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README.md

Python LSM-DB

Fast Python bindings for SQLite4's LSM key/value store.

Features:

  • Embedded zero-conf database.
  • Keys support in-order traversal using cursors.
  • Transactional (including nested transactions).
  • Single writer/multiple reader MVCC based transactional concurrency model.
  • On-disk database stored in a single file.
  • Data is durable in the face of application or power failure.
  • Thread-safe.
  • Python 2.x

Limitations:

  • Not tested on Windoze.

The source for Python lsm-db is hosted on GitHub.

If you encounter any bugs in the library, please open an issue, including a description of the bug and any related traceback.

Quick-start

Below is a sample interactive console session designed to show some of the basic features and functionality of the lsm-db Python library. Also check out the API documentation.

To begin, instantiate a LSM object, specifying a path to a database file.

>>> from lsm import LSM
>>> db = LSM('test.ldb')

Key/Value Features

lsm-db is a key/value store, and has a dictionary-like API:

>>> db['foo'] = 'bar'
>>> print db['foo']
bar

>>> for i in range(4):
...     db['k%s' % i] = str(i)
...

>>> 'k3' in db
True
>>> 'k4' in db
False

>>> del db['k3']
>>> db['k3']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "lsm.pyx", line 973, in lsm.LSM.__getitem__ (lsm.c:7142)
  File "lsm.pyx", line 777, in lsm.LSM.fetch (lsm.c:5756)
  File "lsm.pyx", line 778, in lsm.LSM.fetch (lsm.c:5679)
  File "lsm.pyx", line 1289, in lsm.Cursor.seek (lsm.c:12122)
  File "lsm.pyx", line 1311, in lsm.Cursor.seek (lsm.c:12008)
KeyError: 'k3'

By default when you attempt to look up a key, lsm-db will search for an exact match. You can also search for the closest key, if the specific key you are searching for does not exist:

>>> from lsm import SEEK_LE, SEEK_GE
>>> db['k1xx', SEEK_LE]  # Here we will match "k1".
'1'
>>> db['k1xx', SEEK_GE]  # Here we will match "k2".
'2'

LSM supports other common dictionary methods such as:

  • keys()
  • values()
  • update()

Slices and Iteration

The database can be iterated through directly, or sliced. When you are slicing the database the start and end keys need not exist -- lsm-db will find the closest key (details can be found in the LSM.fetch_range() documentation).

>>> [item for item in db]
[('foo', 'bar'), ('k0', '0'), ('k1', '1'), ('k2', '2')]

>>> db['k0':'k99']
<generator object at 0x7f2ae93072f8>

>>> list(db['k0':'k99'])
[('k0', '0'), ('k1', '1'), ('k2', '2')]

You can use open-ended slices. If the lower- or upper-bound is outside the range of keys an empty list is returned.

>>> list(db['k0':])
[('k0', '0'), ('k1', '1'), ('k2', '2')]

>>> list(db[:'k1'])
[('foo', 'bar'), ('k0', '0'), ('k1', '1')]

>>> list(db[:'aaa'])
[]

To retrieve keys in reverse order, simply use a higher key as the first parameter of your slice. If you are retrieving an open-ended slice, you can specify True as the step parameter of the slice.

>>> list(db['k1':'aaa'])  # Since 'k1' > 'aaa', keys are retrieved in reverse:
[('k1', '1'), ('k0', '0'), ('foo', 'bar')]

>>> list(db['k1'::True])  # Open-ended slices specify True for step:
[('k1', '1'), ('k0', '0'), ('foo', 'bar')]

You can also delete slices of keys, but note that the delete will not include the keys themselves:

>>> del db['k0':'k99']

>>> list(db)  # Note that 'k0' still exists.
[('foo', 'bar'), ('k0', '0')]

Cursors

While slicing may cover most use-cases, for finer-grained control you can use cursors for traversing records.

>>> with db.cursor() as cursor:
...     for key, value in cursor:
...         print key, '=>', value
...
foo => bar
k0 => 0

>>> db.update({'k1': '1', 'k2': '2', 'k3': '3'})

>>> with db.cursor() as cursor:
...     cursor.first()
...     print cursor.key()
...     cursor.last()
...     print cursor.key()
...     cursor.previous()
...     print cursor.key()
...
foo
k3
k2

>>> with db.cursor() as cursor:
...     cursor.seek('k0', SEEK_GE)
...     print list(cursor.fetch_until('k99'))
...
[('k0', '0'), ('k1', '1'), ('k2', '2'), ('k3', '3')]

It is very important to close a cursor when you are through using it. For this reason, it is recommended you use the LSM.cursor() context-manager, which ensures the cursor is closed properly.

Transactions

lsm-db supports nested transactions. The simplest way to use transactions is with the LSM.transaction() method, which doubles as a context-manager or decorator.

>>> with db.transaction() as txn:
...     db['k1'] = '1-mod'
...     with db.transaction() as txn2:
...         db['k2'] = '2-mod'
...         txn2.rollback()
...
True
>>> print db['k1'], db['k2']
1-mod 2

You can commit or roll-back transactions part-way through a wrapped block:

>>> with db.transaction() as txn:
...    db['k1'] = 'outer txn'
...    txn.commit()  # The write is preserved.
...
...    db['k1'] = 'outer txn-2'
...    with db.transaction() as txn2:
...        db['k1'] = 'inner-txn'  # This is commited after the block ends.
...    print db['k1']  # Prints "inner-txn".
...    txn.rollback()  # Rolls back both the changes from txn2 and the preceding write.
...    print db['k1']
...
1              <- Return value from call to commit().
inner-txn      <- Printed after end of txn2.
True           <- Return value of call to rollback().
outer txn      <- Printed after rollback.

If you like, you can also explicitly call LSM.begin(), LSM.commit(), and LSM.rollback().

>>> db.begin()
>>> db['foo'] = 'baze'
>>> print db['foo']
baze
>>> db.rollback()
True
>>> print db['foo']
bar

Reading more

For more information, check out the project's documentation, hosted at readthedocs:

https://lsm-db.readthedocs.io/en/latest/