
    GET-DB.

- [reader] DB-VALUE-ENCODING DB (:VALUE-ENCODING)

    The ENCODING that was passed as VALUE-ENCODING
    to GET-DB.

- [function] DROP-DB NAME PATH &KEY OPEN-ENV-ARGS (DELETE T)

    Empty the database with NAME in the environment denoted by PATH. If
    DELETE, then delete the database. Since closing a database is
    dangerous (see GET-DB), DROP-DB opens and closes the environment
    itself.
    
    Wraps [mdb\_drop()](http://www.lmdb.tech/doc/group__mdb.html#gab966fab3840fc54a6571dfb32b00f2db).

- [function] DB-STATISTICS DB

    Return statistics about the database.
    
    Wraps [mdb\_stat()](http://www.lmdb.tech/doc/group__mdb.html#gae6c1069febe94299769dbdd032fadef6).

## Encoding and decoding data

In the C lmdb library, keys and values are opaque byte vectors
only ever inspected internally to maintain the sort order (of keys
and also duplicate values if @DUPSORT). The client is given the
freedom and the responsibility to choose how to perform conversion
to and from byte vectors.

LMDB exposes this full flexibility while at the same time providing
reasonable defaults for the common cases. In particular, with the
KEY-ENCODING and VALUE-ENCODING arguments of GET-DB, the
data (meaning the key or value here) encoding can be declared
explicitly.

Even if the encoding is undeclared, it is recommended to use a
single type for keys (and duplicate values) to avoid unexpected
conflicts that could arise, for example, when the UTF-8 encoding of
a string and the :UINT64 encoding of an integer coincide. The same
consideration doubly applies to named databases, which share the key
space with normal key-value pairs in the default database (see
@LMDB/THE-UNNAMED-DATABASE).

Together, :UINT64 and :UTF-8 cover the common cases for keys. They
trade off dynamic typing for easy sortability (using the default C
lmdb behaviour). On the other hand, when sorting is not
concern (either for keys and values), serialization may be done more
freely. For this purpose, using an encoding of :OCTETS or NIL with
[cl-conspack](https://github.com/conspack/cl-conspack) is
recommended because it works with complex objects, it encodes object
types, it is fast and space-efficient, has a stable specification
and an alternative implementation in C. For example:

```
(with-temporary-env (*env*)
  (let ((db (get-db "test")))
    (with-txn (:write t)
      (put db "key1" (cpk:encode (list :some "stuff" 42)))
      (cpk:decode (g3t db "key1")))))
=> (:SOME "stuff" 42)
```

Note that multiple DB objects with different encodings can be
associated with the same C lmdb database, which declutters the code:

```
(defvar *cpk-encoding*
  (cons #'cpk:encode (alexandria:compose #'cpk:decode #'mdb-val-to-octets)))

(with-temporary-env (*env*)
  (let ((next-id-db (get-db "test" :key-encoding *cpk-encoding*
                                   :value-encoding :uint64))
        (db (get-db "test" :key-encoding *cpk-encoding*
                           :value-encoding *cpk-encoding*)))
    (with-txn (:write t)
      (let ((id (or (g3t next-id-db :next-id) 0)))
        (put next-id-db :next-id (1+ id))
        (put db id (list :some "stuff" 42))
        (g3t db id)))))
=> (:SOME "stuff" 42)
=> T
```


- [type] ENCODING

    The following values are supported:
    
    - :UINT64: Data to be encoded must be of type `(UNSIGNED-BYTE 64)`,
      which is then encoded as an 8 byte array in *native* byte order
      with UINT64-TO-OCTETS. The reverse transformation takes place when
      returning values. This is the encoding used for INTEGER-KEY and
      INTEGER-DUP DBs.
    
    - :OCTETS: Note the plural. Data to be encoded (e.g. KEY argument of
      G3T) must be a 1D byte array. If its element type
      is `(UNSIGNED-BYTE 8)`, then the data can be passed to the foreign
      code more efficiently, but declaring the element type is not
      required. For example, [VECTOR][type]s can be used as long as the
      actual elements are of type `(UNSIGNED-BYTE 8)`. Foreign byte
      arrays to be decoded (e.g. the value returned by G3T) are returned
      as OCTETS.
    
    - :UTF-8: Data to be encoded must be a string, which is converted to
      octets by TRIVIAL-UTF-8. Null-terminated. Foreign byte arrays are
      decoded the same way.
    
    - NIL: Data is encoded using the default encoding according to its
      Lisp type: strings as :UTF-8, vectors as :OCTETS, `(UNSIGNED-BYTE
      64)` as :UINT64. Decoding is always performed as :OCTETS.
    
    - A [CONS][type]: Data is encoded by the function in the CAR of the
      cons and decoded by the function in the CDR. For example, :UINT64
      is equivalent to `(CONS #'UINT64-TO-OCTETS #'MDB-VAL-TO-UINT64)`.


- [macro] WITH-MDB-VAL-SLOTS (%BYTES SIZE MDB-VAL) &BODY BODY

    Bind %BYTES and SIZE locally to the corresponding slots of MDB-VAL.
    MDB-VAL is an opaque handle for a foreign `MDB_val` struct, that
    holds the pointer to a byte array and the number of bytes in the
    array. This macro is needed to access the foreign data in a function
    used as *KEY-DECODER* or *VALUE-DECODER*. MDB-VAL is dynamic extent,
    so don't hold on to it. Also, the pointer to which %BYTES is bound
    is valid only within the context of current top-level transaction.

- [type] OCTETS &OPTIONAL (SIZE '\*)

    A 1D SIMPLE-ARRAY of `(UNSIGNED-BYTE 8)`.

- [function] MDB-VAL-TO-OCTETS MDB-VAL

    A utility function provided for writing *KEY-DECODER* and
    *VALUE-DECODER* functions. It returns a Lisp octet vector that holds
    the same bytes as MDB-VAL.

- [function] UINT64-TO-OCTETS N

    Convert an `(UNSIGNED-BYTE 64)` to OCTETS of length 8 taking the
    native byte order representation of N. Suitable as a *KEY-ENCODER*
    or *VALUE-ENCODER*.

- [function] OCTETS-TO-UINT64 OCTETS

    The inverse of UINT64-TO-OCTETS. Use MDB-VAL-TO-UINT64 as a
    *KEY-DECODER* or *VALUE-DECODER*.

- [function] MDB-VAL-TO-UINT64 MDB-VAL

    Like OCTETS-TO-UINT64, but suitable for *KEY-DECODER* or
    *VALUE-DECODER* that decodes unsigned 64 bit integers in native byte
    order. This function is called automatically when the encoding is
    known to require it (see GET-DB's INTEGER-KEY, :VALUE-ENCODING,
    etc).

- [function] STRING-TO-OCTETS STRING

    Convert STRING to OCTETS by encoding it as UTF-8 with null
    termination. Suitable as a *KEY-ENCODER* or *VALUE-ENCODER*.

- [function] OCTETS-TO-STRING OCTETS

    The inverse of STRING-TO-OCTETS. Use MDB-VAL-TO-STRING as a
    *KEY-DECODER* or *VALUE-DECODER*.

- [function] MDB-VAL-TO-STRING MDB-VAL

    Like OCTETS-TO-STRING, but suitable as a *KEY-DECODER* or
    *VALUE-DECODER*.

### Overriding encodings

Using multiple DB objects with different encodings is the
recommended practice (see the example in @LMDB/ENCODINGS), but when
that is inconvenient, one can override the encodings with the
following variables.

- [variable] *KEY-ENCODER* NIL

    A function designator, NIL or an ENCODING. If non-NIL, it overrides
    the encoding method determined by KEY-ENCODING (see GET-DB). It is
    called with a single argument, the key, when it is to be converted
    to an octet vector.

- [variable] *KEY-DECODER* NIL

    A function designator, NIL or an ENCODING. If non-NIL, it is
    called with a single MDB-VAL argument (see WITH-MDB-VAL-SLOTS), that
    holds a pointer to data to be decoded and its size. This function is
    called whenever a key is to be decoded and overrides the
    KEY-ENCODING argument of GET-DB.
    
    For example, if we are only interested in the length of the value
    and want to avoid creating a lisp vector on the heap, we can do
    this:
    
    ```
    (with-temporary-env (*env*)
      (let ((db (get-db "test")))
        (with-txn (:write t)
          (put db "key1" "abc")
          (let ((*value-decoder* (lambda (mdb-val)
                                   (with-mdb-val-slots (%bytes size mdb-val)
                                     (declare (ignore %bytes))
                                     ;; Take null termination into account.
                                     (1- size)))))
            (g3t db "key1")))))
    => 3
    => T
    ```


- [variable] *VALUE-ENCODER* NIL

    Like *KEY-ENCODER*, but for values.

- [variable] *VALUE-DECODER* NIL

    Like *KEY-DECODER*, but for values.
    
    Apart from performing actual decoding, the main purpose of
    *VALUE-DECODER*, one can also pass the foreign data on to other
    foreign functions such as `write()` directly from the decoder
    function and returning a constant such as T to avoid consing.

## Basic operations

- [function] G3T DB KEY

    Return the value from DB associated with KEY and T as the second
    value. If KEY is not found in DB, then NIL is returned. If DB
    supports @DUPSORT, then the first value for KEY will be returned.
    Retrieval of other values requires the use of @LMDB/CURSORS.
    
    This function is called G3T instead of GET to avoid
    having to shadow CL:GET when importing the LMDB package. On the
    other hand, importing the LMDB+ package, which has LMDB::GET
    exported, requires some shadowing.
    
    The LMDB+ package is like the LMDB package, but it has `#'LMDB:G3T`
    fbound to LMDB+:G3T so it probably needs shadowing to avoid conflict
    with CL:GET:
    
    ```
    (defpackage lmdb/test
      (:shadow #:get)
      (:use #:cl #:lmdb+))
    ```
    
    Wraps [mdb\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga8bf10cd91d3f3a83a34d04ce6b07992d).

- [function] PUT DB KEY VALUE &KEY (OVERWRITE T) (DUPDATA T) APPEND APPEND-DUP (KEY-EXISTS-ERROR-P T)

    Add a KEY, VALUE pair to DB within TXN (which must support writes).
    Returns T on success.
    
    - OVERWRITE: If NIL, signal LMDB-KEY-EXISTS-ERROR if KEY already
      appears in DB.
    
    - DUPDATA: If NIL, signal LMDB-KEY-EXISTS-ERROR if the KEY, VALUE
      pair already appears in DB. Has no effect if DB doesn't have
      DUPSORT.
    
    - APPEND: Append the KEY, VALUE pair to the end of DB instead of
      finding KEY's location in the B+ tree by performing comparisons.
      The client effectively promises that keys are inserted in sort
      order, which allows for fast bulk loading. If the promise is
      broken, a LMDB-KEY-EXISTS-ERROR is signalled.
    
    - APPEND-DUP: The client promises that duplicate values are inserted
      in sort order. If the promise is broken, a LMDB-KEY-EXISTS-ERROR
      is signalled.
    
    - If KEY-EXISTS-ERROR-P is NIL, then instead of signalling
      LMDB-KEY-EXISTS-ERROR return NIL.
    
    May signal LMDB-MAP-FULL-ERROR, LMDB-TXN-FULL-ERROR,
    LMDB-TXN-READ-ONLY-ERROR.
    
    Wraps [mdb\_put()](http://www.lmdb.tech/doc/group__mdb.html#ga4fa8573d9236d54687c61827ebf8cac0).

- [function] DEL DB KEY &KEY VALUE

    Delete KEY from DB. Returns T if data was deleted, NIL otherwise.
    If DB supports sorted duplicates (@DUPSORT), then VALUE is taken
    into account: if it's NIL, then all duplicate values for KEY are
    deleted, if it's not NIL, then only the matching value. May signal
    LMDB-TXN-READ-ONLY-ERROR.
    
    Wraps [mdb\_del()](http://www.lmdb.tech/doc/group__mdb.html#gab8182f9360ea69ac0afd4a4eaab1ddb0).

## Cursors

- [macro] WITH-CURSOR (VAR DB) &BODY BODY

    Bind VAR to a fresh CURSOR on DB. Execute BODY, then close the
    cursor. Within the dynamic extent of BODY, this will be the
    @DEFAULT-CURSOR. The cursor is tied to the @ACTIVE-TRANSACTION.
    
    LMDB-CURSOR-THREAD-ERROR is signalled if the cursor is accessed from
    threads other than the one in which it was created.
    
    Wraps [mdb\_cursor\_open()](http://www.lmdb.tech/doc/group__mdb.html#ga9ff5d7bd42557fd5ee235dc1d62613aa)
    and [mdb\_cursor\_close()](http://www.lmdb.tech/doc/group__mdb.html#gad685f5d73c052715c7bd859cc4c05188).

- [macro] WITH-IMPLICIT-CURSOR (DB) &BODY BODY

    Like WITH-CURSOR but the cursor object is not accessible directly,
    only through the @DEFAULT-CURSOR mechanism. The cursor is
    stack-allocated, which eliminates the consing of WITH-CURSOR. Note
    that stack allocation of cursors in WITH-CURSOR would risk data
    corruption if the cursor were accessed beyond its dynamic extent.
    
    Use WITH-IMPLICIT-CURSOR instead of WITH-CURSOR if a single cursor
    at a time will suffice. Conversely, use WITH-CURSOR if a second
    cursor is needed. That is, use
    
    ```
    (with-implicit-cursor (db)
      (cursor-set-key 1))
    ```
    
    but when two cursors iterate in an interleaved manner, use
    WITH-CURSOR:
    
    ```
    (with-cursor (c1 db)
      (with-cursor (c2 db)
        (cursor-first c1)
        (cursor-last c2)
        (if (some-pred (cursor-value c1) (cursor-value c2))
            (cursor-next c1)
            (cursor-prev c2))
        ...))
    ```
    
    Wraps [mdb\_cursor\_open()](http://www.lmdb.tech/doc/group__mdb.html#ga9ff5d7bd42557fd5ee235dc1d62613aa)
    and [mdb\_cursor\_close()](http://www.lmdb.tech/doc/group__mdb.html#gad685f5d73c052715c7bd859cc4c05188).

- [class] CURSOR STRUCTURE-OBJECT

- [function] CURSOR-DB INSTANCE

- [glossary-term] default cursor

    All operations, described below, that take cursor arguments accept
    NIL instead of a CURSOR object, in which case the cursor from the
    immediately enclosing WITH-CURSOR or WITH-IMPLICIT-CURSOR is used.
    This cursor is referred to as the *default cursor*.
    
    To reduce syntactic clutter, some operations thus make cursor
    arguments &OPTIONAL. When this is undesirable - because there are
    keyword arguments as well - the cursor may be a required argument as
    in CURSOR-PUT. Still NIL can be passed explicitly.

### Positioning cursors

The following functions *position* or *initialize* a cursor while
returning the value (*a* value with @DUPSORT) associated with a key,
or both the key and the value. Initialization is successful if there
is the cursor points to a key-value pair, which is indicated by the
last return value being T.

- [function] CURSOR-FIRST &OPTIONAL CURSOR

    Move CURSOR to the first key of its database. Return the key, the
    value and T, or NIL if the database is empty. If @DUPSORT, position
    CURSOR on the first value of the first key.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_FIRST](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-FIRST-DUP &OPTIONAL CURSOR

    Move CURSOR to the first duplicate value of the current key. Return
    the value and T. Return NIL if CURSOR is not positioned.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_FIRST\_DUP](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-LAST &OPTIONAL CURSOR

    Move CURSOR to the last key of its database. Return the key, the
    value and T, or NIL if the database is empty. If @DUPSORT, position
    CURSOR on the last value of the last key.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_LAST](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-LAST-DUP &OPTIONAL CURSOR

    Move CURSOR to the last duplicate value of the current key. Return
    the value and T. Return NIL if CURSOR is not positioned.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_LAST\_DUP](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-NEXT &OPTIONAL CURSOR

    Move CURSOR to the next key-value pair of its database and return
    the key, the value, and T. Return NIL if there is no next item. If
    @DUPSORT, position CURSOR on the next value of the current key if
    exists, else the first value of next key. If CURSOR is
    uninitialized, then CURSOR-FIRST is called on it first.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_NEXT](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-NEXT-NODUP &OPTIONAL CURSOR

    Move CURSOR to the first value of next key pair of its
    database (skipping over duplicate values of the current key). Return
    the key, the value and T. Return NIL if there is no next item. If
    CURSOR is uninitialized, then CURSOR-FIRST is called on it first.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_NEXT\_NODUP](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-NEXT-DUP &OPTIONAL CURSOR

    Move CURSOR to the next value of current key pair of its database.
    Return the value and T. Return NIL if there is no next value. If
    CURSOR is uninitialized, then CURSOR-FIRST is called on it first.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_NEXT\_DUP](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-PREV &OPTIONAL CURSOR

    Move CURSOR to the previous key-value pair of its database.
    Return the key, the value and T. Return NIL if there is no previous
    item. If @DUPSORT, position CURSOR on the previous value of the
    current key if exists, else the last value of previous key. If
    CURSOR is uninitialized, then CURSOR-LAST is called on it first.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_PREV](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-PREV-NODUP &OPTIONAL CURSOR

    Move CURSOR to the last value of previous key pair of its
    database (skipping over duplicate values of the current and the
    previous key). Return the key, the value, and T. Return NIL if
    there is no prev item. If CURSOR is uninitialized, then CURSOR-LAST
    is called on it first.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_PREV\_NODUP](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-PREV-DUP &OPTIONAL CURSOR

    Move CURSOR to the previous duplicate value of current key pair of
    its database. Return the value and T. Return NIL if there is no prev
    value. If CURSOR is uninitialized, then CURSOR-LAST is called on it
    first.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_PREV\_DUP](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-SET-KEY KEY &OPTIONAL CURSOR

    Move CURSOR to KEY of its database. Return the corresponding value
    and T. Return NIL if KEY was not found. If @DUPSORT, position CURSOR
    on the first value of KEY.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_SET\_KEY](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-SET-KEY-DUP KEY VALUE &OPTIONAL CURSOR

    Move CURSOR to the KEY, VALUE pair of its database and return T on
    success. Return NIL if the pair was not found.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_GET\_BOTH](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-SET-RANGE KEY &OPTIONAL CURSOR

    Position CURSOR on the first key equal to or greater than KEY.
    Return the found key, the value and T. Return NIL if KEY was not
    found. If @DUPSORT, position CURSOR on the first value of the found
    key.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_SET\_RANGE](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-SET-RANGE-DUP KEY VALUE &OPTIONAL CURSOR

    Position CURSOR exactly at KEY on the first value greater than or
    equal to VALUE. Return the value at the position and T on success,
    or NIL if there is no such value associated with KEY.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_GET\_BOTH\_RANGE](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

### Basic cursor operations

The following operations are similar to G3T, PUT, DEL (the
@LMDB/BASIC-OPERATIONS), but G3T has three variants
(CURSOR-KEY-VALUE, CURSOR-KEY, and CURSOR-VALUE). All of them
require the cursor to be positioned (see
@LMDB/POSITIONING-CURSORS).

- [function] CURSOR-KEY-VALUE &OPTIONAL CURSOR

    Return the key and value CURSOR is positioned at and T. Return NIL
    if CURSOR is uninitialized.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_GET\_CURRENT](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-KEY &OPTIONAL CURSOR

    Return the key CURSOR is positioned at and T. Return NIL if CURSOR
    is uninitialized.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_GET\_CURRENT](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-VALUE &OPTIONAL CURSOR

    Return the value CURSOR is positioned at and T. Return NIL if
    CURSOR is uninitialized.
    
    Wraps [mdb\_cursor\_get()](http://www.lmdb.tech/doc/group__mdb.html#ga48df35fb102536b32dfbb801a47b4cb0)
    with [MDB\_GET\_CURRENT](http://www.lmdb.tech/doc/group__mdb.html#ga1206b2af8b95e7f6b0ef6b28708c9127).

- [function] CURSOR-PUT KEY VALUE CURSOR &KEY CURRENT (OVERWRITE T) (DUPDATA T) APPEND APPEND-DUP

    Like PUT, store key-value pairs into CURSOR's database.
    CURSOR is positioned at the new item, or on failure usually near it.
    Return VALUE.
    
    - CURRENT: Replace the item at the current cursor position. KEY must
      still be provided, and must match it. If using sorted
      duplicates (@DUPSORT), VALUE must still sort into the same place.
      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.
    
    - OVERWRITE: If NIL, signal LMDB-KEY-EXISTS-ERROR if KEY already
      appears in CURSOR-DB.
    
    - DUPDATA: If NIL, signal LMDB-KEY-EXISTS-ERROR if the KEY, VALUE
      pair already appears in DB. Has no effect if CURSOR-DB doesn't
      have @DUPSORT.
    
    - APPEND: Append the KEY, VALUE pair to the end of CURSOR-DB instead
      of finding KEY's location in the B+ tree by performing
      comparisons. The client effectively promises that keys are
      inserted in sort order, which allows for fast bulk loading. If the
      promise is broken, LMDB-KEY-EXISTS-ERROR is signalled.
    
    - APPEND-DUP: The client promises that duplicate values are inserted
      in sort order. If the promise is broken, LMDB-KEY-EXISTS-ERROR is
      signalled.
    
    May signal LMDB-MAP-FULL-ERROR, LMDB-TXN-FULL-ERROR,
    LMDB-TXN-READ-ONLY-ERROR.
    
    Wraps [mdb\_cursor\_put()](http://www.lmdb.tech/doc/group__mdb.html#ga1f83ccb40011837ff37cc32be01ad91e).

- [function] CURSOR-DEL CURSOR &KEY DELETE-DUPS

    Delete the key-value pair CURSOR is positioned at. This does not
    make the cursor uninitialized, so operations such as CURSOR-NEXT can
    still be used on it. Both CURSOR-NEXT and CURSOR-KEY-VALUE will
    return the same record after this operation. If CURSOR is not
    initialized, LMDB-CURSOR-UNINITIALIZED-ERROR is signalled. Returns
    no values.
    
    If DELETE-DUPS, delete all duplicate values that belong to the
    current key. With DELETE-DUPS, CURSOR-DB must have @DUPSORT, else
    LMDB-INCOMPATIBLE-ERROR is signalled.
    
    May signal LMDB-CURSOR-UNINITIALIZED-ERROR,
    LMDB-TXN-READ-ONLY-ERROR.
    
    Wraps [mdb\_cursor\_del()](http://www.lmdb.tech/doc/group__mdb.html#ga26a52d3efcfd72e5bf6bd6960bf75f95).

### Miscellaneous cursor operations

- [function] CURSOR-RENEW &OPTIONAL CURSOR

    Associate CURSOR with the @ACTIVE-TRANSACTION (which must be
    read-only) as if it had been created with that transaction to begin
    with to avoid allocation overhead. CURSOR-DB stays the same. This
    may be done whether the previous transaction is open or closed (see
    OPEN-TXN-P). No values are returned.
    
    Wraps [mdb\_cursor\_renew()](http://www.lmdb.tech/doc/group__mdb.html#gac8b57befb68793070c85ea813df481af).

- [function] CURSOR-COUNT &OPTIONAL CURSOR

    Return the number of duplicate values for the current key of
    CURSOR. If CURSOR-DB doesn't have @DUPSORT, LMDB-INCOMPATIBLE-ERROR
    is signalled. If CURSOR is not initialized,
    LMDB-CURSOR-UNINITIALIZED-ERROR is signalled.
    
    Wraps [mdb\_cursor\_count()](http://www.lmdb.tech/doc/group__mdb.html#ga4041fd1e1862c6b7d5f10590b86ffbe2).

- [macro] DO-CURSOR (KEY-VAR VALUE-VAR CURSOR &KEY FROM-END NODUP) &BODY BODY

    Iterate over key-value pairs starting from the position of CURSOR.
    If CURSOR is not positioned then no key-value pairs will be seen. If
    FROM-END, then iterate with CURSOR-PREV instead of CURSOR-NEXT. If
    NODUP, then make that CURSOR-PREV-NODUP and CURSOR-NEXT-NODUP.
    
    If CURSOR is NIL, the @DEFAULT-CURSOR is used.
    
    If NODUP and not FROM-END, then the first duplicate of each key will
    be seen. If NODUP and FROM-END, then the last duplicate of each key
    will be seen.
    
    To iterate over all key-value pairs with keys >= 7:
    
    ```
    (with-cursor (cursor db)
      (cursor-set-key 7 cursor)
      (do-cursor (key value cursor)
        (print (cons key value))))
    ```


- [macro] DO-CURSOR-DUP (VALUE-VAR CURSOR &KEY FROM-END) &BODY BODY

    Iterate over duplicate values with starting from the position of
    CURSOR. If CURSOR is not positioned then no values will be seen. If
    FROM-END, then iterate with CURSOR-PREV-DUP instead of
    CURSOR-NEXT-DUP.
    
    If CURSOR is NIL, the @DEFAULT-CURSOR is used.
    
    To iterate over all values that not smaller than #(3 4 5),
    associated with the key 7:
    
    ```
    (with-cursor (cursor db)
      (cursor-set-key-dup cursor 7 #(3 4 5))
      (do-cursor-dup (value cursor)
        (print value)))
    ```


- [macro] DO-DB (KEY-VAR VALUE-VAR DB &KEY FROM-END NODUP) &BODY BODY

    Iterate over all keys and values in DB. If NODUP, then all but the
    first (or last if FROM-END) value for each key are skipped. If
    FROM-END, then iterate in reverse order.
    
    To iterate over all values in DB:
    
    ```
    (do-db (key value db)
      (print (cons key value)))
    ```
    
    This macro establishes a @DEFAULT-CURSOR.

- [macro] DO-DB-DUP (VALUE-VAR DB KEY &KEY FROM-END) &BODY BODY

    Iterate over all values associated with KEY in DB. If FROM-END,
    then iteration starts at the largest value.
    
    To iterate over all values associated with the key 7:
    
    ```
    (do-db-dup (value db 7)
      (print value))
    ```
    
    This macro establishes a @DEFAULT-CURSOR.

- [function] LIST-DUPS DB KEY &KEY FROM-END

    A thin wrapper around DO-DB-DUP, this function returns all values
    associated with KEY in DB as a list. If FROM-END, then the first
    element of the list is the largest value.

## Conditions

- [condition] LMDB-SERIOUS-CONDITION SERIOUS-CONDITION

    The base class of all LMDB conditions. Conditions
    that are LMDB-SERIOUS-CONDITIONs, but not LMDB-ERRORs are corruption
    and internal errors, which are hard to recover from.

- [condition] LMDB-ERROR LMDB-SERIOUS-CONDITION ERROR

    Base class for normal, recoverable LMDB errors.

### Conditions for C lmdb error codes

The following conditions correspond to [C lmdb error
codes](http://www.lmdb.tech/doc/group__errors.html).

- [condition] LMDB-KEY-EXISTS-ERROR LMDB-ERROR

    Key-value pair already exists. Signalled by PUT
    and CURSOR-PUT.

- [condition] LMDB-NOT-FOUND-ERROR LMDB-ERROR

    Key-value pair does not exist. All functions (G3T,
    CURSOR-NEXT, ...) should return NIL instead of signalling this
    error. If it is signalled, that's a bug.

- [condition] LMDB-PAGE-NOT-FOUND-ERROR LMDB-SERIOUS-CONDITION

    Requested page not found - this usually indicates
    corruption.

- [condition] LMDB-CORRUPTED-ERROR LMDB-SERIOUS-CONDITION

    Located page was wrong type.

- [condition] LMDB-PANIC-ERROR LMDB-SERIOUS-CONDITION

    Update of meta page failed or environment had fatal
    error.

- [condition] LMDB-VERSION-MISMATCH-ERROR LMDB-ERROR

    Environment version mismatch.

- [condition] LMDB-INVALID-ERROR LMDB-SERIOUS-CONDITION

    File is not a valid LMDB file.

- [condition] LMDB-MAP-FULL-ERROR LMDB-ERROR

    ENV-MAP-SIZE reached. Reopen the environment with a
    larger :MAP-SIZE.

- [condition] LMDB-DBS-FULL-ERROR LMDB-ERROR

    ENV-MAX-DBS reached. Reopen the environment with a
    higher :MAX-DBS.

- [condition] LMDB-READERS-FULL-ERROR LMDB-ERROR

    ENV-MAX-READERS reached. Reopen the environment
    with a higher :MAX-READERS.

- [condition] LMDB-TXN-FULL-ERROR LMDB-ERROR

    TXN has too many dirty pages. This condition is
    expected to occur only when using nested read-write transactions or
    operations multiple items (currently not supported by this
    wrapper).

- [condition] LMDB-CURSOR-FULL-ERROR LMDB-SERIOUS-CONDITION

    Cursor stack too deep - internal error.

- [condition] LMDB-PAGE-FULL-ERROR LMDB-SERIOUS-CONDITION

    Page has not enough space - internal error.

- [condition] LMDB-MAP-RESIZED-ERROR LMDB-ERROR

    Data file contents grew beyond ENV-MAP-SIZE. This
    can happen if another OS process using the same environment path set
    a larger map size than this process did.

- [condition] LMDB-INCOMPATIBLE-ERROR LMDB-ERROR

    Operation and DB incompatible, or DB type changed.
    This can mean:
    
    - The operation expects a @DUPSORT or DUPFIXED database.
    
    - Opening a named DB when the unnamed DB has DUPSORT or INTEGER-KEY.
    
    - Accessing a data record as a database, or vice versa.
    
    - The database was dropped and recreated with different flags.


- [condition] LMDB-BAD-RSLOT-ERROR LMDB-ERROR

    Invalid reuse of reader locktable slot. May be
    signalled by WITH-TXN.

- [condition] LMDB-BAD-TXN-ERROR LMDB-ERROR

    Transaction must abort, has a child, or is invalid.
    Signalled, for example, when a read-only transaction is nested in a
    read-write transaction, or when a cursor is used whose transaction
    has been closed (committed, aborted, or reset).

- [condition] LMDB-BAD-VALSIZE-ERROR LMDB-ERROR

    Unsupported size of key/DB name/data, or wrong
    DUPFIXED, INTEGER-KEY or INTEGER-DUP. See ENV-MAX-KEY-SIZE.

- [condition] LMDB-BAD-DBI-ERROR LMDB-ERROR

    The specified DBI was changed unexpectedly.

### Additional conditions

The following conditions do not have a dedicated C lmdb error
code.

- [condition] LMDB-CURSOR-UNINITIALIZED-ERROR LMDB-ERROR

    Cursor was not initialized. Position the cursor at
    a key-value pair with a function like CURSOR-FIRST or
    CURSOR-SET-KEY. Signalled when some functions return the C error
    code `EINVAL`.

- [condition] LMDB-CURSOR-THREAD-ERROR LMDB-ERROR

    Cursor was accessed from a thread other than the
    one in which it was created. Since the foreign cursor object's
    lifetime is tied to the dynamic extent of its WITH-CURSOR, this
    might mean accessing garbage in foreign memory with unpredictable
    consequences.

- [condition] LMDB-TXN-READ-ONLY-ERROR LMDB-ERROR

    Attempt was made to write in a read-only
    transaction. Signalled when some functions return the C error code
    `EACCESS`.

- [condition] LMDB-ILLEGAL-ACCESS-TO-PARENT-TXN-ERROR LMDB-ERROR

    A parent transaction and its cursors may not
    issue any other operations than COMMIT-TXN and ABORT-TXN while it
    has active child transactions. In LMDB, @LMDB/BASIC-OPERATIONS are
    always executed in the @ACTIVE-TRANSACTION, but @LMDB/CURSORS can
    refer to the parent transaction:
    
    ```
    (with-temporary-env (*env*)
      (let ((db (get-db "db")))
        (with-txn (:write t)
          (put db #(1) #(1))
          (with-cursor (cursor db)
            (with-txn (:write t)
              (assert-error lmdb-illegal-access-to-parent-txn-error
                (cursor-set-key #(1) cursor)))))))
    ```


* * *
###### \[generated by [MGL-PAX](https://github.com/melisgl/mgl-pax)\]